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The complete guide to fasting diets

Katherine Ellen Foley
Fast today, feast tomorrow.
  • Katherine Ellen Foley
By Katherine Ellen Foley

Health and science reporter

Published Last updated This article is more than 2 years old.

At first, skipping meals, may sound like just another celebrity fad diet. Indeed, Jimmy Kimmel swears by a version of it.

But non-celebrities have had great success with brief periods of fasting, also known as intermittent eating. Mark Mattson, a neuroscientist at the National Institute on Aging who regularly fasts, told the New York Times that he eats all of his food for the day in a six-hour window, but skips breakfast and lunch. “Once you get used to it, it’s not a big deal,” he said. “It’s just a matter of getting adapted to it.”

A growing body of evidence suggests that for many, changing up your eating schedule to include periodic fasting may be a suitable way to lose weight and keep it off.

How they work

Weight loss is a credit and a debit, said Lisa Ashe, a doctor of osteopathic medicine based in Arlington, Virginia. In a nutshell, if you want to lose weight, you need to make sure you use more calories than you consume in a given day.

One pound of weight (in the form of excess fat we store in our bodies) is equivalent to 3,500 calories. So, in order to shrink your fat cells by a pound in a week (assuming you’re not simultaneously putting on extra muscle), you can either burn 500 extra calories a day through exercise while eating the same amount, or you can cut 500 calories a day from your diet.

The challenge, of course, is that your body doesn’t want you to consume fewer calories. When your fat cells become smaller, they produce less of a hormone called leptin, which our bodies use to signal when we’re satiated. It’s therefore harder to feel full. Additionally, an old evolutionary reward system (paywall) wired into our brains gives us a chemical rush when we eat foods that are high in simple sugars that are easy to break down. These foods, which include simple carbohydrates like white bread and foods high in fat like some baked goods, can pack a lot of calories into relatively small portions, and you’re likely want to eat more. They may even be somewhat addictive (paywall), especially when we get used to eating large portions of them at once.

Physical challenges aside, trying to lose weight is difficult mentally. Most people who try calorie-restrictive diets stick to them for six to eight weeks before quitting, said Krista Varady, a nutritionist at the University of Illinois at Chicago. “They get sick of recording food every day and that feeling of daily deprivation.”

Varady and her team started exploring what would happen if, instead of cutting back a little every day, you cut back a lot at once, followed by a break from dieting. In 2009, they showed (paywall) in a small clinical trial that fasting for four weeks followed by four weeks of unrestricted eating helped obese patients lose weight and lower their blood pressure and cholesterol.

This eating regimen has other benefits too. A more recent paper found the same tactic worked to help patients trying to lose just a few pounds. And although the data isn’t published yet, Varady said that she and her team conducted a study on patients to see how they faired for six months using the approach; they all were able to lose weight, and keep the weight off.

It’s called the Every Other Day diet. Three or four days a week, participants consume 500 or 700 calories in a day instead of the usual 1,500 to 2,000. The next day, they eat whatever they want. ”Really giving people that break every other day helps,” she said.

Though it sounds like you’d be tempted to binge the following day after feeling ravenous, Varady and her team have found that that isn’t the case. In her book outlining the diet, she and co-author Bill Gottlieb wrote that following a diet of fasting every other day, most people only ate about 10% more calories than they normally would; in other words, they weren’t bingeing in a way that would undo any of the work they had put in the day before.

How to do it properly

There are many different kinds of fasting diets. Some allow you to eat only between a certain number of hours during the day, like 9am to 5pm. Others, like the 5-2 diet, allow you to eat whatever you want for five days, but then has you restrict for any two you choose. (Varady does not recommend this approach. She’s had clients report that they’ve ended up binging on eating days and gaining weight.) The benefit of having on and off days is that you can plan when to fast to meet your schedule (you may not want to fast on the weekend, for example).

On the days you restrict your eating, don’t forego food entirely; instead, plan on eating about 500 to 600 calories from foods packed with vitamins. ”You want to make sure you’re getting the right amount of nutrients,” said Ashe. This means sticking mostly to fruits and vegetables, which are low in calorie density (meaning you can eat larger quantities). Varady recommends foods with 50 to 70 grams of protein in foods like plain chicken breast, which takes a longer time to digest and can keep you feeling fuller for longer.

