The Importance of Micronutrient Adequacy in Obesity and the Potential of Microbiota Interventions to Support It

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Recommendation for including co-supplementation with micronutrients and pro/pre/synbiotics in obesity treatment plans.

Abstract

Micronutrient deficiencies co-occur with obesity throughout the world. While many factors may contribute to this, microbiota dysbiosis is certainly one that has received a lot of attention in recent years. This work aimed to review the current state of knowledge on the role of micronutrients in obesity and the effects of interventions in microbiota on the micronutrient status of humans. Gathered evidence suggested that the supplementation of most of the deficient micronutrients for people with excess weight may have a considerable, positive impact on lipid and glucose homeostasis and a small effect on weight loss. Interestingly, the doses of micronutrient supplementation that allowed for achieving the best results for most of the minerals and vitamins exceeded the tolerable upper intake levels. To avoid negative effects associated with an overdose of vitamins and minerals, applying microbiota interventions could be considered. Pro- and prebiotics were shown to improve the micronutrient status of humans, and several publications indicated that when applied together with vitamins and minerals, they could give greater benefits than each of these treatments alone. Therefore, supplementation with vitamins, minerals, and pro/pre/synbiotics in obesity treatment plans may be recommended; however, further research is required to mitigate risks and optimize the effects achieved.

1. Introduction

Obesity is a major public health problem worldwide. It shortens life expectancy and predisposes to metabolic diseases [1]. In 2019, the treatment of obesity and related health conditions was associated with costs averaging 1.8% of gross domestic product (mean for eight selected countries) and was predicted to double by 2060 [2]. Hence, the development of effective obesity prevention and treatment approaches is much needed.

One of the conditions co-occurring with obesity, the impact of which on excessive weight gain is understudied, is micronutrient malnutrition. Typical deficiencies include vitamins A, D, C, B1, B6, B9, and B12, and elements Zn, Se, Fe, Ca, and Mg [3,4,5,6]. Out of these deficiencies, possibly the most prevalent one refers to vitamin D; however, it should be stressed that data are highly variable through the studies (see

Table 1. Prevalence (%) of micronutrient deficiencies in people with obesity quoted in selected reviews.

Micronutrient	Via 2012 [3]	Roust and DiBaise, 2017 * [4]	Ciobârcâ et al., 2022 * [5]
Vitamin A	17	0–17	-
Vitamin D	80–90	22–80	20–98
Vitamin C	35–35	-	-
Vitamin B1	15–29	-	-
Vitamin B6	0–11	-	-
Vitamin B9	3–4	0–32	0–63
Vitamin B12	3–8	0–12	5–34
Magnesium	-	0–35	-
Calcium	-	0–14	-
Iron	-	1–9	2–29
Selenium	58	3	-
Zinc	14–30	0–3	-

* In people with morbid obesity prior to bariatric surgery.

) due to heterogeneity of included population groups but also cut-off levels that are referred to as deficient. Meta-analyses summarizing existing evidence in the area are hence in high demand. Recently, such a study was published for vitamin D, which was found to be insufficient in 85% of candidates for bariatric surgery (cut-off concentration 30 ng/mL) [7]. Further works featuring other micronutrients are required.

Through the literature, several authors have emphasized unfavorable dietary habits associated with obesity that could impact the nutritional status of humans. A high volume of ultra-processed foods [8], lower consumption of wholegrain cereals [9], eating away from home [10], and skipping breakfast [11] were a few of them. Furthermore, some studies indicated that the majority of individuals with obesity did not meet the recommended intake for most nutrients [12]. However, dietary habits do not seem to fully explain the development of micronutrient deficiencies in obesity since the condition may not be simply alleviated by providing an adequate concentration of deficient micronutrients to patients who are obese [13].

At present, the cause of micronutrient deficiencies in the population with obesity remains poorly understood. Nevertheless, given that obesity co-occurs not only with micronutrient deficiencies but also with the dysbiosis of intestinal microbiota [14], in recent years, authors started to link both these conditions and give more interest to the role of microbiota in the nutritional status of individuals with obesity [15]. Indeed, microorganisms may interact with micronutrients in various ways, including modulation of their passage through the intestinal epithelium [16]. Therefore, the hypothesis that the microbiota plays a role in the nutritional status of the organism seems justified. However, the interplay between microbiota, micronutrients, and obesity has not been reviewed in detail so far. In particular, to our knowledge, currently, there is no work available that has gathered scientific evidence on the effects of the supplementation of deficient micronutrients in people with obesity and related disorders at adequate doses and discussed whether microbiota interventions could help to replenish the micronutrient status of this population group. Nevertheless, given the pressing need for the development of effective obesity treatment approaches, such works are much needed.

This work aimed to evaluate the role of micronutrients in obesity and review factors that may govern the nutritional status of the body, with a special focus on microbiota. The aim was achieved through gathering the strongest and most recent research evidence on the following:

• Effects of micronutrient-deficient diet on obesity;
• Effects of obesity on micronutrient deficiency;
• Effects of micronutrient supplementation on weight management and metabolic health in obesity;
• Mechanisms used by microorganisms to impact the bioavailability of micronutrients;
• Effects of the interventions into microbiota on the micronutrient status of humans.

The scientific evidence from both human and knowledge-wise complementary animal studies was gathered.

This work was concluded with a highlight of existing knowledge gaps that, when filled, could contribute to the development of effective obesity treatments.

2. Methodology

Relevant works published up to March 2024 were included in this review. Databases used in the search were PubMed, Web of Science, Cochrane Library, and Google Scholar. Combinations of the following terms were applied in the search: “obesity”, “weight loss”, “supplementation”, “prebiotic”, “probiotic”, “micronutrients”, “vitamins”, “minerals”, and individual keywords representing each vitamin and mineral of interest. To gather data on the efficacy of micronutrients and microbiota intervention in supporting weight loss and metabolic health, as well as the micronutrient status of humans, respectively, the strongest research evidence was sought. Therefore, meta-analyses and systematic reviews were prioritized over randomized controlled trials (RCTs). Further selection criteria for these studies were recency, number of reported outcomes of interest, focus on people with excess weight, and reported effect sizes (weighted mean difference—WMD, or mean difference—MD, over standardized mean difference—SMD) to enable comparing outcomes between the works.

Although this review focused on the effects of the mentioned interventions in humans, in cases where no sufficient data were available, some complementary evidence from animal studies was included. Such practice was only applied in cases of insufficient data from human studies or important findings, showing responses in living organisms that for ethical (experiments where obesity or related health disorders were induced in animals on purpose) or physical reasons (for example, in the case of germ-free animals) could not be replicated in humans.

3. Effects of Micronutrient-Deficient Diet on Obesity

Several studies have suggested that the rate of increase in obesity is greater in regions of the world with greater micronutrient deficiencies [17], while others reported significant inverse associations between the dietary intake of micronutrients and Body Mass Index (BMI) and/or obesity [18,19,20,21,22]. These studies were cross-sectional and hence failed to demonstrate the cause-and-effect relationship between micronutrient deficiencies and excess weight unequivocally.

Stronger research evidence showing that a micronutrient-deficient diet may induce obesity in animals was published. Amara et al. (2014) [23] examined the effect of simultaneously reducing the content of vitamins (B1, B2, B3, B5, B6, B7, B9, B12, A, D3, E, and K) in the diet of C57BL/6 J Rj mice by 50% over 12 weeks. At the end of the experiment, test mice were characterized by 6% higher weight and twice as much body fat content when compared to control mice on standard feed. At the same time, no significant difference in feed consumption in both groups was noted. The authors suggested that vitamin deficiencies in the diet favored the deposition of adipose tissue by reducing the intensity of its breakdown processes.

Another murine study that investigated the influence of micronutrient inadequacy in the development of obesity focused on vitamin D and Ca [24]. In this work, mice were fed a “new Western diet” with an elevated fat content to induce obesity. A group of mice with Ca and vitamin D insufficiency developed greater levels of inflammatory markers in blood plasma when compared to the control group. Nevertheless, both groups had similar adiposity and mean body weight throughout the experiment. Therefore, the dietary adequacy of vitamin D and Ca did not protect mice from the development of obesity due to a high-fat diet.

