Intermittent fasting and time-restricted eating have become popular strategies to lose weight, boost insulin sensitivity, improve blood sugar and blood lipids, and encourage ketosis. But there is still a lot of confusion about how to fasti correctly, avoiding possible pitfalls while maximizing the benefits. One recurring point of confusion is working out while fasting—how much to exercise, whether to stop high-intensity training, and so on.
Before I get into the meat of this post, let’s make one thing clear: You SHOULD continue to work out while fasting. Stay active, don’t just sit around. Yes, even during longer fasts (with some caveats that I’ll discuss below).
What Not Exercising Does to Your Fitness During a Fast
It’s actually imperative that you exercise while fasting. Everything we do, or don’t do, sends a message. If you stay sedentary during a fast, you’re sending your body bad messages:
It tells your body you’re too weak to handle a fast. In previous incarnations of the human, fasting preceded food procurement. You didn’t stroll over to the fridge for a meal. You worked for it. You hunted and gathered for it. You exerted yourself to fill that empty belly. Simply lying around during a fast meant you’d die. Don’t tell yourself you’re too weak to handle the fast.
It tells your body you don’t need all that muscle tissue. If you don’t use your muscles during a fast, your body will consider them fair game. Those muscles can provide a big dose of amino acids that convert into glucose, and if you’re not using them, you’ll lose them to make glucose. “This bozo obviously doesn’t need those biceps, let’s dig in!”
Indeed, one of the reasons most people lose muscle when reducing calories and losing weight is that they fail to lift weights. Another big reason is they fail to eat enough protein, of course, but simply by lifting weights during calorie deficits, we can retain muscle mass. Since a fast is by definition an extreme reduction in calories, exercise becomes all the more important.
Working Out While Fasting: Best Practices
Okay, so exercising during a fast is a Good Move. How should you do it to maximize benefits and minimize downsides?
The condensed version is that for short fasts, you should exercise the way you normally do. By “short fasts,” I mean anything from the truncated eating windows of time restricted eating (generally 16 or 18 hours of fasting) to fasts lasting 24 to 36 hours.
For extended fasts, from 48 hours to a week or more, you should still exercise, but a little differently than normal.
Let’s get into it:
How to Exercise During a Short-Term Fast
If you’re doing short-term fasting, train as you normally would. Evidence suggests that not only will your performance not suffer, but the training effect may also be augmented if you time your eating right. Here’s what research tells us:
In one study, male participants practiced 20/4 time-restricted eating for eight weeks. The also trained four days per week, lifting to failure for 8 to 12 reps of four exercises for four sets. Even though they ate 650 fewer calories on average than a non-fasting comparison group, they retained all their muscle mass and even made gains on strength and muscle endurance. That said, the non-fasting group made more size gains.1
Another study in women found that resistance training on a time-restricted eating schedule (16/8) led to muscle gains as long as calorie intake and protein intake were maintained.2
A 2009 study found that, compared to athletes who lifted weights after breakfast, athletes who lifted weights in the morning in a fasted state had an augmented anabolic response to a post-workout protein-and-carb shake.3
Finally, in a small study of elite male cyclists, 16/8 fasting during a four-week training block did not impair their physical performance compared to a control group. In fact, the authors noted an improvement in power-to-weight ratio due to a decrease in body weight (a big deal for cyclists).4
The simplest form of exercise that everyone should do while fasting is walking. There’s no trick or science to walking while fasting. You just walk while not eating.
What are the benefits?
If you have trouble sticking to the fast—if you’re the type who wants to eat because you’re bored and can’t think of anything else to do—you need to walk as much as possible during a fast. It keeps you busy when you’re distracted by hunger, when fasting is becoming a chore.
You can do as much as you can squeeze in, because fasted walking isn’t just easy and not stressful—it’s anti-stress. Research indicates that walking while fasting is no more stressful than walking on eating days; in fact, fasting subjects spontaneously maintain their daily step count without affecting the benefits.5
In those who are already pretty lean but want to get very lean, fasted walking can be effective. This is the hardest body composition transition—from lean to very lean. Lean is what the body “wants,” and going lower requires getting over the natural tendency to hold on to diminished body fat stores. A fasted walk, light jog, or cycling session performed in the aerobic zone almost forces the body fat to release into circulation. Hence the classic bodybuilding trick for cutting body fat: wake up, consume no calories, and go for a brisk 20- to 30-minute walk.
2. Lift weights to preserve muscle.
On an extended fast, lifting weights is imperative. If you’re in the middle of a long-term fast and you want to stave off muscle loss:
Lift at a higher intensity for lower reps.
Don’t lift to failure. Keep several reps in the tank.
Don’t max out. On a scale of 1 to 10, with 1 being effortless and 10 taking all you have to complete, stay around a 6 or 8.
Stick to full-body compound movements rather than isolation. You want to hit the entire body with a powerful stimulus, to send your whole body a message, not just your biceps or your glutes.
This will provide a strong enough stimulus to maintain muscle mass without being so stressful that you start breaking down more muscle mass than you can maintain.
3. Do “easy” cardio.
If you’re going to do “cardio” or endurance training during an extended fast, keep the intensity low to stay in the “aerobic zone”—the heart rate zone where you’re burning primarily body fat. A fasting human should be able to remain active in that zone almost indefinitely without needing much, or any, food.
If you’re 20, your aerobic heart rate zone would be under 160 beats per minute. Don’t exceed 160 BPM when endurance training in a fasted state.
If you’re 50, your aerobic heart rate zone would be under 130 BPM. Don’t exceed 130 BPM when endurance training in a fasted state.
This will feel “easy,” and that’s the entire point. You’re not dipping into glycogen stores or increasing sugar cravings because you aren’t burning much sugar at all. You’re just (mostly) burning your own body fat.
4. Avoid really hard or long workouts.
I’d stay away from more intense endurance training, HIIT, and sprinting during a long fast unless it’s at the end, and you plan on breaking the fast shortly afterwards. Those kinds of activities will make it much harder to fast. While you can definitely lift weights midway through a five-day fast and come out the other end okay, you probably won’t do so well doing a dozen hill sprints midway through that same fast.
There are no hard-and-fast rules to exercising while fasting. After all, you’re still you. You know what works for you. You know what makes you tick. But if you want me to generalize? There’s no need to worry about changing up your workouts if you’re fasting for less than 48 hours. Most people on a longer fast will do best going for a walk every day and lifting at least once or twice.
What about you, folks? How do you exercise on a longer fast?
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.