Benefits of Vegetarian Diets

With the growing popularity of vegetarian diets in the athletic population, researchers have begun to investigate the role of these diets in sports performance and metabolic profile [71].

Studies on vegetarian diets have suggested that these diets may improve endurance performance by increasing exercise capacity and performance, modulating exercise-induced oxidative stress [72], inflammatory processes including anti-inflammatory and immunologic responses [4], and upper-respiratory tract infections (URTI) [73], and providing better cardiovascular function [59].

Studies measuring the aerobic capacity of vegetarian and omnivorous athletes reported controversial results [54,56,58,59]. Two studies showed that VO2max values were higher in vegetarian athletes compared to omnivore athletes [56,59], while a crossover study showed no difference between the groups [54]. Studies supported higher VO2max values in vegetarians designed as a case study and two cross-sectional studies [56,58,59], which are considered as the lowest level of the etiology hierarchy. A cross-sectional study in amateur runners reported that vegetarian female athletes had higher VO2max values than omnivorous female athletes; however, no difference was observed in VO2max values between vegetarian and omnivorous male athletes [58]. We need more high-level studies on the interaction between VO2max and vegetarian diet patterns in endurance athletes.

The availability of studies on vegetarian endurance athletes supports neither a positive nor a negative impact on exercise capacity [52,56]. Comparing the exercise capacity of lacto-ovo-vegetarian, vegan and omnivorous athletes, Nebl et al. [52] measured maximum power output (Pmax) during incremental exercise as the primary outcome of the study in determining exercise capacity, while maximum power output per lean body weight (PmaxLBW), blood lactate and glucose concentration during incremental exercise were evaluated as secondary outcomes. No differences were detected in Pmax, PmaxLBW, blood lactate and glucose concentrations between groups during increased exercise, suggesting that there was no difference in exercise capacity compared to the lacto-ovo-vegetarian (LOV), vegan or omnivorous diet pattern in endurance athletes [52]. In addition, a case

study by Leischik and Spelsberg [56] assessed the exercise performance, cardiac status, and nutritional biomarkers of a male vegan ultra-triathlete and a control group of 10 Ironman triathletes during a Triple Iron ultra-triathlon (11.4 km swimming, 540 km cycling, and 126 km running). Apart from a mild thrombopenia with no pathological consequences in laboratory parameters, the vegan athlete did not have weakened nutritional biomarkers or impaired health symptoms. Additionally, the VO2max value of the vegan athlete was greater compared to the omnivorous athletes. Systolic and diastolic functions also did not differ between vegan and omnivorous athletes. The findings indicate that a well-planned vegan diet can provide adequate nutritional support for an ultra-triathlete [56].

In addition to these aforementioned benefits, vegetarian diets may also provide advantages for exercise capacity by increasing muscle glycogen levels [71], and delaying fatigue [74]. As for increasing glycogen stores, carbohydrate intake is considered the cornerstone of a better endurance performance by enhancing muscle glycogen stores, delaying fatigue, and providing athletes to compete at better and higher levels during prolonged periods [75]. Given the fact that the vegetarian diets are rich in carbohydrates (CHO) [71], such diets may offer more opportunities when considering races or training that can last at least six hours [2]. However, these data bring us to the point where foods high in CHO rather than diet types may be responsible for better performance. Taken together, both studies have shown that vegetarian diets neither benefit nor harm exercise capacity and endurance performance compared to omnivorous athletes. However, more studies are needed due to the small number of studies on the topic.

Studies have shown that the beneficial effects of vegetarian diets in alleviating oxidative stress and regulating the anti-inflammatory response are based on their enormous non-nutrient content called phytochemicals [4,76]. Polyphenols containing flavonoids, phenolic acids, lignans, and stilbenes are the most diverse non-nutrient group of phytochemicals that are produced as secondary metabolites throughout plants and have a broad spectrum of effects on metabolic health [77]. Polyphenol research of the athletic population has often been conducted using various fruits and vegetables, mainly berries [78], including blueberries [79–82], black currant [83], Montgomery cherry [84,85], and pomegranate [86]. Acute polyphenol intake or supplementation of ~300 mg 1–2 h before training or >1000 mg of polyphenol supplementation (equivalent to 450 g blueberries, 120 g blackcurrants or 300 g Montmorency cherries) 3 to more days (1–6 weeks) before and immediately after training is recommended as a countermeasure to improve antioxidant and anti-inflammatory response mechanisms [87]. However, only two studies examined the effect of vegetarian diets on exercise-induced oxidative stress in endurance athletes by comparing them with omnivorous diets, revealing contradictory results [53,55]. An incremental exercise test was applied in both studies. Nebl et al. [53] showed that nitric oxide levels, also known as an important biomarker for inflammation, endothelial and vascular function, did not alter between groups. In addition, exercise-induced malondialdehyde (MDA) concentration, an end product of lipid-peroxidation that is commonly measured to detect oxidative stress, significantly increased in vegan athletes in both studies, and in LOV athletes compared to omnivorous athletes [53]. Further, Potthast et al. [55] found a negative interaction between MDA, and sirtuin activities and antioxidant intakes such as ascorbate and tocopherol. These studies showed opposite results, against expectations, i.e., vegetarian diets increased the antioxidant response while suppressing the oxidant response. One explanation might be that the MDA test may not provide accurate measurement in biological samples due to its high reactivity and cross-reactions with other biochemicals available in the body despite its widely usage as an oxidative stress biomarker [88]. Therefore, studies with a greater sample size and including other oxidant parameters are needed to clarify these findings.