Summarizing, studies suggest that inadequacy of micronutrient intake may lead to excessive weight gain but adequacy may not protect from obesity if other dietary recommendations—such as relatively low-fat intake—are not met.

4. Effects of Obesity on Micronutrient Deficiency

The evidence clearly showing the development of micronutrient deficiencies with the progression of obesity comes from animal studies. For example, Trasino et al. (2015) found impaired vitamin A transcriptional signaling in murine organs, which advanced with increasing adiposity and fatty liver but normalized with weight loss [25], while Chung et al. (2016) showed that Fe storage in murine livers was lowered due to obesity but could be restored by the administration of a low-fat diet [26]. Prospective studies on the development of micronutrient malnutrition in humans with the advancement of excessive weight gain are lacking; however, many works show that weight loss may affect blood levels of vitamins and minerals. Broadly researched in this respect is vitamin D. A meta-analysis of RCTs and controlled trials without randomization reported that serum 25-hydroxyvitamin D increased with weight reduction by a modest mean difference (MD) of 1.24 ng/mL and 1.94 ng/mL in each group of studies, respectively, with no dose-dependent effects for weight loss [27]. The status of other nutrients during the treatment of obesity was also looked at. An overview of the results from exemplary trials is gathered in

Table 2. Effects of weight loss treatments on micronutrient status of people with excess weight.

Micronutrient	Mean or Median % Change within Group	Studied Group	Mean or Median Weight Loss within Group (kg)	Treatment Duration	Reference
Serum vit. A	−22	Danish ^ adults with obesity	12	8 weeks	[28]
Serum vit. E	−25
Plasma vit. C	−27	American ^ adults with obesity	4	4 weeks	[29]
Erythrocyte vit. B1	−12	Australian ^ adults with T2D 1 and regular levels of vit. B1 in the diet	10	16 weeks	[30]
Plasma vit. B6	52	Spanish ^ women with obesity on hypocaloric cereal-based diet	3	6 weeks	[31]
Serum vit. B9	217	Danish ^ adults with obesity	12	8 weeks	[28]
Serum vit. B12	24
Serum Mg	5	Norwegian ^ patients with a BMI in the obese range	10	8 weeks	[32]
Serum Ca	<−1	Jordanian ^ women with obesity	10	3 months	[33]
Serum Fe	35	Jordanian ^ women (18–30 years old)	7	3 months	[34]
Serum Se	<−1	Cypriot ^ adults	1	8 weeks	[35]
Serum Zn	10	Egyptian ^ students on low-carbohydrate diet	6	40 days	[36]

¹ Type 2 diabetes, ^ denotes patients eligible to participate in the study carried out in this country. Color coding means that there was (dark-grey for decrease and white for increase) or there was not (light-grey) a significant change in the status of the particular nutrient. Data that were sourced out from the studies on the effects of micronutrient supplementation are results for control groups where no supplementation was used.

.

With weight loss, blood levels of vitamins A, E, C, and B1 were found to decrease [28,29,30], while no appreciable changes in Se and Ca were noted [33,35]. On the other hand, the status of Mg, Fe, and Zn, as well as vitamins B6, B9, and B12, was shown to improve [28,31,32,34,36].

The impact of dietary interventions that caused weight loss on the micronutrient status of patients participating in studies listed in Table 2 cannot be ignored since these interventions may provide high concentrations of micronutrients. For example, the plasma level of vitamin B6 increased by 52% in women with obesity on a cereal-based diet and only by 3% in women on a vegetable-based diet [31]. In addition, including foods that provide an excess of vitamin B1 in a dietary intervention was shown to prevent its losses [30].

Nevertheless, besides dietary intake, some other reasons related to improvements in the homeostasis of the organism following weight reduction could also be crucial for micronutrient status. For example, the increase of serum Mg could be explained by the decrease in blood glucose levels [32] since one of the major reasons for hypomagnesemia in type 2 diabetes (T2D) is osmotic diuresis that reduces the tubular reabsorption of this macroelement [37].

Among the micronutrients that are effectively restored in people with obesity due to weight loss, some phenomena that govern Fe deficiency were well established. A recent review described multiple mechanisms that could lead to the accumulation of this mineral in the adipose tissue and subsequent dysregulation of its function in obesity [38]. Dietary Fe absorption decreases in obesity due to increased levels of hepcidin, a hormone regulating the uptake of this mineral into the blood [39]. Levels of hepcidin are affected by the low-level inflammation known to co-occur with obesity [40]. Hence, weight loss could result in the improvement of Fe status through both the decrease of adiposity and the reduction of the inflammatory status.

Also, Zn status can be affected by inflammation. Proinflammatory cytokines released by the adipose tissue and oxidative stress that is chronic in obesity and associated with excess fat storage are known to alter the expression of Zn transporters [41,42].

Other than Fe, vitamin D is yet another micronutrient that is known to be sequestered in the adipose tissue [7,27]. Hence, weight loss would be expected to improve its levels in the blood, especially if it is associated with a decrease in adiposity.

Similarly, the serum levels of vitamin B9 were suggested to decrease in obesity due to sequestration in tissues [43]. However, the authors have also pointed out a greater use of this vitamin and differences in urinary excretion and endocrine function in obese compared to normal weight subjects, and the impact of adiposity on its absorption [44].

The number of works demonstrating that micronutrient deficiency may be ameliorated by weight loss is modest, except for vitamin D. However, the gathered evidence is convincing, especially since multiple mechanisms that could lead to the development of these deficiencies with obesity were proposed in the literature. The next important question arising is whether the reduction of micronutrient deficiencies could ameliorate obesity and related conditions that endanger human health.

5. Effects of Micronutrient Supplementation on Weight Management and Metabolic Health in Obesity

The micronutrient supplementation in certain cases of human obesity is necessary. Some patients are forced to eliminate particular food products due to allergies or other conditions like Crohn’s disease (exclusion of milk and vegetables) that may induce nutritional deficiencies (Ca and vitamins C, E, and K) [45]. In addition, increased micronutrient losses through the gastrointestinal tract are typical after bariatric surgery [46,47]. Multivitamin supplements are also recommended to women with excess weight who are pregnant to reduce the risk of preterm births and preeclampsia [48,49] as well as to improve birth outcomes, in particular, to reduce the odds of low birth weight [50].

On the other hand, obesity and associated metabolic dysfunctions are conditions where micronutrient supplementation becomes increasingly appreciated [51,52,53,54].

In addition to micronutrients that are deficient in people with obesity (see Table 1), the effects of supplementation on weight loss, as well as glucose and lipid homeostasis, were intensively studied for vitamin E and Cr.

Vitamin E deficiency was reported not to occur in people with obesity [3]. However, excess weight, insulin resistance, and dyslipidemia showed reverse association with blood levels of vitamin E [6], and the number of studies on the effect of vitamin E supplementation on metabolic disorders was sufficient to enable meta-analyses [55,56,57]. Similarly, numerous clinical trials and meta-analyses looked at the effects of Cr supplementation [58,59,60]. This mineral plays an important role in the metabolism of all macronutrients [61] and has been applied in commercial weight loss supplementation for years.

On the other hand, data on the effects of Fe supplementation on body weight, glucose, and lipid homeostasis in humans are very limited. In excess, Fe promotes the formation of reactive oxygen species and consequent DNA damage and lipid peroxidation, which contribute to the development of type 2 diabetes (T2D), cancer, and cardiovascular diseases [62]. However, longitudinal studies have shown conflicting results regarding the association of Fe intake with the abovementioned conditions [63]. Meta-analyses have indicated an increased risk of disease due to excessive consumption of heme Fe alone, which may therefore be explained by excessive consumption of high-fat meat and offal products [64,65,66,67]. Some evidence linking Fe supplementation with gestational diabetes mellitus was published; however, studies reported ambiguous results indicating positive or no associations [68,69]. On the other hand, animal studies have shown metabolic benefits from Fe supplementation. The positive effects on glucose and lipid homeostasis in obese rodents fed a high-fat diet through the regulation of mitochondrial functions and hepatoprotective effects were demonstrated [70,71]. Furthermore, dietary excess of Fe was shown to reduce the weight gain of fattened mice by 15%, which was explained by increased lipolytic processes and impairment of lipogenesis [72].

In Section 5.1, Section 5.2 and Section 5.3, an overview of the effects of vitamin and mineral supplementation on weight loss and the chosen markers of glucose and lipid homeostasis extracted from meta-analyses and RCTs is given.

