Fuel vs Fast: Navigating Intermittent Fasting in Sport

Table 1: Common intermittent fasting patterns and their structure

From a physiological perspective, fasting periods may lead to reduced glycogen availability, increased reliance on fat oxidation, and transient hormonal shifts such as elevations in growth hormone and cortisol and reductions in insulin levels^(2,3). However, it is important to note that IF does not inherently dictate total energy intake; yet, in practice, many individuals unintentionally reduce their caloric consumption due to the shortened eating window. This distinction is particularly relevant in athletic populations, where maintaining adequate energy availability is essential for performance, recovery, and overall health.

Evidence in the General Population

The general population often uses IF for a range of health reasons, although these are often comparable to those achieved through more traditional continuous energy restriction (see Table 2)^(4–8).

IF in Athletes: Benefits and Drawbacks

Athletes may use intermittent fasting (especially time-restricted eating, TRE/16:8) for body weight control. Research in trained and elite populations suggests it can improve body composition and health markers while usually maintaining performance, but there are caveats around hormones, early adaptation, and specific fasting styles (see Table 3)^(8–11).

Table 3: Common intermittent fasting in male vs female athletes in trained adults

Gender Differences

Research directly comparing male and female athletes under intermittent fasting is sparse. Most IF–performance trials are male-dominated or mixed, with no sex-specific analyses, so any sex differences are only partially understood (see Table 4).

Table 4: Gender differences

Practical implications

Intermittent fasting (IF) has several practical implications for athletes. Time-restricted eating (e.g., 16:8) can modestly reduce body weight and fat mass while preserving lean mass and strength, thereby improving the power-to-weight ratio and maximum power in both endurance and strength/power sports. When total calories, protein, and training are well managed, IF generally does not impair aerobic capacity, strength, or anaerobic performance, and may even enhance VO₂ max and jumping or sprint performance in some protocols. 

Ramadan-style fasting (no food or fluids during the daytime) is more challenging, as most performance variables are maintained. Still, high-intensity exercise, morning sprints, sleep, hydration, mood, and cognitive function can be negatively affected, especially without careful planning. Practically, athletes using IF should prioritize matching energy and macronutrient intake to training load; placing key sessions when fed or well hydrated; monitoring sleep and recovery; and avoiding very high-intensity work during the deepest fasted periods, particularly during Ramadan. Overall, IF is a viable tool for body composition and metabolic benefits in athletes, but requires individualized planning to prevent subtle decrements in high-intensity performance and recovery.

References

1. Nat Rev Endocrinol. 2022;18(5):309-321. 
2. Journal of Diabetes Research. 2023;2023:1-15. 
3. Qual Sport. 2025;40:59420. 
4. Nutrients. 2024;16(2):316. 
5. ArveMED. 2025;15(4). 
6. Orthopaedics and Traumatology: Surgery and Research. 2019;105(2):375-382. 
7. Nutrients. 2019;11(10):2442. 
8. Front Pharmacol. 2024;15:1428601. 
9. Nutrients. 2020;12(5):1390. 
10. Journal of the International Society of Sports Nutrition. 2020;17(1):65. 
11. BMJ Open Sport Exerc Med. 2022;8(1):e001206. 
12. IJERPH. 2022;19(7):4240. 
13. Journal of the American Nutrition Association. 2022;41(8):810-825. 
14. Medicine & Science in Sports & Exercise. 2021;53(12):2577-2585. 
15. Nutrients. 2024;16(1):168. 
16. IJERPH. 2021;18(12):6431. 
17. Nutrients. 2023;15(4):985. 
18. Nutrients. 2022;14(11):2343. 
19. The American Journal of Clinical Nutrition. 2019;110(3):628-640. 
20. Nutrients. 2023;15(22):4841.