**2. Methods**

We searched both the PubMed and Cochrane databases for the terms "diet\*", "trackand-field", "runner\*", "marathoner\*", "cyclist", "cycling", "triathlete", "endurance", and "endurance athletes" in the title, abstract, and keywords to detect the most applied diets between 2015 and 2021 in endurance athletes. We obtained 217 results in PubMed and 80 trials in the Cochrane database. We defined the most recurrent diets in endurance athletes, including "High CHO availability", "High-carbohydrate diet", "Ketogenic diet",

**Citation:** Devrim-Lanpir, A.; Hill, L.; Knechtle, B. Efficacy of Popular Diets Applied by Endurance Athletes on Sports Performance: Beneficial or Detrimental? A Narrative Review. *Nutrients* **2021**, *13*, 491. https:// doi.org/10.3390/nu13020491

Academic Editors: David C. Nieman and Ajmol Ali Received: 11 December 2020 Accepted: 29 January 2021 Published: 2 February 2021

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"Low-CHO diet", "Low-CHO, high-fat diet", "Ketogenic low-carbohydrate, high-fat diet", "Low-carbohydrate ketogenic diet", "Low-carbohydrate, high fat, ketogenic diet", "Highfat, low carbohydrate diet", "Ketone ester supplementation", "time-restrictive eating", "Ketone supplementation", "Intermittent fasting", "fasting during Ramadan", "Vegan diet", "Lacto-Ovo vegetarian diet", "Vegetarian diet", "Low fermentable oligo-, di-, monosaccharide, and polyol diet", and "Gluten-free diet". Since we all know that high-carbohydrate diet is already well proven to enhance endurance performance [2], we targeted other diets for in-depth investigation by categorizing them as "vegan/vegetarian diets", "high-fat diets", "intermittent fasting", "low-FODMAP diet, and "gluten-free diet". We included studies on endurance athletes and popular diets, including vegetarian diets, high-fat diets, intermittent fasting, gluten-free diet, and low-FODMAP diet. Using PubMed, Cochrane Library, and Web of Science databases, we aimed to identify studies on races and endurance training. Two researchers (A.D.L and L.H.) independently reviewed the literature. In cases of conflict, a third investigator (B.K.) resolved the disagreement. We identified the studies published from 1983 to 2021. To define the studies on endurance athletes and diets to be included in the current narrative review, we searched MeSH terms (("Diet, Ketogenic" (Majr); "Diet, High-Fat" (Majr); "Diet, Carbohydrate-Restricted" (Majr); "Ketone Bodies" (Majr); "Diet, Vegetarian" (Majr); "Diet, Vegan" (Majr); "Fasting" (Majr); "Diet, Gluten-Free" (Majr); "athletes" (Majr); "physical endurance" (Majr); "Diet Therapy" (Majr); " Oligosaccharides" (Majr), "Disaccharides" (Majr)) and MeSH terms found below this term in the MeSH hierarchy recommended by PubMed and Cochrane Library. We also searched by adding the terms "FODMAP diet", "low-FODMAP diet", "FODMAP\*", "Fermentable oligosaccharides, disaccharides and polyols", "Fermentable, poorly absorbed, short chain carbohydrates", "Inulin", "Xylitol", "Mannitol", "Maltitol", "Isomalt", "Fructose", "Fructans", "Galactooligosaccharides", "fructooligosaccharides", and "Polyols" to all databases, as no MeSH terms for the low-FODMAP diet were defined. We discussed the findings after determining the clinical and practical relevance of the studies by considering only human studies. We included studies available in English clearly describing the applied diet and investigating the effect of diet on endurance athletes as the primary goal. In addition, we included studies where diets were applied according to the dietary description. We excluded studies not explicitly addressing the impact of the diet on endurance performance or health-related parameters, that were not written in English, and were conducted on animals or in vitro. Based on our inclusion and exclusion criteria, we identified 57 research articles (Table 1). We organized the narrative review by considering both the beneficial and detrimental aspects of all five diets for endurance athletes.

#### **3. Popular Diets Applied to Improve Sports Performance in Endurance Athletes** *3.1. Vegetarian Diets*

Worldwide, it is estimated that around four billion people follow vegetarian diets [9]. In addition to many books and documentaries on vegetarian diets along with various types of practice (Table 1) and many well-known athletes who have adopted vegan diets and improved their performance [10], vegan diets have become more acceptable and feasible in the athletic population [11]. Looking at the athletic population, using a surveybased study conducted with 422 marathon runners, approximately 10% (*n* = 39) of the athletes consumed vegetarian/vegan/pescatarian diets [12]. However, in the NURMI study, the authors used the prevalence of vegetarian diets in ultra-endurance runners, primarily living in Austria, Germany, and Switzerland [13]. The findings revealed that the ratio of vegetarian and vegan athletes was 18.4% and 37.1%, respectively.

**Table 1.** Studies investigating the potential effects of vegetarian, fasting, high-fat, gluten-free, and low-FODMAP diets on athletes' endurance performance.







**Table 1.** *Cont.*



**Table 1.** *Cont.*

↓: A significant decrease after the diet manipulation in the experimental group; ↑: A meaningful rise after the diet manipulation in the experimental group; ↔: No change after the diet manipulation in the experimental group. Abbreviations: K-LCHF: ketogenic low-carbohydrate, low-fat diet; NK-LCHF: non-ketogenic low-carbohydrate, low-fat diet CHO: carbohydrate; HCD: high-carbohydrate diet; TT: time trial; CPT: critical power test; s-IgA: serum immunoglobulin A; Wmax: maximal power output; VO2peak: peak oxygen uptake; VO2max: maximal oxygen uptake; PCD: periodized carbohydrate diet; TTE: time-to-exhaustion; MIE: moderate intensity exercise; HIE: high-intensity exercise; LDL-c: low-density lipoprotein; HDL-c: high-density lipoprotein; CK: creatine kinase; LDH: lactate dehydrogenase; SS: steady state; HCO3: hydrogen bicarbonate; KE: ketone ester; KME: ketone monoester; BW: body weight; βHB: (R)-3-hydroxybutyl (R)-3-hydroxybutyrate); VT: ventilatory threshold; GI: gastrointestinal; MCT: medium-chain triglycerides; RER: Respiratory exchange ratio; HR: heart rate; IMCL: Intra myocellular lipid; LOV: lacto-ovo-vegetarian; MDA: malondialdehyde; NO: nitric oxide; P/S ratio: polyunsaturated/saturated fatty acid ratio; MVC: Maximal Voluntary Isometric Contraction; IL-6: interleukine-6; PPO/BW ratio: peak power output/body weight ratio; DOMS: delayed onset muscle soreness; GFD: gluten-free diet; FODMAP: fermentable oligosaccharides, disaccharides, monosaccharides and polyols.

#### The Impact of Vegetarian Diets on Sports Performance