5.1. Effects of Micronutrient Supplementation on Body Weight Reduction

Supplementation with most micronutrients did not influence body weight reduction (

Table 3. Effects of vitamin and mineral supplementation on weight loss.

Micronutrient	Studied Group	Number of People Included	Measure	95% Confidence Interval	Reference
Vit. A	Iranian ^ OB reproductive-aged women	56	MD −1.5 *	−1.8, −1.3	[73]
Vit. D	OW and OB participants of weight loss programs	947	MD −0.4	−1.1, 0.2	[74]
Vit. E	OW and OB adults	1245	WMD −0.1	−2.1, 2.0	[55]
Vit. C	Iraqi ^ Adults with MetS	120	MD 0.0	0.0, 0.0	[75]
Vit. B1	Mexican OW and OB adults with T2D	36	MD −0.6	N/A	[76]
Vit. B6	Iranian ^ dieting OB and OW women	44	MD −1.4 *	−2.5, −0.4	[77]
Vit. B9	Adults	457	WMD −0.2	−0.5, 0.2	[78]
Vit. B12	Iranian ^ adults with NAFLD	40	DBM −0.6	N/A	[79]
Mg	Adults	2551	WMD 0.2	−0.3, 0.7	[80]
Ca	OW and OB adults	662	MD −0.7 *	−1.0, −0.5	[81]
Se	Iranian ^ women with PCOS given probiotics, Spanish ^ healthy men on isocaloric diet, Italian ^ dieting OB adults	60, 24, and 37, respectively	MD −0.8 in all studies; * only in women with PCOS	Range from individual RCTs	[82,83,84]
Zn	OW and OB adults otherwise healthy	245	WMD −0.6	−1.1, −0.0	[42]
Cr	Adults with T2D	636	WMD −0.3	−0.7, 0.2	[58]

MD—mean difference, SMD—standardized mean difference, WMD—weighted mean difference, DBM—difference between median weight loss for treatment and control group, T2D—type 2 diabetes, OW—overweight, OB—obesity, MetS—metabolic syndrome, T2D—type 2 diabetes, NAFLD—non-alcoholic fatty liver disease, PCOS—polycystic ovary syndrome, * statistically significant difference reported in the study. In grey, data from RCTs were highlighted; remaining data were sourced out from meta-analyses; ^ denotes patients eligible to participate in the study carried out in this country.

).

The meta-analyses showed that only Ca administered to people with obesity and overweight allowed for a modest but statistically significant decrease of body mass by MD of −0.7 kg [81], while RCTs documented such an effect for vitamin B6 (MD −1.4 kg [77]), vitamin A (MD −1.5 kg [73]), and Se (MD −0.8 kg [82]) supplemented to dieting women with excess weight, women of reproductive age with obesity, and women with polycystic ovary syndrome who were given probiotics, respectively.

Several studies have indicated that the characteristics of the study group, the presence of dietary intervention, and the dose and source of micronutrients may influence the extent to which vitamin and mineral supplementation is helpful in reducing body weight.

For example, supplementation with Ca did not contribute to a significant weight loss in meta-analyses where data from studies that included varying population groups were considered [85,86] but, as mentioned, a small significant effect was noted when only data from studies on participants with overweight and obesity were looked at [81]. In addition, two independent meta-analyses have shown that increasing dairy consumption may be more likely to aid weight and fat loss when used in conjunction with a weight loss program [86,87].

Although Mg supplementation did not aid weight loss in a broad range of participants included in the meta-analysis by Askari et al. (2021), the authors reported a statistically significant decrease in BMI (WMD −0.21 kg/m2, 95% confidence interval −0.41, 0.00) [80]. This was because several studies used in the abovementioned meta-analysis did not report the weight of participants but only BMI. Moreover, the authors found that the effects of Mg supplementation were associated with a statistically significant decrease in the body weight of particular groups, namely women, people with excess weight, and people with insulin resistance and/or hypertension.

Similarly, meta-analyses did not report a significant effect of vitamin D supplementation on body weight reduction; however, a decrease in the BMI of adults with excess weight undergoing weight loss treatments [76] and in some specific population groups—such as females and Asians [77]—was noted.

The dose turned out to be a factor influencing the effectiveness of Mg and Zn supplementation. Weight loss was statistically significant when these micronutrients were administered at ≥350 and ≥50 mg/day, respectively [80,88].

Overall studies suggest that the application of micronutrients as a sole weight-loss treatment may give no or small effects, which could, however, be more pronounced if supplementation was an adjuvant to traditional weight-loss treatments and applied at adequate doses.

5.2. Effects of Micronutrient Supplementation on Lipid Homeostasis

The effects of micronutrient supplementation on selected indicators of lipid homeostasis are summarized in

Table 4. Effects of vitamin and mineral supplementation on lipid homeostasis.

Micronutrient	Studied Group	Number of People Included	Measure (95% Confidence Interval)					Reference
			Effect Size	TG (mg/dL)	TC (mg/dL)	LDL (mg/dL)	HDL (mg/dL)
Vit. A	Iranian ^ OB reproductive-aged women	56	MD	6.2 (−0.9, 13.3)	1.2 (−5.0, 7.4)	4.3 (−12.8, 21.3)	−3.1 (−4.3, −1.9)	[73]
Vit. D	OW OB children and adolescents	595	WMD	−4.3 (−20.2, 11.6)	−0.2 (−2.4, 2.1)	−4.6 (−12.8, 3.7)	1.0 (−1.2, 3.2)	[89]
Vit. E	Patients with diabetes	613	WMD	1.3 (−9.2, 11.9)	−0.7 (−15.0, 13.7)	−0.5 (−8.3, 7.3)	0.7 (−1.3, 2.6)	[56]
Vit. C	Adults with T2D	872	WMD	−16.5 (−31.9, −1.1)	−13.0 (−23.1, −2.9)	−7.5 (−17.3, 2.3)	2.2 (−0.5, 5.0)	[90]
Vit. B1	Adults with T2D	364	MD	−20.3 (−44.3 to 3.5)	-	5.4 (−6.6, 17.4)	3.9 (0.4, 7.7)	[91]
Vit. B6	Iranian ^ dieting OB and OW women	44	MD	−11.4 (−19.8, −2.9)	−13.5 (−20.1, −6.9)	−9.4 (−14.8, −4.0)	−0.5 (−3.3, 2.3)	[77]
Vit. B9	Adults	21,718	WMD	−9.8 (−15.5, 4.0)	−4.0 (−6.7, −1.2)	−1.0 (−6.8, 4.9)	0.4 (−0.5, 1.4)	[92]
Vit. B12	Iranian ^ adults with NAFLD	40	DBM	−16.5 (N/A)	-	5.5 (N/A)	−0.1 (N/A)	[79]
Mg	Adults	1192	WMD	−0.9 (−22.1, 3.5)	1.2 (−4.3, 6.2)	−0.4 (−5.0, 4.3)	1.2 (−0.1, 2.3)	[93]
Ca	Adults	1119	WMD	1.7 (−2.7, 7.1)	0.00 (−4.3, 4.6)	−3.1 (−6.1, −0.4)	−0.4 (−1.6, 0.4)	[94]
Se	Adults	2984	WMD	−0.8 (−4.7, 3.1)	−2.1 (−4.1, −0.1)	0.9 (−1.2, 3.0)	0.3 (−0.7, 1.3)	[95]
Zn	Adults with T2D	1357	WMD	−17.4 (−22.6, −12.2)	−19.6 (−28.5, −10.7)	−8.8 (−14.8, −2.8)	4.8 (0.9, 8.8)	[88]
Cr	Adults	12,844	ES	−0.2 (−0.5, 0.1)	−0.1 (−0.4, 0.2)	−0.1 (−0.2, 0.0)	0.1 (−0.1, 0.1)	[59]

TG—Triglycerides, TC—Total cholesterol, LDL—Low density lipoprotein, HDL—High density lipoprotein, OB—obesity, OW—overweight, T2D—type 2 diabetes mellitus, NAFLD—non-alcoholic fatty liver disease, MD—mean difference, WMD—weighted mean difference, ES—effect size; ^ denotes patients eligible to participate in the study carried out in this country. In light grey, statistically significant findings (p < 0.05) were highlighted.

