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In the world of nutrition, there is always a new, “hot” idea that people believe will solve all of their weight, athletic, or disease issues. The ideas are often recycled from an old diet or just a new spin on an old way of thinking. By now, most of you have heard about intermittent fasting and the promise that it will make you leaner, a better athlete, and live a longer life.
The question is, can this method of eating benefit athletes? What about endurance athletes who spend hours at a time swimming, biking, running, etc? In this article, I’ll highlight what athletes need to know about intermittent fasting, including the pros and cons of doing so.
First of all, what is intermittent fasting?
According to Healthline, there are three main types of intermittent fasting.
The 16:8 method is when you fast for 16 consecutive hours daily and allow yourself to eat during the other 8 hours. Many people take advantage of their sleeping hours to be part of this and then skip breakfast. These hours may vary as some participants use a 20:4 or 12:12 method.
The eat-stop-eat method is when you do not eat for an entire 24 hours, once or twice a week on non-consecutive days.
The 5:2 method is when you drastically restrict calories to about 500-600 calories on two non-consecutive days during the week.1
What are the benefits of intermittent fasting?
The majority of people who participate in intermittent fasting are hoping for weight loss and studies support the belief that intermittent fasting will produce that goal.2 Well, I am not surprised. Any type of calorie restriction will certainly lead to weight loss.
What confuses me about this diet is that you are supposed to eat “freely” during your eating hours, yet supporters of this diet warn not to make up for the “lost calories” during the fasting period.
Be aware that just about any calorie restrictive diet will show promising weight loss results. The question is, will it show long-term benefits?
One study took a look at the long-term question and followed 100 metabolically healthy obese adults for one year. Adults were assigned to one of three groups, the 5:2 method, daily calorie restriction, or no intervention.
Both intervention groups decreased their mean caloric intake by 25% of their daily baseline needs. That meant that the alternate-day fasting group was instructed to consume only 25% of their baseline needs on fasting days and 125% of needs on feast days. There was no significant difference between the 5:2 method and daily calorie restriction as far as weight loss or cardio-protection.3
Perhaps one of the most intriguing parts of that study was that the drop-out rate was highest in the 5:2 intermittent fasting group. If you can’t stick to it, is it even worth it? Also, this was the only study I found that compared a calorie-restrictive diet with intermittent fasting.
Other benefits shown in animal studies
Prevention of cancer, improved brain health, and increased life-span have all been shown in animal studies. Before you jump on the bandwagon with this diet, remember that most of these results have not been shown in humans and the studies were short-term.4,5
Inflammation and immunity
Several studies that examined Muslims during Ramadan concluded that inflammation was reduced and immunity improved. While that is promising, they did not compare results to other groups of people who calorie-restricted or experienced weight loss in a different manner. Subjects were simply those who fasted and those who did not.6,7
Before we look at how athletic performance is impacted by intermittent fasting, let’s go over a few basics about sports nutrition.
Which macronutrients does your body need for athletic performance?
During short bursts of activity, such as in resistance training, a few different pathways are used to create energy. The body is heavily reliant on carbohydrates. High-intensity training also heavily relies on the availability of carbohydrates.
This reliance on carbohydrates is due to the fact that the body can quickly break them down to create energy without the presence of oxygen. Your body can use carbohydrates in the form of glucose in the bloodstream, glycogen stored in the muscles, or glycogen stored in the liver.
Endurance training is where it becomes tricky. The body uses many different pathways to create energy. The longer the activity, the greater the amount of fat being broken down for energy and fewer carbohydrates are being used.
Why can fat be used more with endurance training?
Simply, there is more time to break fat down because the activity is being sustained over a longer period.
Some athletes believe that training in a fasting state will allow the body to rely more on fat for energy rather than stored carbohydrates. It is certainly plausible that relying more on fat stores could be beneficial in many ways. Using fat for fuel would obviously decrease body fat, which may be a desire. Also, fat stores essentially could provide energy for you to run for days and days.
So, yes, using fat for fuel is a great thing. However, let’s not forget that the brain’s fuel source is glucose, a form of carbohydrate. Also, if you have completely depleted your glycogen (carbohydrate) stores, it will take longer to create energy from fat. Your body may also begin to break down lean protein, which also creates energy more slowly.
Essentially, when the glycogen stores are gone, your body will not be able to create energy as quickly for your muscles or brain. This leads to fatigue. Fatigue is definitely not what you are looking for when running a marathon or competing in an IronMan.
This need for fast fuel is why endurance athletes drink sports drinks, slurp gooey concoctions of carbohydrates and electrolytes, and chew on sports beans. Our bodies need fuel to perform well.
Let’s take a closer look at what the research says
A review of research by two doctors examined the effects of intermittent fasting on high-intensity exercise, endurance exercise, and resistance training by combing through a number of studies.8
The article first looked at the effects of intermittent fasting on high-intensity exercise. Most of these studies, again, were comprised of Muslim subjects who were fasting for Ramadan.
Most studies showed a small decrease in performance while some showed a significant decrease in 60-minute sprint times. The significant decrease was found even when they controlled for nutrition, sleep, and training load.
Other studies completed outside of the observations of Ramadan were mostly of the cycling variety. It was found that peak power and time to exhaustion were impacted negatively. They also concluded that 4-10 days of adjustment to intermittent fasting were needed to be sure that the negative effects were no longer present.
