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.