.

Meta-analyses reported significant beneficial effects of supplementing Zn, Se, Ca, and vitamins C, B1, and B9 on at least one of the outcomes reported in Table 4, with Zn giving the greatest effect size for triglycerides (TG; WMD = −17.4 mg/dL), total cholesterol (TC; WMD = −19.6 mg/ dL), low-density lipoprotein (LDL; WMD = −8.8 mg/dL), and high-density lipoprotein (HDL; WMD = 4.8 mg/dL) among studied micronutrients [88,90,92,94,95]. An exceptionally high TG decrease was also reported for supplementation with vitamin B1 (WMD = −20.4 mg/dL) but, due to wide confidence intervals, this finding was not statistically significant [91]. Mentioned effect sizes might be of clinical importance for mild cases of dyslipidemia since they allow for the modification of levels considered to be elevated by about 10% (the reference levels are ≥150 mg/dL for TG; ≥190 mg/dL for TC; ≥100 mg/dL for LDL; and for HDL < 40 for men and <50 mg/dL for women [96]).

One of the important properties of micronutrients that could partly explain their beneficial influence on lipid homeostasis is the ability to scavenge free radicals. The antioxidative properties of vitamin C, Se, and Zn may improve the resistance of the adipose tissue to inflammation [97,98,99]. Furthermore, since vitamin C plays a role in the conversion of cholesterol to bile acids, the adequacy of its concentration may regulate blood cholesterol levels, especially LDL fraction [90]. On the other hand, animal research has shown that Zn supplementation exhibits hepatoprotective effects and increases fat excretion with feces [100]. Other minerals, such as Ca and Mg, are known to form soaps with fatty acids and bile in the digestive tract and in this way prevent a part of this macronutrient from entering the bloodstream [80,101]. This mechanism may explain the decrease of the accumulation of adipose tissue in animals on an obesogenic diet when supplemented with Ca [102], but in addition, the suppression of parathyroid hormone and 1, 25-dihydroxy vitamin D3 expression associated with a greater intake of Ca may contribute to the effect [103].

The supplementation of diet with vitamins B9 and B12 is known to decrease the concentration of homocysteine [104]—an amino acid that when present in excess may lead to dysregulation of lipid homeostasis and fat accumulation in tissues [105]. Unlike for vitamin B9, the number of studies on the efficacy of vitamin B12 supplementation in supporting the homeostasis of lipids in humans is very scarce. The available works did not show significant beneficial effects on lipid profile in T2D and non-alcoholic fatty liver disease (NAFLD) patients [79,106], but clearly more research on the topic is required.

Vitamin B1 is a cofactor for enzymes catalyzing the metabolism of not only lipids but also glucose and amino acids [107]. Therefore, its role in maintaining lipid homeostasis is crucial, with recent animal research showing that when supplemented at high doses, it protects against the development of NAFLD [108]. The dose could be of importance for the effect of supplementation in humans, as a meta-analysis by Muley (2022) found that administration of vitamin B1 only at 120 mg/day allowed for a statistically significant and greater reduction of TG levels in T2D patients (MD −97.4 mg/dL) [91]. Other micronutrients that showed dose-dependent performance in the regulation of lipid homeostasis were Ca (better effects at ≥1 g [94]), Se (at 200 µg/day [109]), Zn (at ≥50 mg/day for TG but <50 mg/day for TC, LDL and HDL [88]), Cr (at >500 µg/day [59]), vitamin D (at ≥4000 IU/day [89]), vitamin C (at <1000 mg/day [90]), and vitamin B9 (at ≥5 mg/day [92]).

Not many studies looked at the effect of vitamins A, B6, and B12 on lipid homeostasis in humans. Therefore, for these micronutrients, Table 4 shows data from exemplary RCTs. Vitamin A was the only nutrient for which a significant negative outcome (decrease of HDL level; MD −3.1 mg/dL) on lipid homeostasis was reported [73]. In the cited study, supplementation with 25,000 IU/day retinyl palmitate in women with obesity for 4 months, besides a mild decrease of HDL, resulted in a statistically significant increase of TG, TC, and LDL. These findings are difficult to interpret since two control groups (one, women with obesity receiving a placebo; two, women with normal weight supplementing retinyl palmitate at the same dose) also exhibited negative changes in lipid homeostasis, similar to those in the treatment group. Other human studies on the association of vitamin A and carotenoids with weight loss and metabolic markers reported variable results, and currently, no clear recommendations for supplementing these substances in obesity are available [110,111]. Furthermore, due to the known dose-dependent, hepatotoxic effects of synthetic forms of this vitamin, recommending suitable doses poses a challenge [112]. On the other hand, maintaining adequate vitamin A levels may be crucial for the health of adipose tissue since this substance was shown to promote angiogenesis and de novo adipogenesis [113]. Healthy adipose tissue contains more abundant, well-vascularized adipocytes for fat storage [114]. Such a structure, to a certain extent, protects adipocytes from overgrowth and inflammation, which are risk factors contributing to the development of metabolic dysfunctions, especially insulin resistance [98]. Since the formation of adipocytes is most intense during fetal development and their number gets established in childhood and early adulthood, providing vitamin A adequacy might be especially crucial in early life [113].

Vitamin B6 supplementation at 80 mg/day in dieting women with excess weight for 8 weeks was shown to aid in the reduction of TG, TC, and LDL (see Table 4). A few of the mechanisms explaining the regulatory functions of vitamin B6 on lipid homeostasis are antioxidative properties, its role in enzymatic reactions, as well as its influence on calcium ion signaling associated with enhanced lipolysis and decreased synthesis of fatty acids [77].

In summary, the influence of the majority of micronutrients on lipid homeostasis is significant and supplementation may be recommended in mild cases of dyslipidemia or as an adjuvant to conventional treatments in more severe cases. In addition, research on the combinations of micronutrients at doses that were found to be effective would be of interest to determine whether additive or synergistic outcomes could be achieved.

5.3. Effects of Micronutrient Supplementation on Glucose Homeostasis

The effects of micronutrient supplementation on selected parameters indicative of glucose homeostasis status are given in

Table 5. Effects of vitamin and mineral supplementation on glucose homeostasis.

Micronutrient	Studied Group	Number of People Included	Measure (95% Confidence Interval)					Reference
			Effect Size	FG (mg/dL)	FI (µIU/mL)	HOMA-IR	HbA1c (%)
Vit. A	Iranian ^ OB reproductive-aged women	56	MD	0.5 (−1.6, 2.7)	-	-	-	[73]
Vit. D	Patients with diabetes	2006	MD	−2.7 (−7.8, 2.4)	-	−0.1 (−0.5, 0.4)	−0.0 (−0.1, 0.1)	[51]
Vit. E	Patients with diabetes	2171	MD	−3.4 (−8.1, 1.4)	−1.1 (−1.5, −0.6)	−0.4 (−0.8, −0.1)	−0.2 (−0.3, −0.1)	[57]
Vit. C	OW and OB adults with T2D	1447	WMD	−10.7 (−18.5, −2.9)	−1.7 (−3.2, −0.3)	−0.9 (−2.0, 0.3)	−0.5 (−0.8, −0.2)	[115]
Vit. B1	Adults with T2D	364	MD	−3.6 (−12.4, 5.2)	-	-	0.0 (−0.4, 0.3)	[91]
Vit. B6	Iranian ^ dieting OB and OW women	44	MD	−1.0 (−5.0, 3.0)	−1.5 (−2.2, −0.8)	−0.5 (−0.7, −0.3)	-	[77]
Vit. B9	Adults	34,646	WMD	−2.2 (−3.7, −0.7)	−0.2 (−0.4, −0.1)	−0.4 (−0.7, −0.1)	−0.3 (−0.7, 0.2)	[116]
Vit. B12	Iranian ^ adults with NAFLD	40	DBM	−3.5 (N/A)	−1.3 (N/A)	−0.3 (N/A)	-	[79]
Mg	Adults	1362	WMD	−3.6 (−8.1, 0.9)	−0.3 (−1.4, 0.7)	−0.7 (−1.2, −0.1)	0.0 (−0.1, 0.1)	[117]
Se	Adults	1411	WMD	−1.3 (−4.0, 1.4)	−3.0 (−5.1, −0.9)	−0.8 (−2.1, 0.5)	0.1 (−0.2, 0.3)	[118]
Zn	OW and OB adults	651	WMD	−8.6 (−14.0, −3.1)	−0.8 (−2.5, −0.9)	−0.5 (−0.8, −0.3)	−0.3 (−0.4, −0.1)	[119]
Cr	Patients with T2D	1350	WMD	−19.0 (−36.2, −1.9)	−1.8 (−2.6, 1.0)	−1.5 (−2.4, −0.7)	−0.7 (−1.2, −0.2)	[60]

FG—Fasting glucose, FI—Fasting insulin, HOMA-IR—Homeostatic model assessment for insulin resistance, HbA1c—Glycated hemoglobin, OB—obesity, OW—overweight, T2D—type 2 diabetes mellitus, NAFLD—non-alcoholic fatty liver disease, MD—mean difference, WMD—weighted mean difference; ^ denotes patients eligible to participate in the study carried out in this country. In light grey, statistically significant findings (p < 0.05) were highlighted.