Two groups of researchers studied rowers using the 16:8 method. One found that the intermittent fasting group was negatively impacted in their rowing. The other concluded that this type of intermittent fasting may be used for rowers; however, their evening rowing times were still impacted negatively.
Back in 1993, a researcher did a review of studies that examined fasting from 24 hours to 4 days and its impact on endurance exercise. He found negative effects in all but one study.
Since that time, the majority of studies have focused on the 16:8 method where athletes are typically just skipping breakfast. These studies really showed a lot of mixed data. Some athletes were able to perform just as well in the fasting state and others weren’t.
One thing you should note is that none of these athletes performed better due to intermittent fasting. So maybe it won’t hurt your time, but it certainly will NOT help you.
Studies that examined the effects of intermittent fasting on resistance training were mostly focused on the 16:8 method or other similar variations with slightly different hours of fasting versus eating. Some studies found a decrease in fat mass, but all of the studies found no change in muscle mass or strength. The results were consistent as long as participants did not alter daily caloric intake and sleep patterns.
Benefits vs costs of intermittent fasting for your body
As we discussed before intermittent fasting does show some promise for weight loss. Some athletes could benefit from weight loss, but the question remains if this is the proper method. When there is a possibility of your athletic performance being harmed, my answer is a simple “no”.
Athletes involved in resistance training may not experience negative effects in their sport; however, that doesn’t mean this is the right method for them either.
I am not convinced that weight loss occurs for any other reason than pure calorie restriction. Calorie restriction can be achieved in many other ways that would not negatively impact athletic performance. Increasing portions of high nutrient-dense foods, such as vegetables, fruits, and whole grains; and decreasing portions of high caloric foods is a positive way to achieve weight loss.
Many research studies on intermittent fasting involve animals, are short-term, and do not include many women. Not only is it difficult to make definite conclusions about the effectiveness of intermittent fasting, but after 16 years of being a registered dietitian, I can attest to the fact that restrictive diets are difficult for people to follow. Diets that remove entire food groups also fall into the category of being difficult to follow.
It is my professional opinion that this type of dieting only feeds our disordered relationship with food as a society. Avoiding hunger signals and adhering to strict guidelines of when you should eat do not promote a positive, long-term result.
Athletes tend to be at a higher risk for disordered eating, which is another reason to avoid this method. The focus should be on fueling their athletic needs appropriately. If weight loss would be beneficial for an athlete, working with a sports dietitian would be appropriate and extremely helpful.
Remember that the research showed that intermittent fasting may or may not hurt your performance, but it will not help you. There truly is no magical and perfect solution to nutrition. There is also no one-size-fits-all approach to perfecting your nutrition.
Learning to eat in a way that supports your activity and health is a process that should evolve over time. Patience, while difficult, is key to changing habits and living better.
A sports dietitian can help you determine your individual needs and work on slow positive changes to achieve your goals. Habits and lifestyles weren’t formed in a day and can’t be changed in one day. Incorporating more nutrient-dense foods that support your health and activity over time will be more beneficial than stretches of totally avoiding food.
We can’t just look at a diet on paper and decide if it works or not. We have to consider our lifestyles happiness, ability to adhere to changes, and mental health.
What has your experience been like with intermittent fasting? Share your thoughts in the comments below!
Author bio: Erin Kesterson is a registered dietitian with a bachelor of science degree in food and nutrition sciences and a bachelor of music in bassoon performance. In addition, she obtained a master of science degree in exercise science. She is also a certified running coach with the RRCA. Erin loves working with others to help them achieve their goals in nutrition and fitness, which is why she founded Fuel Your Sport, LLC. She is a seven-time marathon runner with four Boston Qualifying times. In 2019, she completed the Boston Marathon. Erin and her husband are the proud parents of five thriving kiddos.
- Gunnars, Kris. “Intermittent Fasting 101 – The Ultimate Beginner’s Guide.” Healthline, 25 July 2018, https://www.healthline.com/nutrition/intermittent-fasting-guide#methods.
- Barnosky, Adrienne R, et al. “Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings.” Translational Research, vol. 164, no. 4, Oct. 2014, pp. 302-311.
- Trepanowski, John F. et al. “Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial.” JAMA Internal Medicine, vol 177, no. 7, July 2017, pp. 930-938. https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2623528
- Mattson, Mark P. “Energy Intake, Meal Frequency, and Health: A Neurobiological Perspective.” Annual Review of Nutrition, vol. 25, no. 1, 2005, pp. 237-260.
- Longo, Valter D, and Mark P Mattson. “Fasting: molecular mechanisms and clinical applications.” Cell metabolism, vol. 19, no. 2, 2014, 181-92. doi:10.1016/j.cmet.2013.12.008
- Johnson, James B et al. “Alternate day calorie restriction improves clinical findings and reduces markers of oxidative stress and inflammation in overweight adults with moderate asthma.” Free radical biology & medicine, vol. 42, no. 5, 2007, pp. 665-74. doi:10.1016/j.freeradbiomed.2006.12.005
- Aksungar F, et al. “Interleukin-6, C-Reactive Protein and Biochemical Parameters during Prolonged Intermittent Fasting.” Annals of Nutrition and Metabolism, vol. 51, 2007, pp. 88-95. doi: 10.1159/000100954
- Levy, E, et al. “Intermittent Fasting and It’s Effects on Athletic Performance.” Current Sports Medicine Reports, vol. 18, no. 7, July 2019, pp. 266-269. doi: 10.1249/JSR.0000000000000614