*6.2. Evidence in Human Studies*

Currently, there is little research on the effects of fasting on patients with diagnosed MetS [29,31]. However, several small-scale and pilot trials on obese patients and patients with cardiometabolic risk clustering (but no MetS diagnosis) have provided promising results [29,31,82]. Similarly, a recent systematic review on the effects of fasting on cardiometabolic risk factors supports the role of IF in improving metabolic health [83]. Small but significant improvements were detected in risk factors such as body weight, waist circumference, fat mass, BMI, blood pressure, total cholesterol, triglycerides, fasting blood glucose, fasting insulin, and insulin resistance [83].

On the other hand, there are also some contradictory results, as one randomized control trial on obese or overweight adults produced only modest weight loss (below 2% of initial body weight), but no real benefits of IF on metabolic parameters or fat loss in the absence of controlled food intake [84].

**Figure 1.** Pathways through which intermittent fasting/time restricted feeding may affect the constituents of metabolic syndrome. Abbreviations: IF: intermittent fasting; TRF: time-restricted feeding.

A single-arm, paired-sample trial by Wilkinson et al. investigated the effects of 14 h fasts for 12 weeks on 19 patients with MetS who received the standard of care (antihypertensive therapy and/or statin) [31]. To track caloric intake and the adherence to TRE intervention, a validated app—myCircadianClock (mCC) was used [31]. Although there were no recommendations to reduce caloric intake, an 8.62% decrease in mean daily caloric intake was recorded during the intervention. Significant bodyweight reduction from baseline was achieved (−3%), along with desirable reductions in body fat percentage (−3%), along with significant decreases in visceral fat rating (−3%) and waist circumference (−4%) [31]. Importantly, the authors also noted significant decreases in systolic and diastolic blood pressure, total cholesterol, LDL-C, and non-HDL-C [31]. Despite the study's limitations (it was an unblinded, single-arm pilot study with a relatively small sample size), it still provided valuable evidence on the safety, adherence, and probable efficacy of TRF in MetS management.

In a proof-of-concept study, Sutton et al. investigated the effects of 5 week early TRF (eTRF, eating period from 8 a.m. to 2 p.m.) in men with prediabetes, while controlling for weight loss (feeding the participants enough food to maintain their weight) [84]. In the study, eTRF reduced insulin levels, blood pressure, and oxidative stress, while it improved β-cell responsiveness and insulin sensitivity. Although the study was conducted on just eight participants, excellent adherence was achieved, since the participants were required to eat all meals under supervision [84,85]. The importance of this randomized controlled trial was that it demonstrated the benefits of IF independent of food intake and weight loss in humans.

In another study, researchers examined the effects of 8 h TRF over 12 weeks on 23 obese adults, compared to a matched historical control group [82]. There were significant decreases in body weight, energy intake, and systolic blood pressure in the TRF group (−2.6% ± 0.5; −341 ± 53 kcal/d; −7 ± 2 mm Hg, respectively). Interestingly, there were no significant differences in fat mass, lean mass, visceral fat mass, diastolic blood pressure, LDL cholesterol, HDL cholesterol, triglycerides, fasting glucose, fasting insulin, HOMA-IR, or homocysteine [82].

Kesztyüs et al. conducted a pilot study on the effects of TRF in abdominally obese participants (waist-to-height ratio, WHtR ≥ 0.5) in a general practitioner's office [86]. Forty participants were asked to restrict their daily eating time to an 8–9 h window. On average, the participants reached the 15–16 h fasting target in 86 ± 15% of all recorded days. In addition, the participants achieved moderate weight loss (−1.7 ± 2.5 kg), along with a marked reduction in waist circumference (−5.3 ± 3.1 cm), leading to a reduction in WHtR (−0.03 ± 0.02) [86].

Moreover, a randomized controlled trial that researched the effects of two-month 4 and 6 h TRF in obese adults, resulted in promising outcomes concerning weight and cardiometabolic health [87]. Of 58 participants, 19 were randomized into the 4 h TRF group, 20 into the 6 h TRF group, and 19 into the no-intervention control group. Both the 4 and 6 h TRF regimens achieved a similar reduction in daily calorie intake (−550 kcal), while also producing similar weight loss (−3% body weight). Additionally, they reported a marked reduction in fasting insulin, insulin resistance, and oxidative stress. However, the significance of insulin and insulin resistance reductions was partly driven by a worsening in the control arm [87].

A trial by Parr et al. investigated the feasibility of TRF for individuals with T2DM [88]. The intervention consisted of a 2 week Habitual period to establish a baseline dietary intake, followed by a 6 week TRF intervention. Of the 24 enrolled participants, 19 completed the study. Overall daily dietary intake did not change between habitual and TRF periods. Moreover, the compliance with the 9 h TRF period was 72 ± 24% of 28 days (i.e., ~5 days/week). Interestingly, TRF did not significantly influence glycemic control or body mass, whereas the participants described hunger, daily stressors, and emotions as the main barriers to adherence. [88]. Furthermore, in a study by Chow et al., the authors examined and compared the effects of TRF (8 h target eating window) to an unrestricted (non-TRF) diet in overweight individuals [89]. Compared to non-TRF, the TRF group exhibited significantly reduced weight, lean mass, and visceral fat. Furthermore, when the TRF group was compared to their pre-intervention state, marked reductions in weight (−3.7%), fat mass (−4%), lean mass (−3.0%), and visceral fat (−11.1%) were observed. Interestingly, metabolic measures (lipids, blood pressure, 2 h oral glucose tolerance test, 2 week continuous glucose monitoring), and physical activity (actigraphy-assessed) remained unchanged [89]. The same research group performed a secondary analysis of the aforementioned trial, where they examined the effects of TRE on quality of life (QoL) measures [90]. TRE did not adversely affect QoL, and it even led to modest QoL improvements relative to baseline and unrestricted eating [90].