.

The meta-analyses pointed to significant beneficial effects of supplementing Mg, Se, Zn, and Cr, as well as vitamins E, C, and B9, to at least one of the outcomes reported in Table 5, with Cr giving the greatest reduction of fasting glucose (FG; WMD = −19.0 mg/dL), homeostatic model assessment for insulin resistance (HOMA-IR; WMD = −1.53), and glycated hemoglobin (HbA1c; WMD = −0.71%); and Se of fasting insulin (FI; WMD = −3.0 µIU/mL) among the studied micronutrients [60,118]. These values constituted more than 10% of threshold levels considered normal, which are < 100 mg/dL for FG, <5.7% for HbA1c [120], <25 µIU/mL for FI [121], and from <1.6 to <3.8 (depending on population) for HOMA-IR [122]; hence, they could be of clinical relevance in mild cases of disturbed glucose homeostasis.

Table 5 does not include data on the influence of Ca supplementation on glucose tolerance. This is because the majority of the available studies applied Ca jointly with vitamin D or were concerned with the increase in dairy product intake. Meta-analyses of these studies reported improvements in glycemic control such as the decrease of the HOMA-IR index by a WMD of −0.37 and MD −1.21, respectively [87,123].

Calcium and Mg are both important in the control of insulin secretion. This process requires an influx of Ca that is regulated by extracellular Mg [80,124]. Furthermore, Mg is a cofactor for enzymatic reactions, including glycolysis [125].

The antioxidative properties of vitamins E and C as well as Se and Zn may ameliorate the inflammation of adipocytes that is thought to contribute to insulin resistance [57,97,98,99,115]. Furthermore, the role of selenoproteins in the regulation of cellular functions and of Zn in the insulin-signaling pathway and clearance were mentioned in the literature as important mechanisms that could help to explain the effects of supplementation with these minerals on glucose metabolism [99,126]. On the other hand, Cr is known to enhance the transport of glucose into cells through the trafficking of glucose transporter type 4 (GLUT-4) [127], whereas vitamins B9 and B12 normalize homocysteine levels and in this way contribute to the maintenance of insulin sensitivity [104].

The effects of some of the micronutrients on glucose homeostasis were dose-dependent. The doses recommended in meta-analyses to achieve optimal effects were ≥4000 IU/day for vitamin D [89], between 400 and 700 mg/day for vitamin E [57], ≥1000 mg/day for vitamin C [115], ≥5 mg/day for vitamin B9 [116], 200 µg/day for Se [118], and ≥30 mg for Zn [119].

The number of published studies on the effects of supplementation with vitamins A, B6, and B12 on glucose homeostasis is small, and no meta-analyses are available to date. Hence, for these micronutrients, the data in Table 5 were outsourced from RCTs.

Vitamin A did not have a significant impact on the homeostasis of glucose in reproductive-aged women (see Table 5). However, it was found to increase the phosphorylation of insulin receptors in WNIN/Ob rats [128]; therefore, it could possess insulin resistance modulatory properties, and further research should be carried out to optimize effects achieved in humans.

The recent findings of clinical trials demonstrated the positive effects of vitamin B6 on glucose homeostasis. For example, RCT with the participation of women with excess weight undergoing an 8-week weight reduction program (see Table 5) showed that such supplementation had a significant, positive effect on FI and HOMA-IR compared to the control group receiving a placebo [77]. In addition, other studies demonstrated lower postprandial glucose levels in healthy adults when vitamin B6 was administered 20 min before the oral glucose tolerance test (OGTT) [129] and a decrease of FG, FI, HOMA-IR, and HbA1c in T2D patients undergoing a 4-week supplementation [130]. In the latter study, the authors used vitamin B6 supplementation at a high dose of 300 mg/day compared to 80 mg/day and 50 mg/single dose in the former two studies. respectively. The supplementation was applied as an adjuvant to metformin and lifestyle modification resulting in mean reductions (treatment vs. control group with metformin and lifestyle modification only) of FG by 37 vs. 25%, FI by 30 vs. 18%, HOMA-IR by 56 vs. 38%, and HbA1c by 17 vs. 11%. Therefore, vitamin B6 presents a great potential for therapeutic use, and further work that will help to establish an optimal dose to treat insulin resistance is required.

Overall, micronutrient supplementation displays significant beneficial effects on glucose homeostasis. These effects are increasingly appreciated, with recent big-data studies exploring their potential in treating T2D [51,53]. Therefore, micronutrient supplementation may be recommended for inclusion in the treatment of obesity, if not as an effective weight loss aid, then to influence the regulation of co-occurrent metabolic dysfunctions.

5.4. Doses of Micronutrients That Were Found to Be Effective in Aiding Weight Loss and Metabolic Health

Micronutrient supplementation displayed dose-dependent effects on weight loss and metabolic health. Further research on implementing vitamins and minerals in the treatment of obesity should hence consider these doses that were found to be effective. In

Table 6. The effective doses of micronutrients for weight loss and metabolic health in obesity and dietary reference values recommended by the European Food Safety Authority [131].

Micronutrient	Studied Group	Range in Studies Included in Analysis (Dose/Day)	Parameter for Which the Effective Dose was Proposed	Suggested Effective Dose per Day	Reference	UL per Day	AR per Day	AI per Day
Vit. D	OW and OB children and adolescents	357–42,857 IU	HDL, CRP	≥4000 IU	[89]	4000 IU (100 µg) 2		600 IU (15 µg)
Vit. E	Patients with diabetes	90–1620 mg	HbA1c, FI	400–700 mg	[57]	300 mg 3		11 and 13 mg 7
Vit. C	OW and OB adults with T2D	250–2000 mg	HOMA-IR	≥1000 mg	[115]	N/A	80 and 90 mg 7
Vit. B1	Adults with T2D	100–900 mg	TG	120 mg	[91]	N/A	0.6–0.8 and 0.7–1 mg 7
Vit. B6	Iranian ^ dieting OB and OW women	80 mg	TG, TC, LDL, FI, HOMA-IR, Body weight	80 mg 1	[77]	25 mg 4	1.3 and 1.5 mg 7
Vit. B9	Adults	0.25–15 mg	HOMA-IR, TC, LDL	≥5 mg	[92,116]	1 mg 5	0.25 mg
Mg	Adults	48–450 mg	BMI	≥350 mg	[80]	250 mg 6		300 and 350 mg 7
Ca	Adults	800–2000 mg	LDL	≥1000 mg	[94]	2500 mg	750 and 860 mg 7
Se	Adults	100–300 µg	TG, TC	200 µg	[109]	300 µg		70 µg
Zn	T2D adults with Zn sufficiency	22–660 mg	Body weight, TG	≥50 mg	[88]	25 mg	6.2–8.9 and 7.5–11.0 mg 7
Cr	T2D Adults	200–740 µg	TG	>500 µg	[59]	N/A	N/A	N/A

¹ Based on RCT, no recommendations from meta-analyses are available; ² as ergocalciferol and cholecalciferol; ³ mg α-tocopherol equivalents; ⁴ as pyridoxamine, pyridoxine, pyridoxal, and phosphorylated forms of these substances; ⁵ synthetic folic acid; ⁶ magnesium in supplements, does not include dietary intake; ⁷ for women and men, respectively, for vitamin B1 calculated based on the physical activity level dependent on average energy requirement for young adults aged 18–29, for Ca values for young adults aged 18–24; for Zn depending on phytate intake level. Abbreviations: UL—tolerable upper intake level, AR—average requirement, AI—adequate intake. All dietary reference intake values outsourced from Ref. [131]. ^ denotes patients eligible to participate in the study carried out in this country.

