Intermittent Fasting for Athletes: Benefits, Risks, and Performance Insights

Intermittent fasting is one of the most popular nutrition strategies today. From marathon runners to CrossFit athletes, many are trying out fasting to improve performance, burn fat, and simplify nutrition. But does intermittent fasting actually help athletes perform better, or could it hurt endurance training and recovery?

This article breaks down the science behind intermittent fasting for athletes, the potential benefits and drawbacks, and practical tips if you’re considering adding fasting to your training plan.

What Is Intermittent Fasting?

Intermittent fasting is an eating schedule that cycles between periods of fasting and eating. Unlike traditional diets, it doesn’t prescribe specific foods, just the timing of your meals.

Popular methods include:

16:8 Method – 16 hours fasting, 8-hour eating window.

5:2 Diet—Eat normally 5 days, restrict calories on 2 days.

Alternate-Day Fasting—Fast every other day.

Many athletes are drawn to intermittent fasting because it may aid in fat loss, enhance energy levels, and promote fat adaptation during fasted training sessions.

What happens in your body while fasting?

⏱ 0–4 Hours After Eating

Your body uses glucose from your last meal as fuel.

Insulin rises to help store nutrients.

⏱ 4–12 Hours After Eating

Insulin levels start to drop.

The body begins tapping into glycogen stores in the muscles for energy and in the liver to maintain blood sugar levels.

⏱ 12–24 Hours of Fasting

Glycogen runs low, the body switches to burning more fat.

Fat cells release free fatty acids, which muscles use as fuel.

Liver produces ketones for the brain and muscles.

⏱ 24+ Hours of Fasting

Fat burning becomes the main energy source.

Human Growth Hormone (HGH) rises, helping preserve muscle.

Autophagy kicks in, and cells start recycling old or damaged parts.

Benefits of Intermittent Fasting for Athletes

1. Fat Adaptation for Endurance Sports

Training in a fasted state can encourage your body to rely on fat as a primary fuel source, potentially improving endurance in long races.

2. Body Composition Goals

For athletes looking to lose body fat while maintaining muscle, intermittent fasting can help control calorie intake without strict dieting.

3. Improved Metabolic Health

Research suggests intermittent fasting may improve insulin sensitivity, blood sugar regulation, and inflammation, which is key for recovery and long-term performance.

4. Simplicity and Routine

Many athletes find it easier to stick to a structured eating window, reducing mindless snacking and making meal prep more efficient.

Drawbacks of Intermittent Fasting for Athletes

1. Reduced Training Intensity

High-intensity workouts and sprint sessions rely heavily on carbohydrates. Training fasted may make speed work feel harder and reduce performance.

2. Slower Recovery

Skipping post-workout meals delays muscle repair and glycogen replenishment, which is especially risky for athletes training daily.

3. Risk of Underfueling

Endurance athletes often need high calorie intake. Eating within a restricted window may not provide enough energy, leading to fatigue, injuries, decreased performance, or RED-S (Relative Energy Deficiency in Sports).

4. Not Ideal for Competition Season

During peak training or racing, performance, not fasting, should be the top priority.

Who Might Benefit from Intermittent Fasting?

• Endurance athletes in the off-season who want to improve fat metabolism or body composition.

• Recreational athletes with moderate training loads.

• Athletes who prefer structure and consistency in eating habits.

Who Should Avoid or Be Cautious?

• Marathoners, triathletes, and cyclists in heavy training or peak season.

• Younger athletes still growing and developing.

• Anyone with a history of low energy availability or disordered eating.

Tips for Athletes Trying Intermittent Fasting

1. Schedule intermittent fasting around easy workouts. Save long runs, rides, intervals, and high-intensity sessions for days when you’re fueled.

2. Break your fast with recovery fuel: Prioritize protein and carbohydrates immediately after workouts.

3. Hydrate well: Water is allowed during fasting windows.

4. Track your performance: If strength, speed, or recovery declines, fasting may not be right for you.

Bottom Line: Does Intermittent Fasting Work for Athletes?

Intermittent fasting can be a useful tool for some, particularly in the off-season or for those with body composition goals. However, for endurance athletes in heavy training, underfueling is a bigger risk than overeating. If you’re considering fasted training or intermittent fasting, align your nutrition with your performance goals, not just diet trends.

Thinking about trying intermittent fasting as an athlete?

As a sports nutritionist, I create personalized fueling strategies for runners, cyclists, and triathletes. Book a consultation today, and let’s design a plan that supports both your training and your health.

Marine Lenehan, sports nutritionist