How To Start Intermittent Fasting as an Athlete Without Losing Energy

High-intensity male athlete moves through the outdoor monkey bars as part of his intermittent fasting workout

If you’re an athlete logging a lot of hours on the road or in the gym, fasting can seem a bit counter-intuitive. You are burning a ton of calories every week, and you need to maintain your energy levels by consuming plenty of nutrient-dense foods. 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 some suggested benefits of intermittent fasting, the methodology is not as simple as it sounds. High-intensity athletes require highly focused nutrition, and if intermittent fasting is not done correctly, it can decrease the number of 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. 

What Is Intermittent Fasting For Athletes?

So, what exactly is intermittent fasting for athletes anyway? Is it something that can increase your performance capacity? This may depend on the type of athlete you are, as well as your specific goals and routines. 

The basic idea behind intermittent fasting 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 is 8 hours, and 16 hours of fasting. But you can adjust this according to your unique goals and needs. 

Athletes may choose to use intermittent fasting (IF) for several reasons:

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 IF 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 “remarkable reduction of plasma inflammatory factors”, suggesting that IF could improve cognitive function and “preserve the brain against distress.” 

Reduce Body Fat

Studies have found that IF combined with high-intensity interval training is an effective way for improving 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 IF can be effective in weight gain prevention. IF also generally leads to a reduced caloric intake, which may further explain its fat/weight loss benefits. 

Increase the Production of Energy-Producing Organelles

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). 

Studies have suggested that “nutritional manipulation” or fasted workouts increase the energy production efforts by the body since glycogen availability is reduced. In layman’s terms, 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 rather than a consistent routine.  

Increase Growth Hormone 

Fasting after a workout seems to help increase growth hormone release. Growth hormones are responsible for wound healing, muscle growth, 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 fasting. 

How Does Intermittent Fasting For Athletes Work?

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

First, it 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 promotes longevity. 

As an athlete, you are constantly putting your body under stress through intense workouts and training sessions. For your body to effectively recover and adapt, it needs adequate time for the autophagy process. 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 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 animals who eat fewer calories (while still eating enough for good function) live longer than those who consume 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 persons may choose to eat selectively during their fasting period or incorporate some supplements, versus eliminating any intake entirely. 

Can Intermittent Fasting Impact Performance?

There seems to be a decline in performance in athletes who practice fasting 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 as well as typical sleep patterns, they would likely not experience a decline in performance. 

After studying the research, one can conclude that intermittent fasting can have several benefits for athletes 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. 

What About Protein?

Another 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. Some studies suggest 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. 

This leads us to our next topic: supplements. For high-intensity athletes, using specific protein supplements during fasting periods may remove concerns about getting enough protein. 

Can I Take Supplements While I’m Fasting?

You might be wondering how supplements can fit into an intermittent fast without skewing the results. 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 (i.e. a liquid protein shot) with low or no carbohydrates in order to reach your daily recommended intake. This will also ensure you are maximizing your protein absorption rate. 


Don’t worry- you can still use caffeine during an intermittent fast. Black coffee or unsweetened green tea are a great source of natural 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.  

The Bottom Line On Intermittent Fasting For Athletes

Is intermittent fasting for athletes of all types?

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.

Best Sellers

Knowledge & Resource

The Science of FrogFuel

See what makes Frogfuel stand out
from the rest of the pack.
See the Science
Knowledge & Resource

Advanced Training Tips

Take your training to the next level
View More Blogs