, a summary of the supplementation levels suggested by meta-analyses was presented. In addition, Table 6 contains tolerable upper intake levels (UL), average requirements (AR), and adequate intakes (AI) recommended by the European Food Safety Authority (EFSA) for adults [131].

The choice of the literature data was focused primarily on the doses that showed significant weight or BMI reduction effects, and then doses affecting glucose and lipid homeostasis were considered. The mentioned dietary reference intake values (DRIs) were available for all nutrients except for vitamins C and B1 (no UL) and Cr (no DRIs available). For Cr, AI was proposed by the American Food and Nutrition Board at 25 and 35 μg/day for young women and men, respectively, but no UL was established due to insufficient data at the time of document publication (in 2001) [132]. Despite the popularity of supplements containing trivalent Cr compounds and the large number of RCTs studying their effects in humans, the toxicity of these compounds is still a subject of research [133].

Also, due to insufficient data, the Food and Nutrition Board did not propose UL for vitamin B1 [134]; however, for vitamin C, it was established at 2000 mg/day, as above this intake, a risk of osmotic diarrhea exists [135].

Doses for all micronutrients suggested as effective in aiding weight loss and/or improving metabolic health were all greater than the mentioned AR or AI for healthy adults proposed by EFSA and the Food and Nutrition Board. However, most of these values, namely for vitamins E, B6, B9, Mg, and Zn, exceeded UL thresholds as well. The dose established as UL is the “maximum level of total chronic intake of a nutrient from all sources judged to be unlikely to pose a risk of adverse health effects in humans” [136]. Therefore, above UL, there is a risk of micronutrient overdose.

To enable the application of vitamins and minerals in obesity at doses greater than UL, more data are needed. Even though the majority of the RCTs that implemented such doses did not report adverse effects, care has to be taken to interpret the results since studies were performed on a limited number of participants meeting all exclusion and inclusion criteria, creating quite homogenous groups.

A good example of the risk that micronutrient overdosing may pose is presented in a study by Said et al. (2021) [137]. The authors reported two cases of hair loss among 71 T2D patients who received vitamins A and E at doses of 50,000 IU (corresponding to 15.015 µg, which is approximately 5-fold higher than EFSA’s UL) and 100 mg, respectively, as well as 25 mg of Zn (only 35 out of 71 participants). Even though the authors reported the beneficial effects of the mentioned supplementation on glucose homeostasis and insulin secretion, they have admitted that the dose of vitamin A will need refinement in future studies.

While establishing effective and safe doses of micronutrient supplementation for the treatment of obesity and associated metabolic disorders might be challenging, another option to consider is enhancing the bioavailability of the minerals and vitamins that are clearly reduced in obesity.

The research from the last two decades gives convincing evidence that the bioavailability of micronutrients could be improved by the modulation of microbiota. Within the next section, the mechanisms through which the microbiota may influence the use of minerals and vitamins by the organism of the host are summarized.

6. Mechanisms Used by Microorganisms to Impact the Bioavailability of Micronutrients

The role of microbiota in the modulation of the nutritional status of the host is of great importance. This is best demonstrated by the studies on germ-free animals displaying low levels of some micronutrients that are enhanced after introducing the microbiota [138,139].

Understanding mechanisms by which the microorganisms may impact the bioavailability of nutrients is crucial for realizing the potential of microbiota in modulating the absorption and use of these nutrients in the human body.

The microbiota influences intestinal function and absorption processes through a variety of actions. Beneficial microorganisms produce various metabolites, including short-chain fatty acids (SCFAs) that were found to accelerate the proliferation of intestinal epithelial cells and thus increase the absorptive surface of the intestine, as well as to improve the intestinal blood supply [140]. Nevertheless, the best-known mechanism through which probiotics may impact the regulation of the intestinal barrier is the antagonism to pathogens that damage these functions [141]. The inflammatory status caused by the imbalance of microbiota in obesity may also indirectly impact the bioavailability of micronutrients. For example, levels of hepcidin that are known to depend on this status have an impact on the absorption of Fe [40].

Other than regulating intestinal absorption, gut microorganisms could be considered a bioreactor in the human intestinal tract, transforming various compounds into beneficial or harmful metabolites, thus having a crucial role in maintaining the availability of micronutrients for use and storage in the body [142]. Some of the water-soluble vitamins may be synthesized de novo by the microbiota. For example, vitamin B12 is produced by Propionibacterium freudenreichii, Salmonella enterica, and Listeria innocua; some lactobacilli, such as L. reuteri; and others [143], while bifidobacteria, especially B. bifidum and B. longum, are renowned for the synthesis of vitamin B9 in high concentrations [144]. Other B-group vitamins produced by beneficial members of intestinal microbiota that may impact the nutritional status of the host include thiamin (B1) [145] and riboflavin (B2) [146].

Gut microbiota was shown to influence the bioavailability of fat-soluble vitamins in several studies. Some microorganisms were capable of performing ex novo synthesis of carotenoids through the mevalonate pathway, as well as the production of proteins involved in vitamin A transport into the enterocytes [147]. Similarly, the intestinal uptake of vitamin D was improved by microbial stimulation of the vitamin D receptor expression [148], as well as through modulation of the activity of fibroblast growth factor 23 produced mainly by osteocytes and osteoblasts [149]. Furthermore, gut microbiota was shown capable of initiating the cytochrome P450-dependent metabolism of vitamin E in the intestinal lumen [150], while few commensal bacteria (Bacteroides, Enterobacter, Veillonella, and Eubacterium lentum) could synthesize de novo several forms of vitamin K (menaquinones) [151].

Of importance are also metabolites of microorganisms that may either aid the absorption of micronutrients or decrease it. For example, members of gut microbiota, including pathogens such as Salmonella, produce lipopolysaccharides, which contribute to the inflammatory response of the intestine and also decrease the uptake of vitamin C by downregulating the intestinal transporters of this vitamin [152]. Furthermore, microbiota, especially in dietary Fe deficiency, was shown to produce Fe-binding metabolites that helped the microorganisms utilize this micronutrient but limited its bioavailability to the host [153]. On the other hand, microbial metabolites may capture minerals and promote their intestinal uptake through active transport. Although research on exact compounds with such properties is currently underway, some of them, for example, SCFAs that create salts with minerals [154] and more complex organic compounds, i.e., siderophore enterobactin produced by E. coli [155] to bind Fe, are already known. The potential of microbial metabolites to raise the uptake of minerals is significant since it may go beyond an order of magnitude. In an exemplary work, L. reuteri LB26 (DSM 16341) and B. lactis Bb1 (DSM 17850) increased the bioaccessibility of Se by up to 65 and Zn by up to 31.5 times compared to organic and inorganic compounds containing these minerals (sodium selenate, seleno l-methionine, seleno l-cysteine, zinc sulfate, and zinc gluconate) through internalization of these minerals [156]. In addition, the uptake of minerals may be facilitated by a change in their solubility or oxidation state. For example, L. plantarum 299v was shown to stimulate the intestinal tissue to produce iron reductase—an enzyme that reduces plant-derived Fe³⁺ to more soluble and bioavailable Fe²⁺ [157]. Other lactobacilli displayed the ability to produce compounds with iron-reducing properties (e.g., p-hydroxyphenyllactic acid from L. fermentum) [158]. Furthermore, enzymes released by some members of the microbiota allowed for digesting phytates and releasing Ca, Mg, Zn, and Fe for absorption [159], while SCFAs increased the solubility of mineral compounds by lowering the pH of the intestinal lumen [154].

The number of in vitro studies where detailed information on how different mechanisms through which the gut microbiota may influence vitamin and mineral bioavailability is limited. Rather, we know that such phenomena occur; therefore, work in this area should be continued to aid the selection of microbial strains that could be used for amelioration of micronutrient deficiencies in obesity.

