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You need food to fuel your workouts, but when you’re fasting, you can’t eat. That makes fasting and exercise incompatible, right? “Not if you approach it right,” says Angelo Poli, ISSA, co-owner of Whole Body Fitness in Chico, CA, creator of the MetPro diet and exercise app.
Intermittent fasting (IF) involves organizing your eating around the time of day, eating normally for certain hours of the day (or days of the week), and fasting for others. To maximize fat loss and muscle gain while following a diet like this, you’ll need to schedule your exercise sessions strategically around those fueled or fasted windows.
How to Work Out When You’re Intermittent Fasting
Don’t try to build muscle while fasting.
Whether it’s a 5/2 protocol (eat for five days, fast for two) a 16/8 (fast for 16 hours, eat for eight), or any other version of IF, most people on a fasting diet wind up losing weight. That’s because it’s much harder to overeat if all your daily calories are crammed into an eight-hour window than if you can spread them out over 15-16 hours. That’s what makes IF such an effective weight-loss tool: by restricting the time frame in which you can eat, you effectively restrict the number of calories you take in as well.
But if your main goal is maximizing muscle, fasting isn’t a great idea. “Unless you’re a real novice, you can’t build appreciable muscle in a caloric deficit,” says Poli. A pound here and there? Maybe. But you won’t build anywhere near as much as you would if you consume a few hundred extra calories above and beyond what your body needs each day. So don’t try. Your primary goal while fasting should losing fat.
But you should still lift.
Even if your main goal is losing fat, you still need to lift, which prevents your body from burning through muscle to fuel your daily activities. You won’t gain much muscle if you’re fasting, but if you lift, you won’t lose it, either. “The same activities that build muscle when you’re fueled help preserve it when you’re in a caloric deficit,” says Poli.
Since you’re only trying to maintain the muscle you have—not pack on additional beef—you can get away with a fairly infrequent lifting schedule—2 to 3 times per week, exercising your whole body each workout (try this routine).
And you should eat before you lift.
Lifting weights, sprinting, doing CrossFit WODS, and other high-intensity activities all depend on carbohydrates for fuel, explains Poli. If you perform any of these activities during (or worse, at the end of) your fast, your performance will suffer. Instead of getting stronger and faster, you may well get weaker and slower.
What to do? “If you’re a big guy with a lot of weight to lose, no big deal,” says Poli. “Go ahead and lift on an empty stomach. You might lose a little bit of muscle, but you’ll burn fat, too—and that’s your main goal.”
But if you’re a slimmer guy with less muscle mass to spare, Poli recommends you schedule your lifting workouts during your feeding window. So if you eat from noon to 8 PM each day, try to hit the gym around 5, then go home and eat a high-protein meal to ensure adequate recovery from your workout. Also acceptable: lift weights after your feeding window (9 or 9:30 PM in this example). The protein you’ve ingested prior to your workout will support muscle repair after it.
Do your cardio fasted.
Many bodybuilders and other physique athletes swear by “fasted cardio”—jumping on a treadmill or bike for 30 minutes or more before breakfast—as a muscle-chiseling tool. Research is equivocal on whether this practice burns more fat than hitting the pavement after a meal or two. But Poli says it can’t hurt. “As long as you keep that cardio session low-intensity, you may well burn more fat in a fasted state,” he says.
Regardless, it’s less essential that you fuel up with carbs when you do lower-intensity work than it is when you lift or perform other high-intensity activities. Reason? “Slow cardio and other low-intensity activities run primarily on fat,” says Poli. “Even very lean athletes have plenty of fat on their bodies to power them through a long workout,” he says (think of lean ultra-runners who race for hours at a time without a bite to eat.)
The Bottom Line on Intermittent Fasting and Working Out
While fasted lifting is a big fat mistake, fasted cardio is fine, and may help you burn additional fat. So for best results, schedule those lifting sessions during or after your feeding windows, and schedule cardio before them.