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The Science Behind Intermittent Fasting for Athletes

Man doing pull ups across monkey bars as part of his intermittent fasting workout


When you’re logging a lot of hours on the road or in the gym, intermittent fasting for athletes can seem a bit counter-intuitive. You are burning a ton of calories every week, and you need to maintain your energy levels with nutrient-dense foods. Should you really be fasting?

Taking a day to “fast” may be the last thing you want to do, but a lot of athletes are finding benefits from incorporating “eating windows” and “fasted workouts” into their regular routines. 

While there are many potential benefits of intermittent fasting for athletes, it isn’t as simple as it sounds. High-intensity athletes require highly focused nutrition, and if intermittent fasting isn’t done correctly, it can decrease the calories an already depleted athlete is eating. 

For this reason, it is recommended that you do adequate research and planning before implementing a fasting strategy. If you are experienced in your nutritional habits, you may be more ready than someone who is still learning their caloric needs and refining their dietary methods. 

Basic intermittent fasting strategies may be most useful to athletes with lower caloric needs, while competitive athletes might need to take a more complex and strategic approach to intermittent fasting for athletes.

Before we dive into all the benefits of fasting, let’s make sure you understand exactly what intermittent fasting for athletes is and how an intermittent fasting workout works.  

What is intermittent fasting for athletes?

The basic idea behind intermittent fasting for athletes is that you choose a specific window during a 24-hour period where you eat all your calories. This “eating window” can be whatever length of time you determine is most effective for you. 

The standard eating window for intermittent fasting for athletes is 8 hours, with 16 hours of fasting. You can adjust this according to your unique goals and needs. 

How does intermittent fasting for athletes work?

Intermittent fasting for athletes has a couple of different effects on the body. 

First, fasting gives the body the time and energy it needs to properly perform autophagy. Autophagy, or “self-eating”, is the body’s housekeeping process for eliminating damaged cells, bad proteins, and unwelcomed pathogens. 

Through autophagy, the body is also able to recycle old materials into energy, as well as building blocks for new cells. This physiological process is key to the body adapting to stress and promoting longevity. 

Digestion takes a significant amount of time and energy for your body to perform. If you are eating all day, this can interrupt or halt the body’s attempts at “housekeeping”. It’s like you’re using the kitchen all day without giving your maid adequate time to clean in between meals. 

Intermittent fasting for athletes gives your body plenty of time to focus on eliminating and recycling waste so that your workout efforts are maximized. 

Second, intermittent fasting can effectively eliminate impulsive eating and excessive unintended caloric intake. 

If you limit yourself to an 8-hour window for eating, you will be less likely to mindlessly snack in the late hours of the night. 

Our intake of calories as a species has gradually increased over the past several decades, while our movement has steadily decreased. Some research has concluded that eating fewer calories improves longevity more than consuming more calories than needed. 

This can be an argumentative point for high-intensity athletes, however, who by nature of their lifestyle demand more calories than normal. These athletes may choose to eat selectively during their fasting period or incorporate some supplements, versus eliminating intake entirely. 

Can intermittent fasting negatively impact performance?

There seems to be a decline in performance in athletes who attempt intermittent fasting workouts without ensuring an adequate intake of calories and micronutrients. 

The quality of sleep also impacts the effectiveness of fasting. A recent review suggested that as long as athletes maintain good nutrient intake and sleep patterns their performance should not decline with fasting

After studying the research, one can conclude that intermittent fasting for athletes can have several benefits when done correctly. The purpose of intermittent fasting is not to deprive the body of calories, but to consume them within an 8-hour time window. 

Benefits of intermittent fasting for athletes 

Now that you understand what intermittent fasting for athletes is, let’s discuss what some of the benefits of it are. Here are the reasons many athletes choose to undertake intermittent fasting workouts: 

Fasting may improve focus

Have you ever felt like it’s hard to think clearly after eating a large meal? Does it seem like your energy goes straight to your gut and out of your head? There is some research to support the idea that fasting can improve cognitive function. 

In a clinical trial done with mice, researchers found that withholding water and food for a short two-hour block every day caused a significant reduction of plasma inflammatory factors. This suggests that fasting could improve cognitive function and protect the brain against distress.  

Fasting may reduce body fat

Intermittent fasting, combined with high-intensity interval training, is an effective way to improve body composition. This is partially because fasting can increase metabolic flexibility, or your body’s ability to switch between carbs and fat for energy. 

Research suggests that intermittent “energy restriction” improves weight loss efficiency and helps to combat the body’s negative metabolic reaction to calorie restriction. 