7. Effects of the Interventions into Microbiota on the Micronutrient Status of Humans

The dysbiosis of intestinal microbiota and micronutrient deficiencies co-occur with obesity. Interventions in the microbiota to improve the nutritional status of populations suffering from micronutrient malnutrition may thus be an idea worth considering. The two most researched types of such interventions are the supplementation of the diet with probiotics and/or prebiotics. Both are targeted to change the human intestinal microbiota, which consequently may affect the bioavailability of nutrients, and some such interventions were shown to affect the micronutrient levels in humans positively. Unfortunately, most of the published evidence involved healthy people or groups with illnesses not directly associated with obesity. A systematic review of studies that looked at the effect of probiotic supplementation on the micronutrient status of healthy humans found 14 works published up until June 2020 [160]. Reviewed studies reported a significant increase in blood levels of vitamins B9 and B12, as well as Fe, Zn, and Ca, following the administration of probiotics (in the case of a single study of a synbiotic [161]) and ambiguous effects for vitamins A, D, and E and carotenoids. These works were too heterogeneous in terms of measured outcomes, researched populations, and intervention conditions, including applied probiotics, to perform a meta-analysis [160].

The type of probiotic is a very important factor affecting the efficacy of the intervention. For example, a meta-analysis of studies published until February 2019, showed that the use of the probiotic L. plantarum 299v increased the absorption of non-heme Fe from the diet in healthy white Europeans, mainly women [162]. However, the potential effect of probiotics other than L. plantarum 299v on Fe absorption was questionable since none of them had a significant effect on Fe status.

Several studies described the effect of prebiotics and potentially prebiotic substances on the micronutrient status of healthy humans in a variety of populations. For example, Whisner et al. demonstrated the positive effects of galactooligosaccharides (GOS) and soluble corn/maize fiber on the absorption of Ca in adolescents [163,164,165]. In postmenopausal women, different authors found that fructooligosaccharide (FOS) supplementation resulted in increased absorption of Mg and Cu but not Se and Zn [154,166]. In addition, administering inulin to infants fed with formula resulted in increased apparent absorption of Zn, Mg, and Fe but not Ca [167,168]. On the other hand, the improvement of Ca blood levels in response to inulin was observed in young, healthy males [168].

Only a few studies looked at the effects of pro- and/or prebiotic supplementation on the micronutrient status in people with excess weight and/or associated health conditions. A summary of these studies is presented in

Table 7. Studies that reported effects of pro-and/or prebiotic supplementation on micronutrient status of people with obesity or metabolic disorders.

Supplementation in Treatment Group	Micronutrient	Researched Population	Type of Research and Reference
Familact® with L. casei, L. rhamnosus, S. thermophilus, B. breve, L. acidophilus, B. longum, L. bulgaricus, and FOS; MV and mineral supplement (not disclosed if given by the investigators or advised to take), intramuscular vitamin B12	vitamin B12, B9, and D↑	46 Iranian ^ women after one anastomosis gastric bypass/mini gastric bypass surgery	RCT with placebo [169]
Flora Vantage® L. acidophilus NCFM, B. lactis Bi-07, and MV	Vitamin B9, B12, and D *↑	101 Brazilian ^ patients after Roux-en-Y surgery	RCT with placebo [170]
LactoWise® B. coagulans, galactomannans, advised to supplement MV with emphasis on vitamins D, B12, C, and Fe	Vitamin B12 and D	60 American ^ patients after laparoscopic sleeve gastrectomy	RCT with placebo [171]
Puritan’s Pride® containing Lactobacillus, advised to supplement MV and vitamin B12	Vitamin B12↑	44 American ^ patients after Roux-en-Y surgery	RCT, no placebo [172]
L. acidophilus, B. breve, B. longum, L. delbrueckii susp. bulgaricus, L. helveticus, L. plantarum, L. rhamnosus, L. casei, Lc. lactis susp. lactis, S. thermophilus, soluble fiber Nutriose®, and multivitamin	Ferritin and haemoglobin	48 German ^ patients after mini gastric bypass surgery	RCT with placebo [173]
Bio-25® L. acidophilus, B. bifidum, L. rhamnosus, L. lactis, L. casei, B. breve, S. thermophilus, B. longum, L. paracasei, L. plantarum, B. infantis, no information about MV supplement given	Ferritin and haemoglobin *↓	100 Israeli ^ patients after sleeve gastrectomy surgery	RCT with placebo [174]
L. reuteri NCIMB 30242	Vitamin A, D↑, E, β-carotene, Ca	127 Czech ^ hypercholesterolemic adults	Post-hoc analysis of RCT with placebo [175]
L. acidophilus, B. bifidum, L. reuteri, L. fermentum and vitamin D at 50,000 IU every two weeks in addition to vitamin D 1000 IU and B9 400 µg daily supplementation	Vitamin D↑	87 Iranian ^ women with gestational diabetes	RCT with placebo [176]
L. acidophilus La-B5, B. lactis Bb-12 in yogurt enriched with vitamin D	Vitamin D	119 Iranian ^ adults with obesity on low-calorie diet	RCT with placebo [177]
B. bifidum W23, B. lactis W51, B. lactis W52, L. acidophilus W37, L. brevis W63, L. casei W56, L. salivarius W24, Lc. lactis W19, and Lc. lactis W58 at two doses	Serum Fe, Zn *↑, Cu Hair Fe *↓, Zn, Cu	90 Polish ^ postmenopausal women with obesity	RCT with placebo [178]
L. acidophilus, L. casei, L. rhamnosus, L. bulgaricus, B. breve, B. longum, S. thermophilus, and FOS	Ca↑, Zn, Mg, Fe	58 Iranian ^ patients with T2D	RCT with placebo [179]
Inulin from chicory enriched with FOS	Ca↑	46 Iranian ^ women with T2D	RCT with placebo [180]

RCT—Randomized controlled trial; FOS—fructo–oligo saccharides, MV—multivitamin, T2D—type two diabetes mellitus, arrows denote ↑—increase and ↓—decrease of blood levels (except for reference [178], where also hair levels were reported), *—result significantly different compared to baseline but not to control; at the same time, no significant change to baseline (with same direction) detected in the control group. ^ denotes patients eligible to participate in the study carried out in this country.

.

The first six studies mentioned in the table were performed on patients who underwent bariatric surgery [169,170,171,172,173,174]. These studies are, on one hand, a good example of the influence of microbiota intervention on the micronutrient status of people because they include micronutrient supplementation in both the treatment and control groups. On the other hand, bariatric procedures alter the uptake of nutrients from the gastrointestinal tract. Hence, the effects could be different in people with intact digestive tracts. In addition, cited studies for bariatric patients all looked at the effects of probiotics and synbiotics on micronutrient status shortly after the surgery, when changes to body function were greatest due to very intensive weight loss. This, as discussed in Section 4, may have a profound impact on the micronutrient status of the body. Therefore, the results of studies with the participation of patients undergoing bariatric procedures must be discussed separately. A meta-analysis that included these six studies found that microbiota interventions improved the levels of vitamin B12 but did not have a significant effect on the status of vitamins D and B9, as well as ferritin and hemoglobin [181].

The remaining six studies included in Table 7 reported results of pro- and/or prebiotic supplementation on the micronutrient status of people with obesity [177,178], T2D [179,180], hypercholesterolemia [175], and gestational diabetes mellitus [176]. All of these studies demonstrated the effect of microbiota intervention on micronutrient status by employing adequate controls. Other studies where the effect of joint supplementation with micronutrients and pro-/prebiotics was shown in the treatment group versus the placebo were not included. Three of the listed studies reported the effects of microbiota intervention on vitamin D status [175,176,177]. In two studies, vitamin D increased [175,176], and in one, no significant change to its status compared to the control group was noted [177]. Interestingly, studies that reported significant effects both included L. reuteri in the supplementation. Only one out of six mentioned studies looked at other vitamins apart from vitamin D (vitamin A, E, and β-carotene) but failed to demonstrate a statistically significant change in their status in response to microbiota intervention [175].

Among works that studied the impact of microbiota intervention on the absorption of minerals, two reported improvements in the Ca status of T2D patients who were given either synbiotic with FOS [179] or chicory inulin enriched with FOS [180], but no effect was found in hypercholesterolemic adults taking L. reuteri NCIMB 30242 alone [175]. Furthermore, two studies reported no effect of the intervention on serum Fe levels in T2D patients [179] and postmenopausal women with obesity [178]. Out of these two studies, only one showed that employed supplements could increase the serum concentration of Zn compared to baseline levels but not to the control [178]. On the other hand, the levels of Cu [178] and Mg [179] remained unaffected.

