Fasting for five days with juice will just leave you cranky and hungry. But fasting for 16-24 hours throughout the week? That’ll leave you energized and leaner.
Intermittent fasting, as this diet method is known, is just as effective as daily calorie restriction for improving body composition, according to a 2017 study analysis from the International Society of Sports Nutrition.
Unlike calorie restriction, though, intermittent fasting doesn’t require you to count calories all the time—and it’s better at preventing hunger.
Furthermore, intermittent fasting is especially good at helping you lose fat while maintaining muscle, rather than losing both (as you would with traditional calorie restriction), says Krista Varady, Ph.D., an associate professor of kinesiology and nutrition at the University of Illinois at Chicago and key researcher on intermittent fasting. According to a study analysis in Nutrition Reviews, intermittent fasting also helps improve total cholesterol and triglyceride levels in both overweight and normal-weight people. The best part: There’s virtually no downside. (You can learn all about the basics of intermittent fasting here.)
There are three types of intermittent fasting: whole-day fasting, alternate-day fasting, and time-restricted feeding.
You’ll see the same benefits from each—that is, losing fat without losing muscle—but, since each type incorporates a different fasting period and fasting frequency, some are better-suited to certain guys and fitness lifestyles.
One note: Though you can use intermittent fasting as a short-term diet, intermittent fasting is better described as a lifestyle—like being on the Paleo or keto diet, says Grant Tinsley, Ph.D., C.S.C.S., assistant professor of exercise physiology and intermittent fasting researcher at Texas Tech University. And, just like those, intermittent fasting won’t work for everyone.
“If you’re the type of guy who wakes up ravenous or who needs to eat every three hours, this may not be for you,” Tinsley says. “You don’t want to do something that makes it harder for you to adhere to eating healthy.” But if you’re over counting calories and tired of thinking so much about counting your macros, intermittent fasting may be a good way for you to reach the same goal with a different kind of effort.
Interested? Here are the three types of intermittent fasting, and whether each is right for you.
Whole-day fasting (5:2 diet)
How it works: For one to two nonconsecutive days per week, you consume just water plus 500 calories, (200 of which are protein), either in one meal or spread out over the day. The other five or six days a week, you can eat whatever you want, whenever you want (though we obviously encourage that to be mainly fruits, vegetables, lean protein, and whole grains).
Pros: There’s evidence that longer fasts of 24 hours or so are the most beneficial for boosting your metabolic rate and fat burn, Tinsley says. Since your fast days aren’t locked into a rigid schedule, you also have more control over pairing heavy lifting days with regular-eating and rest days with fasts.
Cons: Whole-day fasting is difficult to stick with, and you’ll probably be more irritable, because you’re going a full 24 hours without food.
Who it’s right (and not right) for: “For some people, it’s easier to wrap their head around only having to be strict one or two days a week and then not having to worry about what they eat the other five to six days,” Tinsley says. If you love your cheat days, don’t want to give up your nightly glass of wine, or have a more erratic schedule that sometimes calls for late dinners, this one’s for you. If you thrive on predictability, not so much.
How it works: For the first 24 hours, you consume just water plus 500 calories, (200 of which are protein), either in one meal or spread out over the day. For the second 24 hours, you can eat whatever you want, whenever you want. Repeat the cycle every two days.
Pros: Every other day, you can eat whatever you want and still lose weight. A study of Varady’s published in JAMA Internal Medicine found that people who didn’t track their food but tried alternate-day fasting for one year lost 6% of their bodyweight—roughly the same amount as people who restricted their calories for the same period of time.
Cons: Back-to-back lifting days or HIIT sessions will probably suffer thanks to less fuel, particularly if you hit the gym at night.
Who it’s right (and not right) for: If you like routine but want the freedom to indulge regularly, one day on, one day off might be a good fit for you.
How it works: Every day you fast for 16-20 hours, drinking nothing but water or zero-calorie tea. Then you eat whatever you want, all within a four- to eight-hour window, like 10 a.m. to 6 p.m. or 12 p.m. to 8 p.m. (most people opt for an eight-hour feed option).
Pros: Since you’re eating at roughly the same time every day, adherence is probably a little easier, says Tinsley. And if you’re a hungry human, this one will keep you happier than the 24-hour fast options. Plus, you still lose weight without ever having to count calories—in a 2016 study in the Journal of Translational Medicine, fit guys who consumed all their calories in an eight-hour window, fasting for the other 16, lost more fat while maintaining muscle over eight weeks compared to guys who ate normally. And a forthcoming study from Varady’s team found that people who ate within an eight-hour window every day for 12 weeks not only lost weight but also naturally decreased their calorie intake by about 300 calories a day.
Cons: You’ll probably have to give up your nightcap, unless you can hold out until 1 or 2 p.m. for your first meal of the day.
Who it’s right (and not right) for: If you like structure, routine, and predictability, you’ll fare great here. Same if you hate tracking calories and love snacking, Varady says. But if compromise isn’t your strong suit, this probably isn’t your eating plan—you’ll have to choose between early breakfasts with her and late dinners with the boys. Also, if your schedule requires you to hit the gym pre-sunrise, that means you’ll probably start your feeding window earlier, which can make post-work socializing a lot harder.