Intermittent Fasting For Athletes: Benefits and Concerns

To the average person, the idea of elite athletes skipping meals sounds like pure madness. Athletes are fine-tuned, well-oiled machines. Machines need fuel. You don’t see race car drivers running on empty to “promote training adaptations” in their vehicles. No, high performance requires high energy reserves.

Athletes need to eat, and eat well. Right?

But humans aren’t machines. We’re biological. The car doesn’t respond to training stress, but we do. We adapt, grow, recover, and build new capabilities in response to the stress we endure. You expose yourself to a ton of stress, recover from that stress, and end up stronger/fitter/faster on net. That’s training. And sometimes, high stress is exactly what we need to progress—a few heavy sets of squats, some rounds on the Airdyne, a killer CrossFit workout—as long as you can recover from it. A major modulator of our stress is the amount of food we have coming in. At least in theory, exercising in a fasted state could provoke a powerful adaptive response that athletes would find helpful.

So, does it stack up? What exactly can intermittent fasting offer athletes?

Benefits Of Fasting For Athletes

Increases In Growth Hormone

Growth hormone helps spur, well, growth. It improves immune function. It builds muscle, bone, and cartilage. Kids are swimming in the stuff, and they heal like Wolverine. Older adults who inject it enjoy improved wound healing and workout recovery. That’s why it’s a banned substance in professional athletics, and it’s why natural ways to augment growth hormone secretion can be very helpful to athletes of all stripes.

Fasting increases growth hormone, most likely as a way to limit harmful tissue degeneration and preserve muscle; so does exercise. Once or twice a week, I like to fast after workouts to extend and expand the GH release. That’s a slightly more extreme version of post-workout carb abstention, but it’s the same idea: withholding food and forcing your body to adapt. This increases growth hormone (important for fat burning and cellular repair) and speeds up fat adaptation.

Improvement Of Metabolic Flexibility

In experienced male lifters (5-year history of 3-5 days/week training upper and lower body, drawn from advertisements placed in bodybuilding gyms), fasting for 16 hours a day and eating for 8 increased metabolic flexibility.

Metabolic flexibility is the ease with which a person is able to switch between sources of energy—from carbs to fat and back again. For the average person interested in health and longevity, maintaining metabolic flexibility is an important way to live a healthy life. For an athlete interested in performance, health, and longevity, metabolic flexibility is absolutely essential.

If you’re metabolically flexible, you can burn fat for longer before switching over to carbs. You can burn carbs when you actually need them, right away. And afterwards, you can switch back into passive fat-burning mode to keep unnecessary carb cravings and insulin low and improve recovery.

Reduction Of Inflammation

To attain the training effect, an athlete must incur a big blast of inflammation (from the exercise) and then recover from that inflammation. Blunting the initial inflammatory response with drugs and even mega-doses of vitamins will impair the training effect. You can also reduce the training effect by training too soon after a workout, thereby stacking inflammation.

You need the inflammation, but you also need the inflammation to subside. Both sides of the coin matter. What fasting does is improve your natural ability to dampen inflammation. You get the big inflammatory response of a tough workout.

This is where a fasted workout can really shine. When you’re fasted, you’re in a state of very low inflammation. And then you introduce the workout, and inflammation spikes. It’s a big response, a heightened response—and you must adapt to it. Oscillating between fasting, training, and feeding lets you hit those extremes, those margins where peak performance occurs.

Maintenance Of Energy Expenditure

There’s something revitalizing about going without food for a decent period of time and then feasting. You could spend the week restricting calories each day or use fasting to arrive at the same weekly caloric load and the effects will be different. Chronic calorie restriction enervates. Intermittent calorie restriction peppered with intermittent feasting energizes.

For an athlete, chronic calorie restriction spells doom. They need energy. They need to be able to expend energy when they need it. Luckily, studies show that intermittent fasting is one way to “reduce calories” without reducing energy expenditure. Perhaps the main reason is that IF doesn’t necessarily lower calories; it just changes when you get them. In the bodybuilder study, the athletes in both the fasting and the control groups ate about the same number of calories. But only the fasting group lost a lot of body fat, and they did this without suffering a drop in energy expenditure. Pretty cool stuff.

That said, you can overdo it. Too much fasting for too long will depress energy expenditure, as would happen with any kind of chronic calorie reduction. It’s just that fasting seems to stave off the drop in energy longer than other forms of “dieting,” especially if you maintain your calorie intake.

Concerns About Fasting For Athletes

May Reduce Testosterone

In the bodybuilder study, the group with the 8-hour eating window experienced a drop in testosterone. As T is essential for muscle protein synthesis, performance, strength, and general vitality, this could be problematic for athletes (particularly male ones). Despite the drop in testosterone, though, they still gained lean mass, lost fat, and got stronger—so it may not be practically relevant.

May Be Hard To Get Enough Calories To Gain Muscle or Recover

Athletes do need more fuel than the average person. A big draw of fasting for weight loss is that it makes it easier to reduce calories by erecting illusionary barriers that we nonetheless adhere to. If you only have an 8-hour eating window, you can’t eat outside of it. If you’re “fasting today,” you simply can’t eat. It makes things really simple for people who otherwise have trouble limiting food intake.