Overall, the effects of microbiota interventions on micronutrient status reported in the studies with the participation of patients with obesity and metabolic disorders are highly heterogeneous. However, so are the employed intervention conditions and researched groups. In this situation, more research is required to establish effective interventions that could improve the micronutrient status of target populations. Nevertheless, it needs to be noted that both pre- and probiotics were shown to improve the status of some micronutrients in groups of interest, and no negative results were reported, except for the Fe status of postmenopausal women with obesity that was measured in hair [178]. This is quite an interesting result, especially since no serum concentration of this mineral changed significantly in the same study. Although the authors stated that they have measured the concentration of elements in hair to assess the nutritional status of participants, this is not a conventional way of achieving their aim, especially since the guideline levels of micronutrient adequacy refer to their blood concentration. Nevertheless, further studies of changes in mineral status measured in parallel in blood and hair may be of interest since the human body may sequester micronutrients in different parts to different extents, and hair sampling is much less invasive compared to blood sampling.

Since microbiota interventions may impact the micronutrient status of humans and were shown not only to improve body composition and anthropometry [182] but also glucose and lipid homeostasis [183,184], combined treatments of pro- and/or prebiotics with micronutrients could bring even better effects than each of these supplements on its own. Apart from studies on bariatric patients, the research summarized in Table 7 answered the question about the combined efficacy of microbiota and micronutrient intervention on metabolic parameters in only two cases. Jamilian et al., 2019, found that not only did vitamin D status improve more in women with gestational diabetes mellitus who were administered this vitamin along with probiotics, but also that the decrease of TG, TC, and very low-density lipoprotein cholesterol, as well as an increase in HDL, were all greater in the treatment group compared to the control that received probiotics only [176]. On the other hand, the study by Hajipoor et al., 2021, reported that yogurt enriched with probiotics L. acidophilus La-B5, B. lactis Bb-12, and vitamin D did not have an advantageous effect on the lipid profile and anthropometric indices of adults with obesity compared to the control that was given plain non-probiotic yogurt with vitamin D or the placebo (only non-probiotic yogurt) [177].

There are other studies that demonstrated the favorable effects of combining microbiota and micronutrient interventions in people with obesity and metabolic disorders. For example, an 8-week intervention with synbiotic (Protexin^® containing L. casei, L. rhamnosus, S. thermophilus, B. breve, L. acidophilus, B. longum, L. bulgaricus, and FOS) and vitamin E, at approx. 360 mg a day, brought a greater decrease in FG, FI, TC, and LDL levels of patients with NAFLD than each of these supplements alone [185].

Further, synbiotic (GeriLact ^® containing L. casei, L. rhamnosus, S. thermophilus, B. breve, L. acidophilus B. longum, L. bulgaricus, and FOS) and vitamin D administered daily and weekly at 50,000 IU, respectively, were shown to reduce body fat percentage in women with excess weight more than each of these supplements on its own (although, in the case of synbiotic alone, the difference was not statistically significant) in an 8-week trial, where no explicit lifestyle modification was applied [186].

Also, a meta-analysis of five studies published up to February 2022 where joint probiotic and Se (at 50–200 µg/day) supplementations in groups of adults representing various populations were applied showed that the treatment resulted in a significant WMD of FG (−4.0 mg/dL), FI (−2.5 µIU/mL), HOMA-IR (−0.6), TG (−14.4 mg/dL), TC (−12.8 mg/dL), and LDL (−7.1 mg/dL) [187]. All of these values except for FI and HOMA-IR were much greater than those found by meta-analyses studying the effects of Se alone (see Table 5 and Table 6), suggesting that joint Se and probiotic treatment may indeed be more effective in improving the homeostasis of glucose and lipids.

On the other hand, in an 8-week study, no significant change in metabolic parameters of adults with excess weight was reported in response to joint Ω-3, vitamin D (at 200 IU/day), and synbiotic (L. paracasei LCP-37, L. acidophilus NCFM, B. lactis Bi-07, and B. lactis Bi-04) supplementation when compared to the placebo (no vitamin or probiotic) [188]. However, the authors of this study did not see a significant increase in serum levels of vitamin D in participants of either group; hence, it can be suspected that both the dose of vitamin D was too low (20 times lower compared to the effective dose suggested in Table 6) and the probiotic did not have an appreciable influence on its status.

Despite many scientific reports on the positive impact of prebiotics and probiotics on the regulation of the concentration of micronutrients in the human body and improving the outcomes of micronutrient supplementation alone, there are still no significant comprehensive clinical trials that reveal how the composition of the intestinal microbiota influences the bioavailability of micronutrients in people with obesity. In addition, the studies where interventions into microbiota and micronutrient supplementation are combined to treat micronutrient malnutrition are low in number and often do not contain controls that allow for demonstrating the efficacy of applied pro- and/or prebiotics. Therefore, more research is required to explore this promising area of potential therapies in the treatment of obesity and related metabolic disorders.

8. Conclusions

In this review, the role of micronutrients in obesity and factors that may govern the nutritional status of the body with a special focus on microbiota was presented. Human and animal research suggests that micronutrient malnutrition may be both a cause and a consequence of obesity. The deficient intake of vitamins with diet may cause increased adiposity, and the induction of obesity through an energetically imbalanced diet contributes to micronutrient deficiencies even if their intake is sufficient. On the other hand, given that some micronutrients were shown to selectively deposit in the adipose tissue, a simple correction of DRIs for weight might not be adequate. The findings of this review indicate a need for the development of DRIs applicable to obesity that would be safe. Current meta-analyses and RCTs indicate that dietary supplementation of some micronutrients known to be deficient in people with obesity may bring considerable improvements in glucose and lipid homeostasis and support weight loss. Nevertheless, to achieve these positive effects, micronutrient doses applied in successful studies were often higher than UL. Such doses should not be recommended for routine application in obesity treatments since patients could experience side effects from overdosing on micronutrients. Nevertheless, research findings indicate that the reduction of vitamin and mineral doses considered to be effective for the population with obesity could be feasible if these treatments were combined with microbiota interventions. Pro- and prebiotics were shown to improve the micronutrient status of humans when applied alone or jointly with vitamins and minerals. This could be attributed to multiple mechanisms that members of intestinal microbiota use to impact the bioavailability of micronutrients for the human host. The results of these interventions are promising since available current clinical trials that looked at the metabolic and weight-loss effects of pro/pre/synbiotic application jointly with micronutrients showed that in combination, these supplementations were more effective than each of them separately. On the other hand, only a few studies employed controls that allowed for comparing the efficacy of joint and separate treatments of these kinds. Therefore, further research in the area is indispensable.

9. Recommendations for Further Research

Multiple human trials documented the improvement of nutritional status in various groups of the population when probiotics or prebiotics were introduced into the diet, while in vitro studies helped to explain how microorganisms may impact the bioavailability of the micronutrients. Still, there are many knowledge gaps that require filling in order to develop effective approaches for the management of obesity. There are no studies evaluating the impact of whole, complex, obese, and lean microbiota on the bioavailability of micronutrients. Such studies should preferably start with well-controlled in vitro research focused on answering the following three questions:

• What is the effect of micronutrient supplementation on the lean and obese microbiome?
• What does the lean and obese microbiome do with micronutrients?
• What fraction of micronutrients that were exposed to obese and lean microbiome passes through the intestinal epithelium?

The answers to these questions are key to understanding how effective interventions targeted at micronutrient deficiencies in obesity might be designed. Such interventions could be even more effective than supplementation with micronutrients alone, providing numerous benefits, from weight loss through the improvement of glucose and lipid homeostasis to the amelioration of secondary illnesses developed in obesity. Furthermore, given the potential of pro/pre/synbiotics to improve the micronutrient status of humans, the reduction of effective doses of vitamins and minerals for the population with obesity could be achieved. To further the subject, more studies on the use and deposition of micronutrients in bodies with excess weight are needed, including studies on the link between microbiota and the bioavailability of micronutrients.

The findings of this review suggest that future research on effective obesity treatments should consider manipulating the microbiome to modulate the absorption of micronutrients.