In addition to polyphenols, Interleukin 6 (IL-6) has often been identified as an inflammatory biomarker associated with fatigue, skeletal muscle inflammation, and differentiation of immune response, as well as an inducer of the metabolic acute phase response to infection [4,89–91]. It has been suggested that endurance athletes consuming vegetarian diets may have lower IL-6 concentrations and a lower IL-6 increase in response to endurance

performance [4]. These data are explained by the positive interaction between muscle glycogen and IL-6 concentration, based on the information that higher muscle glycogen stores cause lower IL-6 elevations [92]. The higher CHO content of vegetarian diets may increase muscle glycogen stores, resulting in a down-regulated IL-6 response to endurance performance [4]. However, there are no data comparing the vegetarian and omnivorous diets for IL-6 concentration in endurance athletes.

One further point is the possible roles of vegetarian diets in URTI [73]. It is well known that endurance athletes are at greater risk for URTI due to prolonged and excessive training or races that cause immunosuppression and immune deficiency [93]. The possible link between URTI and a vegetarian diet may be explained with an emphasis on its polyphenolic content [94]. Polyphenol supplementation is also preferred in endurance athletes because of its debilitating role in URTI, one of the risk factors that often arise after immunosuppressive endurance exercise. A meta-analysis by Somerville et al. [73] reported that flavonoid supplementation reduced the incidence of URTI by 33% compared to a control group. Researchers also examined all factors that may cause a bias between studies, indicating that the risks for sequence generation, allocation concealment, and reporting bias are unclear in the included studies in the systematic review [73]. On the other hand, in a crossover design, Richter et al. [54] compared the influence of a 6 week LOV diet versus a meat-rich Western diet on in vitro measurements of immunologic parameters in male endurance athletes. The findings reported that no change was detected in CD3+ (pan T-cells), CD8+ (mainly T suppressor cells), CD4+ (mainly T helper cells), CD16+ (natural killer cells), CD14+ (monocytes) after the two diet trials and none of the immunological parameters differed from each other after the two diets. Studies have commonly focused more on diet content rather than diet pattern whether vegetarian or omnivorous. Therefore, the potential immunological benefits of vegetarian diets need to be investigated further.

A review investigating the effect of vegetarian diets on cardiovascular health in endurance athletes highlighted that vegetarian diets can provide better cardiovascular protection by reducing plasma lipid levels, exercise-induced oxidative stress, inflammation and blood pressure, and improving endothelial function and arterial flexibility [71]. One cross-sectional study confirmed the information by investigating the difference in heart morphology and function according to the vegan and omnivorous diets in amateur runners [59]. The results showed that vegans had better systolic function, determined by longitudinal strain (vegan: −20.5% vs. omnivore: −19.6%), and diastolic function in vegans, determined by higher E-wave velocities (87 cm/s vs. 78 cm/s), compared to omnivorous athletes [59]. Therefore, we can confirm that vegetarian diets may have a beneficial impact on cardiovascular function; however, we still need further investigation on endurance athletes.

Potential Risks of Vegetarian/Vegan Diets

Vegetarian and vegan diets offer several beneficial privileges for athletic populations [9,71]. However, the underlying mechanisms linking vegetarian diets to metabolic processes that may lead to undesirable effects on sports performance and, more importantly, metabolic health, should be considered beyond their beneficial functions [95]. In cases where athletes follow a vegetarian diet, issues related to the micronutrient deficiency, diet's energy availability [96], relative energy deficiency syndrome (RED-S) [11], serum hormones [97,98], and protein quality/quantity [99,100] are topics that need to be addressed first.

Athletes who adhere to vegetarian diets are considered at high risk for deficiency of certain nutrients, especially when their dietary composition is not well-structured [10]. These risks are mainly due to the restriction of some food groups with a high nutrient density such as milk, meat, and eggs, the inability to access vegetarian foods when needed, or the development of early satiety and loss of appetite due to the high fiber content of vegetarian foods [95,101]. Furthermore, due to these dietary restrictions, athletes are at a higher risk for several micronutrient deficiencies including omega-3, iron, zinc, iodine, calcium, vitamin D, and vitamin B12 [101].