By contrast, in a prospective, randomized controlled trial conducted on 116 overweight or obese adults, Lowe et al. compared the effects of TRF (eating from 12–8 pm) with consistent meal timing (CMT-3 structured meals per day) over 12 weeks [84]. There was no recommendation for calorie and macronutrient intake or physical activity, so the study only compared the effects of different meal timing. There was a significant decrease in weight in the TRF group (−0.94 kg; 95% CI, −1.68 kg to −0.20 kg; *p* = 0 .01), but no significant differences in weight change between groups (−0.26 kg; 95% CI, −1.30 kg to 0.78 kg; *p* = 0.63). There were no significant within-group or between-group differences in glucose and lipid metabolic parameters [84].

In a randomized controlled trial, Parvaresh et al. investigated the differences between a modified alternate-day fasting regime (ADF) (a very low-calorie diet, 75% energy restriction on Saturday, Monday, and Wednesday accompanied by 100% energy intake on Sunday, Tuesday, Thursday, and ad libitum feeding on Friday) and a standard caloric restriction diet (consuming 75% of energy needs each day) during an 8 week period [91]. Of 70 patients with MetS, 69 completed the study, and the analysis showed significant reductions in body weight, waist circumference, systolic blood pressure, fasting plasma glucose in the ADF group. Interestingly, there were no significant differences in triglyceride, cholesterol (total, HDL, LDL), HOMA- IR, or fasting insulin concentrations [91]. The strengths of this study are in the number of participants (69, compared to the 23 in the second-largest study on MetS/fasting) and its randomized design [31,91].

In summary, a common theme in all of the aforementioned studies is a significant reduction in body weight and waist circumference (Table 2). At the same time, there are mixed results regarding the improvement in insulin sensitivity, glucose and lipid homeostasis, and blood pressure. Interestingly, there are only two studies conducted on participants with diagnosed MetS. In addition to significant weight and waist circumference reduction, both studies also reported a significant reduction in blood pressure levels, albeit with mixed results in cholesterol reduction [31,91]. However, it is possible that the observed improvements using IF/TRF may not be sustained unless close monitoring and follow-up of adherence are performed. Therefore, a combination of patient education with dietary, medical, and coaching staff familiar with the IF protocol is critical in achieving the putative results. On the other hand, the potential downsides of IF/TRF that require attention are hypoglycemic reactions, cardiac arrhythmias, muscle wasting, menstrual irregularities, gout, postural hypotension, peptic ulcers, and upper gastrointestinal bleeding [92,93]. As expected, hypoglycemic reactions represent the main issues among patients with diabetes, especially in T1DM [92].

**Table 2.** Studies examining fasting in patients with cardiometabolic risk factors.



**Table 2.** *Cont.*

Abbreviations: ADF = alternate-day fasting, eTRF = early time-restricted feeding, TRE = time-restricted eating, HOMA-IR = Homeostatic Model Assessment for Insulin Resistance.

#### **7. Conclusions and Future Perspectives**

Due to the high MetS prevalence (around 1/3 of the world population), its increasing trend, and high economic burden (\$260.6 billion aggregate costs due to obesity in the US), we are in dire need of new dietary, lifestyle, and therapeutic options [3,94]. Diets such as the Mediterranean diet and DASH have proven beneficial, but quality healthy foods such as fruit, vegetables, and fish are more expensive, and hence less accessible to a large part of the population [95,96]. Therefore, diets such as fasting/TRF that do not necessarily need a change in diet quality (with expensive foods) to exert beneficial results could prove valuable in combating this epidemic. A significant amount of evidence was gathered on the efficacy of IF/TRF in animal models [73–76,78]. These studies showed how IF/TRF exerts beneficial effects on the gut microbiota, glucose and insulin metabolism, weight and visceral fat, and lipid metabolism. In other words, fasting affects all the crucial pathophysiological points in MetS/diabetes type II development [4]. However, it is important to acknowledge that different species and the different ages of the tested animals affected the response to IF/TRF, and this variability should therefore be taken into account when assessing metabolic effects of IF/TRF. Considering studies on humans, while the results are promising, they are still scarce (there are only two studies on patients with diagnosed MetS). Most of the studies were performed on overweight/obese individuals who often present at least one more cardiometabolic risk factor, so we can expect similar results in larger studies on MetS patients. It is important to note that the viability and safety of IF/TRF methods have been established in small-scale, pilot studies [31,82,85,89]. The results obtained from human studies are somewhat conflicting, as weight loss was achieved in all studies, whereas in some studies, there was no significant effect on insulin resistance, cholesterol/lipid metabolism, or blood pressure [31,82,84,85,89,91]. However, in some cases, the weight loss was as low as 1–2% of initial body weight. Some of the observed differences might be due to the different genetic makeup of the researched population, differences in the IF/TRF protocols used, and differences in the time of day in which the eating period was set (circadian rhythms).

In conclusion, to establish the significance of fasting in controlling metabolic risk factors and the potential treatment of MetS, there is a need for more randomized control trials on larger cohorts of patients with MetS to gather higher yield evidence. Additionally, economic analysis is required to confirm the presumed cost-utility of IF/TRF.

**Author Contributions:** J.V., J.B., M.K., and T.T.K. for conceptualization, original draft preparation, and supervision; J.A.B., M.V., D.M., and V.R. for literature review and visualization. All authors contributed to the final draft of the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