On a typical fasting day, you may want to hold off on breakfast, and eat one meal around lunch. Foods like chicken salad that are high in protein make up a good meal, and vegetarians can try to eat legumes like beans with soup or a salad to keep their energy levels up.

It’s really important to make sure you’re drinking plenty of fluids, said Ashe. She explained that in addition to the water we drink during the day, we get a lot of our fluid from food, especially fruits and vegetables. If we’re eating less over all, we’re going to be drinking less water. She recommended increasing water intake from eight eight-ounce glasses per day to 10 or 12.

If you’re really feeling up for it, you can exercise, too—but do it in the morning, said Varady, and for 45-60 minutes at most. You’ll feel acutely hungry for about 30 minutes immediately afterward, but the feeling should eventually pass. That said, working out could increase your chances of having low blood sugar, or hypoglycemia—a risk that comes with fasting anyway.

On days when you decide to eat regularly, you can eat whatever you feel like eating. Varady recommends still sticking to a Mediterranean diet, which includes fish, whole grains, fruit, vegetables, and nuts.

Why most people don’t binge the next day is something of a mystery. When observing participants in her studies, Varady said that ”people became more mindful eaters and noticed what they were eating more.” But she also said that for whatever reason, they tended to become much fuller faster. She suspects that it has to do with the fact that food is traveling more slowly through the body, but she and her colleagues have yet to test this hypothesis.

Not for everyone

There are, of course, inherent risks to fasting. In the short term, not eating can cause excruciating hunger, dizziness and fainting, headaches, and exhaustion (all symptoms of hypoglycemia, too). Varady said in a press release that in some of her studies, patients drop out before 10 days are up because they found it too difficult. Some have called it as a miserable experience, filled with a combination of physical discomfort and overall irritability.

Improper fasting can have myriad detrimental consequences. Fasting for more than three days in a row can signal to the body to start to break down muscles for energy instead of sugars stored in the liver as glycogen and fat reserves. After even longer durations without food, you increase your risk of developing chronic health problems like low bone density, kidney and liver failure, irregular heartbeats, and infertility. It can even cause death in some cases.

And, though it’s poorly understood, there is a risk that extreme weight loss can contribute to slowing your metabolism overall, which means that you could gain weight quickly in the future. Earlier this year, there was a sobering study (paywall) following the progress of contestants who had been on The Biggest Loser, many of whom regained a lot of the weight they had lost on the show. The rate at which they burned energy was much slower than expected for a person of their size and statue.

There are some people who should never try fasting. Varady said that patients who are pregnant or nursing, or who have Type 1 diabetes, should not fast, because they need to keep their nutrients and blood sugar levels constant. Endurance athletes should not fast because their bodies likely need more energy overall (though Varady said she has heard of weight lifters and wrestlers who enjoy fasting just before a competition to shed a few pounds). And patients with histories of eating disorders, especially anorexia and binge eating disorder, should also not fast.

Even among healthy patients, fasting may not be the right plan for everyone. Most importantly, you have to find what works for you, said Ryan Andrews, a registered dietician and coach at Precision Nutrition. Although Andrews has seen some success with skipping meals, he recommends that his clients take longer breaks in between meals and snacks before trying all-out fasting. “The idea of just allowing a decent amount of time [maybe 10 to 12 hours] to pass between the dinner and the breakfast? That’s a fast in and of itself,” he said.

Andrews said that he has seen success with patients who take a limited approach to fasting because they’re better able to listen to their body’s hunger cues. “Fundamentally, when people don’t trust those body cues, they start to look for external rules and prohibitions to guide their eating,” he said. “If I can get them to notice, recognize, and have a new awareness of their body cues and respond to them in a healthy way, that seems to be a healthy thing in the long run.” By taking time to allow ourselves to be hungry, he said, you can feel exactly what you’re craving and eat accordingly.

“I think it’s about that person’s finding a way to be reasonable about their overall food intake,” he said.

Ultimately, the best way to think of food is as fuel, said Ashe. You have to be able to put in the right amount of energy and nutrients to be able to do what you want and need to do, but it shouldn’t become a point of obsession.

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