Nebl et al. [102] investigated the food consumption of vegan, lacto-ovo-vegetarian (LOV) and omnivorous (OMN) athletes according to the intake recommendations of the German, Austrian, and Swiss Nutrition Societies for the general population. Most athletes did not reach the recommended energy intake. Although omnivorous athletes consumed lower CHO compared to the recommended intake, vegetarian athletes consumed adequate amounts. For micronutrient intake, vegans achieved adequate iron levels by consuming only foods high in iron, while female LOV and OMN athletes achieved the recommended amount after supplementation. The results showed that all groups consumed enough of most nutrients. However, an analysis of the circulating state of nutrients is also needed to better interpret the effectiveness of dietary intake, particularly for vegetarian athletes [102]. A cross-sectional study by the same researchers [103] then compared the micronutrient consumption of LOV, vegan, and omnivorous recreational runners and found that 80% of each group had adequate vitamin B12 and vitamin D levels, and these parameters were higher in supplement users. Red blood cell folate exceeded the reference range; however, there was no difference in red blood cell folate among all groups [103]. No iron deficiency anemia was detected in any group, and less than 30% of each group were found to have depleted iron stores. The results sugges<sup>t</sup> that a well-planned vegetarian diet can meet the athlete's iron, vitamin D, and vitamin B12 needs [103]. These findings have been confirmed in case reports on vegan mountain bikers and ultra-triathletes [56,57]. Additionally, vegetarian diets are often inferior in quality compared to omnivores; this is due to anti-nutritional factors such as trypsin inhibitors, phytate, and tannins in those rich in vegetarian diets [104]. However, these challenges can be overcome by applying pre-cooking techniques described in detail in another review [105]. Therefore, it is obvious that vegetarian diets require careful monitoring in endurance athletes whose energy, macro and micronutrient needs are higher than their omnivore counterparts. However, with a well-planned diet and close monitoring, the nutritional needs of athletes can be successfully met, even ultra-endurance athletes.

Various metabolic risks such as iron deficiency anemia, menstrual disorders, musculoskeletal injuries, immunity, and hormonal irregularities occur in endurance athletes as a result of insufficient energy and nutrient intake following high-intensity endurance performance [106,107]. Relative energy deficiency syndrome has been found more often in vegetarian athletes, which causes endocrine and eating disorders that cause harmful diseases to metabolic health, reduces bone mineral density, and causes menstrual dysfunction [108,109]. Relative energy deficiency syndrome was developed to replace the Female Athlete Triad by broadening the definition to include male athletes and impaired physiological function caused by relative energy deficiency [109]. The key etiological factor of RED-S is a low energy availability which results in, but is not limited to, impairments of metabolic rate, menstrual function, bone health, immunity, protein synthesis and cardiovascular health [109]. In a study, researchers attribute this to either vegetarians' food choices for low-energy-dense, high-fiber foods, even in high-energy situations, or restricted food intake behaviors by indicating dietary rules to mask vegetarians' eating disorders [110]. Since low energy availability has already presented a challenging problem for endurance athletes independent of the diet pattern [111] and even healthy endurance athletes often cannot fully meet their body energy and vitamin requirements [112], nutritional adequacy and the quality of vegetarian diets are often questioned. However, studies examining the nutritional efficiency of vegetarian diets claimed the opposite results. Examining the diet adequacy and performance parameters of a vegan ultra-triathlete with 10 Ironman counterparts, a case report has revealed that a vegan athlete has no nutritional deficiencies or health disorders [56]. Researchers examined the spiroergometric, echocardiographic, or hematological parameters of a vegan ultra-endurance triathlete that has been vegetarian for 22 years and vegan for the past nine years. It has been found that a long-term vegetarian diet is not detrimental to metabolic health for a long-distance triathlete, even at micronutrient parameters associated with anemia. Although being a vegan athlete who consumes a well-planned diet does not have a detrimental impact in terms of cardiometabolic health

