**4. Conclusions**

This review discusses in detail the effectiveness of five popular diets, namely vegetarian diets, HFD, IF, GFD, and the low-FODMAP diet, on endurance performance and metabolism. Considering all findings from the review, all five diets discussed in detail appear to have both beneficial and detrimental effects on endurance performance (Figure 1). For vegetarian diets, we sugges<sup>t</sup> that when adjusting the athlete's diet a sports dietitian is to (a) determine which vegetarian diet the athlete is consuming; (b) control the athlete's micronutrients and related biomarkers, especially vitamin B12, folate, vitamin D and iron; (c) regulate the athlete's energy needs and all macro and micronutrient needs to prevent any deficiency, and (c) monitor the diet consumption and adjust it according to the needs based on individual- and sports-specific needs. While reviews of the HFD and sports performance have controversial results, the scientific evidence on the effectiveness of HFD on endurance performance is not strong enough to recommend these diets to endurance athletes. The evidence for IF diets and endurance performance and health-related parameters also needs to be improved by further investigation. We need more evidence before recommending the IF diet to endurance athletes. Considering all the relevant study results [66,68–70], we can say that a low-FODMAP diet may benefit more from GFD unless athletes have celiac disease. However, it should be kept in mind that the implementation steps of the low-FODMAP diet are complex and require careful monitoring by a trained dietitian. In addition, only lactose and fructose elimination from the diet should be considered in endurance athletes prior to adopting a low-FODMAP diet. We sugges<sup>t</sup> that a short-term (1–6 days) low-FODMAP diet can be planned at first before planning a long-term strategy, especially before endurance racing or strenuous exercise. In summary, all five diets discussed in the review can be applied to endurance athletes in accordance with the athletes' current metabolic demands. Before deciding on a popular diet, considering the current metabolic and sport-specific situation of endurance athletes will result in healthier and more beneficial results.

**Figure 1.** Possible beneficial and detrimental effects of popular diets on endurance athletes. Statements presented in green boxes show the beneficial effects of diets, while red boxes indicate the potential risks of diets. (**a**): Vegetarian diets; (**b**) highfat diets; (**c**) Intermittent Fasting; (**d**) Gluten-free diet; (**e**) low-FODMAP diet. Abbreviations: URTI: Upper-respiratory tract infections; RED-S: relative energy deficiency syndrome; FA: fatty acids; KB: Ketone bodies; GI: Gastrointestinal; FODMAP: fermentable oligosaccharides, disaccharides, monosaccharides and polyols.

> **Author Contributions:** Conceptualization, A.D.-L. and B.K., writing—original draft preparation, A.D.-L., writing—review and editing, A.D.-L., L.H., and B.K. All authors have read and agreed to the published version of the manuscript.

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

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