Athletes.

Fasting and athletic performance is a genuinely contested topic in sports science. On one side: fasting improves metabolic flexibility, teaches the body to use fat efficiently, and can improve body composition for weight-class and aesthetic sports. On the other: fasting impairs high-intensity performance, reduces training capacity, and can compromise recovery and lean mass. Both sides have evidence. The answer depends entirely on the sport, the training phase, and the athlete's goals.

The strongest argument for fasting in athletes is improved metabolic flexibility — the ability to efficiently switch between carbohydrate and fat oxidation depending on exercise intensity. Athletes who train in a fasted state upregulate fat oxidation pathways, which improves endurance performance at moderate intensities and reduces dependence on exogenous carbohydrate fueling. This is well-supported in endurance sports (marathon, ultra-running, cycling) and is the basis for the 'train low, compete high' approach used by elite endurance coaches.

Fasting consistently impairs high-intensity performance — sprinting, heavy lifting, interval training, and any effort above lactate threshold. Glycogen depletion reduces the capacity for intense muscular contraction, and no amount of metabolic flexibility compensates for this at high intensities. Athletes in power sports, team sports, or any discipline requiring maximal effort should not train fasted for their key sessions. The compromise used by many coaches is to train fasted for low-intensity base work and fuel for high-intensity sessions.

Wrestlers, boxers, MMA fighters, weightlifters, and bodybuilders commonly use fasting protocols for weight management between competitions. The most common approach is 16:8 during the training phase with a shift to more aggressive protocols (24-hour fasts, modified ADF) during the cut phase. The risk is lean mass loss — aggressive fasting during a caloric deficit accelerates muscle catabolism, particularly when protein intake is insufficient. High protein intake (1.6 to 2.2 g/kg body weight) during the eating window is essential for athletes using fasting for body composition.

Post-workout nutrition — protein and carbohydrate intake in the 2 hours after training — supports muscle protein synthesis and glycogen replenishment. Fasting through this window compromises both. For athletes prioritizing training adaptation and recovery, the eating window should overlap with the post-training period. A practical approach is to schedule training at the end of the fasting window, breaking the fast immediately after the session. Fasting after a hard training session is generally counterproductive.

Endurance athletes: fasted low-intensity training is well-supported; fuel for intervals and races. Strength athletes: eat before and after heavy sessions; TRE can be used on rest days. Team sport athletes: do not fast on training or game days. Combat sport athletes: TRE for weight management between fights, physician supervision during weight cuts. Recreational athletes: any TRE protocol is fine as long as the eating window covers the post-training period.

What sport do you train for, and what are your current performance goals? Is your primary goal body composition, metabolic flexibility, or general health? Are you in a training phase where recovery is critical? Are you consuming enough protein within your eating window? Are you working with a coach or sports dietitian who understands fasting? Are you tracking performance metrics to ensure fasting is not impairing your training?