Skipping meals to save calories is a sure-fire way to tank your metabolism and sabotage weight loss. Condense those food-free time periods to set intervals, however, and watch the fat melt away.
Thatâ€™s the principle behind the diet method known as intermittent fasting. Unlike many popular diets, intermittent fasting has the backing of scientific research (including a massive 2017 study analysis by the International Society of Sports Nutrition), which suggests that intermittent fasting is just as effective as daily calorie restrictionâ€”sometimes betterâ€”at improving body composition. Furthermore, intermittent fasting doesnâ€™t involve any calorie counting, and may suppress hunger better than traditional low-calorie diets.
What is intermittent fasting?
Intermittent fasting really just means a period of eating followed by a period of not eating, repeated over time, explains Krista Varady, Ph.D., an associate professor of kinesiology and nutrition at the University of Illinois at Chicago, who has studied the effects of intermittent fasting on human body composition.
There are three types of intermittent fasting: whole-day fasting (5-6 feed days, 1-2 nonconsecutive fast days, repeated every week), alternate-day fasting (1 feed day, 1 fast day, repeated), and time-restricted feeding (4-8 feed hours, 16-20 feed hours, repeated daily).
All three types offer the same fat-burning benefits of intermittent fasting. The real differences are just in how long youâ€™re fasting and what youâ€™re allowed to eat during the fast, Varady says. On both whole-day fasting and alternate-day fasting, for example, you consume 500 calories per fast day to spark fat loss but maintain muscle. During time-restricted feeding, however, youâ€™re consuming zero calories during your fast period (water and tea, for example, are allowed). In all types, youâ€™re free to go hog-wild during the feeding windowâ€”though, obviously, all nutritionists would recommend you ditch the junk and stick to healthy fare.
About that: People donâ€™t usually binge that much on their feed day. â€śMost people report they can only eat about 10% more coming off a fast than they normally wouldâ€”their body just wonâ€™t let them overeat,â€ť Varady says. Thatâ€™s why an increasing number of studies show that intermittent fasting may work just as well as limiting your calorie intake when it comes to weight loss.
Now, intermittent fasting isnâ€™t magicalâ€”your calories still count. Itâ€™s just an alternate way to reach that same calorie restriction, says Grant Tinsley, Ph.D., C.S.C.S., an intermittent fasting researcher and assistant professor of exercise physiology at Texas Tech University. â€śWith traditional calorie restriction, youâ€™re following a normal, healthy eating pattern but eating less at each meal. With intermittent fasting, youâ€™re eating roughly the same number of reduced calories, just in a confined time frame. Youâ€™re just giving yourself different parameters to live within.â€ť
Whatâ€™s the payoff of intermittent fasting?
The main benefit of intermittent fasting is weight lossâ€”fat loss, specifically. â€śInsulin increases when you eat, and when insulin is high, you cannot burn fat. When you fast, insulin falls, which allows your body to access its stores of food (i.e., body fat) for energy,â€ť explains Jason Fung, M.D., a Toronto-based nephrologist and author of The Complete Guide to Fasting.
Study after study confirms that any form of intermittent fasting will help reduce weight and improve body composition, though the results are probably about the same as youâ€™d see from restricting your calories. One study Varady ran, published in 2017 in JAMA Internal Medicine, found that after dieting for one year, people lost about 5â€“6% of their bodyweight whether they restricted their calories or tried alternate-day fasting.
Other studies suggest that intermittent fasting may help you lose more weight. One study in the Journal of Translational Medicine found that fit people who tried time-restricted feeding lost more fat and maintained more muscle than people who ate the same number of calories but over 12 hours instead of eight.
The science is mixed on which is better. What is confirmed, though, is that both methods work.
One advantage of intermittent fasting: helping you retain muscle mass. â€śWhen people lose weight, typically 75% is fat loss and 25% is muscle mass. But with fasting, the ratio actually changes so that 90% of weight loss is fat and 10% is muscle,â€ť Varady says.
And because you retain this muscle, your metabolism wonâ€™t drop the way it might with calorie-restricted weight loss. In fact, fasting actually boosts your metabolism, says Tinsley. â€śThereâ€™s a misconception that your metabolic rate will decrease if youâ€™re not eating. If youâ€™re fasting, your body views it as a mild stress, so research actually that shows your resting metabolism is actually higher after an overnight fast, and 16-24 hours is the window in which you see the largest increase in fat burn,â€ť he explains.
Plus, time-restricted feeding can help control cravings better than the normal, all-day graze, according to research out of the University of Alabama at Birmingham.