One study found that regardless of the exercise level of an individual, intermittent fasting can be effective in weight gain prevention. Fasting also generally leads to a reduced caloric intake, which may further explain its weight loss benefits. 

Fasting may increase energy production

Endurance athletes require more energy than the average individual. The body contains billions of energy-producing machines inside most functioning cells called mitochondria. 

When endurance athletes train and energy stores get low, a signaling factor called AMPK is released. AMPK can stimulate many functions, including mitochondrial biogenesis (mitochondria production). 

Because the body cannot rely on recent nutrient intake for energy, it must become more efficient at creating energy with what it already has. However, fasted workouts can have an impact on overall energy and performance capacity, and are best used as a short-term strategy.

Fasting may boost growth hormones 

Fasting after a workout seems to help increase growth hormone release. Growth hormones are responsible for wound healing, muscle growth, post workout recovery, muscle preservation, fat adaption, and cellular repair. 

Children have much higher levels of this hormone than adults, for obvious reasons. This is one reason why kids heal from injuries so rapidly and can seem to grow several inches overnight. 

Adults still have growth hormones and can increase their amounts through controlled stress such as exercise and intermittent fasting. 

You can see now that there are many potential benefits of intermittent fasting that athletes can enjoy. But is there a dark side to fasting? Let’s take a look at the potential side effects. 

Side effects of intermittent fasting

A lot of research still needs to be done about the effects of intermittent fasting, within the athletics sphere and otherwise. 

If you know anything about nutrition for runners, you’ll understand that fasting while leading up to a big race may not be the best idea for your body. High-intensity or high-endurance athletes will have to think carefully about the timing and duration of their fasting schedules. 

Fasting before high-intensity exercise, such as sprints and resistance training, may result in reduced performance.

You also want to make sure that fasting doesn’t limit your protein intake, since a lack of protein can cause you to burn muscle for calories instead of fat. 

If you want to try intermittent fasting, you should speak with a sports nutritionist to ensure you are meeting your nutritional needs and that you are getting enough protein and carbs in your diet. 

The bottom line on intermittent fasting for athletes

It would seem fasting has numerous benefits, but should be used with caution amongst those with high caloric needs. As long as one maintains adequate nutrition day to day, fasting shouldn’t negatively impact performance, and may actually enhance it. 

Those training for major athletic events should consider supplementing with liquid protein shots during their fasting period, which is a quick way to get the protein you need without the additives and extras you don’t.

FAQs about intermittent fasting for athletes 

Here are some common questions about fasting for athletes: 

How can I get enough protein while fasting?

One conundrum athletes face with consistent fasting is getting adequate protein. 

Most nutritional experts agree that protein needs to be consumed throughout the day in smaller amounts, versus a large amount of protein all at once. This is due to your protein absorption rate, or the amount of protein your body can absorb and utilize in one sitting. 

Your protein absorption rate may be as low as 8-10 grams per hour. 

Intermittent fasting limits one’s ability to spread protein intake throughout the day. This may not be a problem if your protein needs fit effectively within an 8-hour eating window. But if you have extremely high protein needs (say, you’re training for a triathlon and spend 3+ hours/day training) you may need to supplement with protein even during a fasting period. 

If you are looking for easy ways to get more protein while fasting, a single-serving liquid protein shot is the perfect option. You can toss it in your gym bag, or put it in your pocket, and have it ready and available for you during your eating window. 

What are the best supplements to take while fasting?

Taking certain supplements during a fast can be helpful, especially if you will be engaging in exhaustive training. 

While some vitamins, minerals, and prescription drugs need to be consumed alongside food (please consult your physician if you have questions), there are a few things you should consider incorporating into your fast based on your metabolic needs and training intensity. 


As mentioned above, protein intake is an important consideration during a fast. If you have a high protein demand, it would be wise to use high-quality protein supplements. Our best recommendation is collagen. 

There are many potential liquid collagen benefits, including improved muscle mass, injury prevention, faster healing times, less joint pain, and more! 


Don’t worry- you can still use caffeine during an intermittent fast. Black coffee or unsweetened green tea are great sources of caffeine that can provide an energy boost for your workout without many calories or unnecessary ingredients. 

You might also consider combining your pre-workout protein and caffeine with a supplement like Frog Fuel Energized.  

What is the 5:2 fasting for athletes?

5:2 fasting is simple. Eat what you normally eat for 5 days a week. On 2 non-consecutive days, eat two meals, twelve hours apart, for a total of 800 calories a day. Just make sure your fasting days are non-workout days.

Do any pro athletes fast?

NFL star Ndamukong Suh attributes his lack of injury during his career to intermittent fasting. His daily routine includes intermittent fasting, kombucha, and a high-protein diet.