Is intermittent fasting for endurance athletes a helpful practice? Will it actually improve performance and aid in health? This post breaks everything you want to know down about running and intermittent fasting.

As an Amazon Associate, I may earn from qualifying purchases.

Intermittent fasting has been trending over the last few years. Its benefits have been proclaimed by healthcare and fitness professionals alike.

As a runner, you may be wondering if running and intermittent fasting, low carb running or keto running are synonymous to improved performance and weight loss. There’s quite a bit to consider with intermittent fasting and exercise timing. 

Maybe you’ve even witnessed some other intermittent fasting athletes. But, is this pattern of eating really what is best for training?

Typical sports nutrition protocols and recommendations recommend a certain amount of carbohydrates before exercise based on weight and time before exercise. 

For example, 1 gram per kg of body weight 1 hour before exercise, up to 4 grams per kg of body weight 4 hours before exercise (likely, in a marathon). See more of our top nutrition tips for marathon runners. 

Here are some more ideas for what to eat before a long run. It is very individualized and will depend on many factors. 

Let’s get in to what intermittent fasting is and what it may look like for the average runner or athlete. 

What Is Intermittent Fasting (IF)?

Intermittent fasting, commonly abbreviated as IF, is an eating pattern where a person abstains from eating for a certain period of time but is allowed to freely eat for the rest of the day.

There are a few different patterns or eating time periods (written as ratios), where people may limit their eating between 5-14 hour windows.

The goal of the diet is to mimic the metabolic effects of a true fasting period without actually engaging in a full-on fast. Some of the different IF eating styles are: 

• 16:8 – 16 hours of eating and 8 hours of fasting.
• 5:2 – Eat for five days, fast for two. Fasting days are usually up to the person to decide. 
• Alternate Day Fasting – People may eat on alternate days and fast on alternate days. 
• Overnight Fasting – Allow about 12 hours of fasting overnight. This is the easiest to start, if you eat dinner at 7pm and eat breakfast at 7am the following morning. 

Some people may be more or less strict around the fasting rules for intermittent fasting and running. For example, some may allow coffee, while others may not. For more information on how coffee and caffeine may impact running, check out this post about coffee vs. pre-workout drinks.  

Benefits Of Intermittent Fasting and Running

Let’s start with some of the positive findings from the research about intermittent fasting and running. 

• Increased Muscle Mass -Some research has seen increased muscle mass, or at least no effect on losing muscle strength. 
• Lowered Fat Mass – The main effect on the body that has been seen in several studies on intermittent fasting is lowered fat mass. Intermittent fasting combined with 8 weeks of resistance training may lead to lower fat mass without compromising muscular strength.
• Insulin and Biomarkers –  Intermittent fasting may improve insulin resistance, leading to lower blood glucose levels. Lipid profiles have also shown to be improved in some studies.

However, we don’t know if these benefits transfer to running, so intermittent fasting for athletes may not have the same benefits, given the higher energy needs runners have. 

Cons of Intermittent Fasting for Active Lifestyles

While some studies tout the benefits of intermittent fasting for improved body composition, some studies state that exercising while in a fasted state is no more helpful for fat loss than exercising in a fed state.

• In one study of male middle and long distance runners who underwent a 16:8 fast, the runners lost some weight, but they did not experience improved performance or an improvement in lab values such as glucose or cholesterol, suggesting that intermittent fasting for endurance athletes may not transfer any benefits.
• While intermittent fasting may have little effect or benefit, it may also have undesirable effects. Limiting eating to one specific time window does not allow for proper nutrient timing and may limit opportunities for adequate nutrient consumption. There could be challenges for intermittent fasting and marathon training, for example, if your post-run recovery period doesn’t fall within the allotted hours to eat.
• Additionally, intermittent fasting, like any diet, encourages black and white thinking and may interfere with the body’s natural hunger and fullness cues. 

Intermittent fasting can also be classified as a “fad diet” by some, especially if followed for a short time period in efforts to lose weight. We have an abundance of data about why fad diets don’t work and are not sustainable. 

They often lead to eating disorders and disordered eating, which can be very serious and will impact performance in a slew of ways. 

Impact of Running and Intermittent Fasting

Intermittent fasting may not necessarily be helpful for runners, especially if you’re trying to PR. If you’re going hours without eating, your body may try to play “catch up” when you are able to eat. This could lead to overeating, bingeing and feelings of discomfort. 

Most studies done on intermittent fasting in active people have been done on people observing Ramadan, the religious holiday in the Muslim faith during which adherents fast from sunrise to sunset. In one study, intermittent fasting was found to lead to lower aerobic capacity.

As discussed in the position paper of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine on Nutrition and Athletic Performance, eating before exercise, as opposed to exercising in the fasted state, has been shown to improve performance. 

Furthermore, we have to consider the internal systems that rely on glucose and fuel. A systematic review on studies involving military members who were fasting for Ramadan found that intermittent fasting was associated with hypohydration and electrolyte imbalances, sleep disruption, and energy deficits, all of which are known to negatively impact performance.

Fasting can impact hydration for runners, since we know carbohydrates can help improve hydration. 

Electrolyte imbalances or deficiencies can cause major problems. Magnesium, for example, is involved in over 300 reactions in the body. Read more about magnesium for runners here. 

Could Intermittent Fasting Help My Performance?

When you take a look at all of the research on intermittent fasting athletes and exercise while intermittent fasting, it appears to be more harmful than helpful for your running performance. 

While it may lead to some weight loss, weight is far from the most important factor in performance, and the side effects associated with that weight loss may be more monumental for the body to adjust to. 

Intermittent fasting is likely to lead to under-fueling, bonking in a marathon, and relative energy deficiency in sport, hypohydration, and an unhealthy mindset around food, all of which can lead to undesired performance and health outcomes. Fueling properly for training will lead to improved running more than following any diet.

If you want a sample long distance runners diet plan or even a vegan ultramarathon runner plan, we’ve got you covered!

A diet for athletes to lose weight needs to meet your basic energy needs without drastic calorie cutting, which can lead to nutrient deficiencies, dehydration, electrolyte imbalance and more. 

Reach out to one of our dietitians to individualize your runners diet plan!

References:

1. American College of Sports Medicine; American Dietetic Association; Dietitians of Canada. Joint Position Statement: nutrition and athletic performance. American College of Sports Medicine, American Dietetic Association, and Dietitians of Canada. Med Sci Sports Exerc. 2000 Dec;32(12):2130-45. doi: 10.1097/00005768-200012000-00025. Erratum in: Med Sci Sports Exerc 2001 Jan;33(1):following table of contents. PMID: 11128862.
2. Brady, A. J., Langton, H. M., Mulligan, M., & Egan, B. (2020). Effects of 8 weeks of 16:8 time-restricted eating in male middle- and long-distance runners. Medicine and Science in Sports and Exercise, 53(3), 633-642.
3. Correia, J. M., Santos, I., Pezarat-Correia, P., Minderico, C., & Mendonca, G. V. (2020). Effects of intermittent fasting on specific exercise performance outcomes: A systematic review including meta-analysis. Nutrients, 12, 1390-1412
4. Linderman, J. K., & O’Hara, R. B. (2020). Effects of intermittent fasting on performance in U.S. military personnel while operating OCONUS: A review. Journal of Exercise Physiology, 23(3), 1-12