and sports performance [56], findings need to be explored with a larger athletic cohort. These findings are similar to those of Wirnitzer et al. [57], who evaluated the food intake of a vegan mountain biker in the Transalp Challenge race (42 h). The researchers have highlighted that a carefully planned vegan diet strategy ensures that the race goals are achieved, and thus the race is completed in a healthy state [57]. Therefore, a well-planned vegan diet can be a grea<sup>t</sup> alternative for ultra-endurance athletes who endure extreme conditions such as psychological, physiological, endocrinological, and immunological stress-related metabolic challenges during prolonged training periods. In the last statement by the Academy of Nutrition and Dietetics on vegetarian diets, it was stated that vegetarian diets seemed more sustainable for all stages of life [113]. Researchers have suggested that well-planned vegetarian and vegan diets containing certain micronutrients such as high-quality plant protein, iron, n-3 fatty acids, Zn, Ca, iodine, vitamin B12, and vitamin D provide various health benefits regarding diseases such as hypertension, ischemic heart disease, diabetes and obesity [113]. In addition, given the content of vegetarian diets that can contain milk, eggs, or fish, vegetarian diets may be a better option for providing better nutritional density and quality than a vegan diet [99]. It is recommended that vegans carefully monitor blood vitamin B12 concentrations and supplement their diets, if necessary, with supplements or fortified foods [113]. Vegetarian and vegan nutrition programs should be planned by considering the above-mentioned data.

For many years, there have been claims that vegetarian diets negatively affect serum sex hormones [97,98], but data on the interaction between serum sex hormones and vegetarian diets remain controversial. In a crossover study conducted in 1992, Raben et al. [114] studied the effects of a 6 week lacto-ovo-vegetarian and omnivorous diet on serum sex hormones and endurance performance in eight endurance athletes. Although endurance performance did not differ according to the diet model, serum testosterone levels slightly decreased after six weeks of consuming a lacto-ovo- vegetarian diet. The researchers stated that these results may be related to dietary fiber binding to sex hormones and higher fiber intake in the lacto-ovo-vegetarian diet [114]. Considering the evidence in the literature that testosterone triggers muscle protein anabolism and lean body mass [115], a decrease in testosterone levels would cause an undesirable situation. However, a recent study in men from the national health and nutrition examination survey (NHANES) database, but not on athletes, found that a vegetarian diet did not link to serum testosterone levels [116]. Along with all data, the interpretation of the vegetarian diet as an attenuating factor to sex hormones by disregarding other confounding factors such as age, gender, training intensity, and emotional stress would be inappropriate [116] and needs further investigation.

The issue of the protein quality and quantity of vegetarian diets has long been controversial [99,117]. While some researchers note that vegetarian proteins have some missing specific amino acids [118], others state that including high-quality protein-rich foods such as legumes, seeds, nuts, and grains in a vegetarian diet is sufficient to meet the body's amino acid requirement [119]. A vegan diet structure should be created by examining the protein content of the food consumed, especially in terms of quality and quantity. Determining the dietary protein quality using the Digestible Indispensable Amino Acid Score (DIAAS) method in omnivore and vegetarian athletes, Ciuris et al. [100] analyzed the diet content of 38 omnivore- and 22 vegetarian athletes. Vegetarian athletes had significantly lower lean body mass (LBM) compared to omnivores ( −14%). Available protein was significantly correlated with strength (*r* = 0.314) and LBM (*r* = 0.541). The main findings revealed that vegetarian athletes needed to consume an additional 10 g of protein per day to achieve the recommended protein intake of 1.2 <sup>g</sup>·kg−<sup>1</sup> body weight (BW) and an additional 22 g of protein to reach 1.4 <sup>g</sup>·kg−1·kg−<sup>1</sup> BW [100]. Data on vegetarian proteins such as hemp, soy, potato, and rice proteins highlight that these vegetarian proteins contain sufficient high-quality protein content to increase muscle protein synthesis and post-workout recovery [119]. Rogerson [10] suggested that vegan athletes could improve their protein intake towards the higher limit of the International Society Of Sports Nutrition's (ISSN) protein recommendation for athletes up to 2.0 <sup>g</sup>·kg−<sup>1</sup> body mass per day. However, given

that there is little evidence in the literature that vegetarian proteins are inadequate to provide an athlete's needs or that vegetarian athletes need a higher protein intake [11,117], this recommendation needs further clarification with clinical research.

Additionally, the potential benefits of vegetarian diets are often attributed to their polyphenolic contents [4]. The intake of polyphenols with food may be the best choice for regulating body hormesis in the case of antioxidants due to the fact that polyphenol supplements may compromise the body's antioxidant defense metabolism [87,120]. However, at this point, the bioavailability of polyphenols taken with food comes into question [121]. While some researchers have suggested that the recommended polyphenol intake can be achieved by consuming polyphenol-rich foods or as a polyphenol supplement [122], others claimed that some polyphenols, such as quercetin, cannot be taken naturally with food [123]. Keeping all this in mind, it is necessary to further clarify the possible mechanisms for how the bioavailability of polyphenols in the body and their effects on sports performance change with their consumption naturally.

With all the data obtained from studies, there is currently no certain evidence that omnivorous or vegetarian diets provide better metabolic health and performance benefits [52,53,55–57,59]. Therefore, more research is needed to clarify the optimal dietary recommendations for macro and micronutrients, as well as polyphenols, to maintain health and improve performance in endurance athletes following vegetarian diets.
