The ketogenic diet, which induces a state of ketosis by severely restricting carbohydrates (typically below 50 grams per day), has been promoted for endurance sports. The rationale is that ketosis shifts the body’s fuel source from glucose to fatty acids, which are abundant even in lean athletes. A 2023 meta-analysis of 24 studies found that ketogenic diets did not improve performance in high-intensity sports (sprints, team sports) but did show a small benefit for ultra-endurance events (marathons, triathlons), likely due to glycogen sparing. However, the adaptation period (2–3 weeks) is debilitating; athletes often experience “keto flu” (fatigue, brain fog), and performance during this period drops by 10–15%. The long-term effects on bone density and immune function remain unclear, with some studies suggesting increased risk of stress fractures. The IOC recommends that ketogenic diets be used only under professional supervision and for specific periods (e.g., 3–4 weeks before a marathon), not as a year-round strategy.
The vegan and plant-based diets have gained popularity, partly driven by ethical and environmental considerations, but also by performance claims. A 2022 study in the Journal of the International Society of Sports Nutrition followed 150 endurance athletes and found that vegan athletes had comparable performance to omnivorous athletes, but required more careful planning to meet protein and micronutrient needs. The vegan diet is typically higher in carbohydrates, which is beneficial for endurance, but may be deficient in vitamin B12, iron, calcium, and omega-3s. Elite vegan athletes, like F1 driver Lewis Hamilton and tennis player Venus Williams, employ registered dietitians to supplement these nutrients, but the average recreational athlete may not have access to such support. The “plant-based” trend has also led to an over-reliance on processed substitutes, which can be high in sodium and low in nutrient density.
The “periodization of nutrition” is the emerging consensus. This approach aligns carbohydrate intake with training load: high-carb days (5–7 g/kg) for intense training, moderate-carb days (3–5 g/kg) for recovery, and low-carb days (1–2 g/kg) for active recovery. The concept of “training low, competing high”—training with low glycogen availability to stimulate mitochondrial adaptations, but competing with high glycogen to maximize performance—has been validated in multiple studies. The “carb loading” before competition is still standard, but the “micro-loading” method (consuming 1.5 g/kg of carbohydrate 15–30 minutes before exercise) has been shown to be equally effective in some studies. The timing of protein intake is also critical; the post-exercise “anabolic window” (30–60 minutes) is still recommended, though the window may extend to 2–3 hours if a meal is consumed.
The “intermittent fasting” trend has attracted athletes seeking weight management, but its performance impact is mixed. A 2024 study in the European Journal of Applied Physiology found that time-restricted eating (e.g., 16:8 protocol) did not affect strength in trained athletes but did reduce endurance performance by 5% in long-duration events, likely due to reduced glycogen stores. The “warrior diet” (eating only one meal per day) has been shown to impair cognitive function and reaction time, which is particularly relevant in team sports. The IOC recommends that intermittent fasting be avoided during competition periods, unless it is part of a religious or cultural practice.
The “personalisation” of nutrition is the next frontier. Genetic testing, microbiome analysis, and metabolic profiling are now available to elite athletes, offering insights into optimal macronutrient ratios and supplement needs. The 2025 introduction of “AI-based nutrition coaching” by companies like Nutrigenomix and Viome has made personalised nutrition more accessible, though the evidence for genetic-based dietary advice is still evolving. The “food-first” approach remains the foundational principle; supplements should only be used when dietary gaps are identified, and whole foods provide the synergy of micronutrients that isolated supplements lack.
The most important lesson is that nutrition is not a “magic bullet.” It is one component of a holistic performance strategy that includes training, recovery, psychology, and sleep. The fad diets—keto, vegan, intermittent fasting—can be effective if properly implemented and individualised, but they can also be detrimental if adopted without professional guidance. The sports nutrition landscape is complex, and athletes should be wary of “quick fix” promises. The best diet is one that is sustainable, balanced, and aligned with the athlete’s goals, preferences, and metabolic profile. As the IOC consensus concludes: “Nutrition is a modifiable factor, but it is not a substitute for hard work or talent.”