The flip-side is that it can make eating enough calories difficult, especially for athletes who do need more fuel than the average person. In a recent study, lifters who ate inside a 4-hour eating window had a 650 calorie daily deficit, lost a little bit of body fat but failed to gain any lean mass, while the control group—who ate more calories and protein—did gain lean mass. The fasting group simply wasn’t able to eat enough food or protein. Despite that, the 4-hour eating window group still gained upper and lower body strength, and they didn’t lose muscle mass. I suspect they could have gotten great results with a few hundred more calories of protein.

As is the case with every study that attempts to collate the individual experiences and results of hundreds of humans into “trends” and “averages,” there’s a wide variety of personal responses to fasting among athletes. The name of the game is experimentation—you have to see what works for you. This week I’ll give some specific recommendations for specific types of athletes, as well as my own experiences utilizing fasting in the pursuit of better physical performance.

For now, though, how has fasting worked for you and your athletic pursuits? Does it seem to help or hinder?


Moro, T., Tinsley, G., Bianco, A., Marcolin, G., Pacelli, Q.F., Battaglia, G., … & Paoli, A. (2016). Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males.J ournal of translational medicine, 14(1), 290.

Tinsley, G.M., Forsse, J.S., Butler, N.K., Paoli, A., Bane, A.A., La Bounty, P.M., … & Grandjean, P.W. (2017). Time-restricted feeding in young men performing resistance training: A randomized controlled trial. European journal of sport science, 17(2), 200-7.

TAGS:  calories

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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13 thoughts on “Intermittent Fasting For Athletes: Benefits and Concerns”

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  1. I know this post is for athletes, but I’m curious about the intersection with longevity. The ramped up metabolism idea has always seemed a little flawed to me because it has to mean a decrease in cell life-span. I wonder if there’re any guidelines for the optimal balance between fueling and fasting when trying to maximize performance and longevity.

    1. “it has to mean a decrease in cell life-span”.

      I believe this has been shown to not be true. I don’t remember the details but in Josh Mittledorf’s blog I remember him mentioning that “rate of living” (which is what I think you’re referring to) as a driver of aging has been dis-proven. I remember some studies that seemed to equate slower metabolisms with longer life, but as with many other things that is likely a u-shaped curve and most people will likely see a net benefit from an increase.

  2. I have tried this last year during my summer training for cross country and found some pros and cons to it. I did lose a lot of body fat and ended up seeing my times significantly drop all summer. However, towards the end of my summer training when I was hitting peak mileage (about 65 miles a week), I found myself very fatigued and having a difficult time recovering. I was worried that I was not eating enough to maintain my training/mileage so I backed off on intermittent fasting. I do want to try this again due to the benefits that I did see initially. What suggestions would you have for doing this for long distance athletes?

    1. There could be a few reasons for this. Are you keto-adapted or are you relying on carbohydrates for fuel? Overtraining may also factor into it.

  3. Hello!

    Wow this is exactly what I need it to hear. Can I asked to really help me out to explore more myself about everything what I am doing wrong? I’ve been doing fasting for years, I mostly train twice per day, loss a lot of hair, Lost my period, and now I see my skin without any healthy appeareance.
    I thought I was doing things perfect, but this article help me to open my eyes. Can somebody reach me and teach me how to really do Things pretty good? Because doctors are so wrong everytime I soeack with them and then run test myself. They really don’t get me!

    1. Training twice a day with fasting is not going to work. Not enough calories and not enough rest. Either eat all day long and keep the double workouts, or fast but drop to one workout a day.

  4. Ben Greenfield did interesting mass building phase while fasting recently. He used lot of essential amino acids that Dr Minkoff suggested to use (in podcast they made, lot of interesting and useful informations) because EAA has almost no negative effect on fasting (no insulin spike, almost no calories) except blunting autophagy. EAA basically helps with anything that body needs (preserving/building muscle, better immunity, gut, etc). It seems to be working for me, my regeneration is better (I always had problem with that on fasting and low carb diets). I also use carnitine for better energy and performance, glutamin (+ whey protein) post workout for better glucose replenishment because I’m not eating carbs. Colostrum for better anabolic respond and gut health and lastly digestive enzymes, betain hcl, bitters for better digestion if my eating window is small and I have bigger meal portion.

    1. That doesn’t sound correct to me. From what I remember the insulin spike from amino acids can be quite pronounced, now this doesn’t have to be a negative. Did he just break his fast with Amino Acids or did he consume EAA all the time?

      This is a problem I have with the topic of fasting in general, people have very conflicting definitions of what fasting is and so use very imprecise language. To me, fasting means taking in (almost) zero calories, but to many it’s more of an elimination diet kind of thing. Are there more precise terms?

  5. I’ve been doing 16 hour fasting for over 2 years. It has kept me lean at 57 when most others are gaining weight. I’m addicted to that benefit but I do worry that some of my joint pain could be not enough either recovery or maybe nutrition…amino acids. I do hard kettle bell workout 3 times a week for about an hour. Swings, squats, lunges, pull-ups, pushups etc. I keep thinking I should try stopping and see how I feel but as I said I’m addicted to the easy leanness that it delivers.

    1. It may not be the fasting affecting your joints but repetive movements that don’t alter the angles your joints experience. Changing grips, foot stance etc can do wonders for your joints, even while working the same prime mover muscles.

  6. Just curious if intermittent fasting would work different in females affecting our hormone cycles?