Additionally, alternate-day fasting and whole-day fasting have been shown to not only reduce body fat but also total cholesterol and triglycerides in both overweight and normal-weight people, according to a study analysis in Nutrition Reviews. Because intermittent fasting can stabilize insulin levelsâ€”if you donâ€™t eat, your blood sugar will come downâ€”thereâ€™s reason to believe it could be a good diet for people with Type 2 diabetes, Varady adds. Research also suggests that intermittent fasting may even help prevent Alzheimerâ€™s and cancer, adds Fung.
Intermittent fasting: Whatâ€™s the catch?
All three of our experts agree: There are no real downsides to intermittent fasting. â€śItâ€™s quite safe in terms of how it affects your health and biomarkers, and it doesnâ€™t lead to eating disorders,â€ť says Varady.
Fung concurs: â€śWe, as physicians, tell people to fast all the time. Prior to surgery, prior to colonoscopy, for ultrasounds, even for fasting blood work, people need to fast and nothing bad happens. Thereâ€™s no more risk with extended fasting.â€ť
The biggest drawback is irritabilityâ€”the first five fast days on alternate-day fasting and whole-day fasting are difficult for many people, Varady adds. â€ś[Irritability] subsides after the first week or so, though, after which people actually say they have a boost of energy on their fast day,â€ť she adds.
What if you sneak a few beers with the boys during a fast window? The only real risk of cheating on your time frames is the same as cheating on any dietâ€”you wonâ€™t lose weight as quickly as if you had stuck to it perfectly, Varady says.
Who should try it?
Intermittent fasting is safe for anyone to try. If you take medications, especially for diabetes, talk to your doctor first, Fung advises.
Itâ€™s important to remember intermittent fasting is an eating plan, not a diet. â€śPeople certainly use it as a short-term plan to lean up and thatâ€™s fine, but the majority of researchers prefer to think of it as a lifestyle youâ€™d continue indefinitely as an alternate feeding plan,â€ť Tinsley adds.
That means thereâ€™s certainly a type of person who wonâ€™t fare as well on it. â€śIf youâ€™re the type of person who wakes up ravenous, who loves breakfast, or who loves to snack, this may not be a good diet for you,â€ť says Tinsley. â€śYou wonâ€™t want to do something that makes it hard for you to adhere to. If this doesnâ€™t gel with something you enjoy, you donâ€™t need to fit yourself into this box just because intermittent fasting is popular right now.â€ť
Ultimately, if youâ€™re trying to lose fat or maintain a low body-fat percentage, the most important question is whether intermittent fasting is an easier day-to-day eating strategy than counting calories or focusing on macros, Tinsley says.
Ready to try intermittent fasting?
So: The first two weeks of intermittent fasting will suck. â€śLike anything else, fasting becomes easier the more you do it,â€ť Fung says. Youâ€™ll likely experience side effects (lack of concentration, irritability, headaches, maybe constipation), and you may feel hungry, but you shouldnâ€™t feel weak, he adds. Learn how to cope with these and know that if you can power through the first 7-14 days, youâ€™re golden.
If you canâ€™t decide which strategy is best for youâ€”whole-day fasting, alternate-day fasting, and time-restricted feedingâ€”try each for a month, Varady advises.
Just remember: If youâ€™re trying whole-day or alternate-day fasting, your goal is to eat 500 calories on your fast dayâ€”and thatâ€™s accounting for 50g of protein (200 caloriesâ€™ worth) to maintain muscle mass, she adds.
Again: Donâ€™t use your feed days as a free pass to the buffet. â€śDuring the feeding period, people should still stick to real, unprocessed foods and avoid sugar,â€ť Fung adds.
Wait, what about working out?
You can still work out on fast days, but stick to low-intensity work and try to get it in toward the start of your fast window rather than the end when your carb stores are totally depleted, Tinsley advises.
Also: Save your fast-day meal till after the workout, Varady adds. â€śIn our studies, people got really hungry after they exercised, so a lot would cheat if theyâ€™d already eaten their 500 calories for the day,â€ť she explains.
Save high-intensity interval training and heavy weight training for feed days/during your feed window, if you can, when you have the calories for fuel and nutrients for recovery. But if your training schedule calls for a grueling workout on a fast day that canâ€™t be moved, add the calories burned to your count for the day so you donâ€™t burn through muscle. At 200 or 300 calories, you donâ€™t need to compensate, but if you burn 500 or more on a fast day, instead of just 500 maintenance calories with 50g of protein, eat 1,000 calories with 80g or more of protein, Varady advises.