June 10, 2023

TEN YEARS AGO, intermittent fasting (IF) was a cutting-edge (and kind of risky) way to change your diet. But now, the fasting movement has gone mainstream: Chris Pratt used intermittent fasting to get in Navy SEAL-worthy shape for The Terminal List. Jimmy Kimmel used it to slim down after years of making fat jokes about himself. And Brooklyn Nine-Nine star Terry Crews credits fasting for helping him maintain his six pack.
“Intermittent fasting has only become popular in the past five years or so,” says Krista Varady, Ph.D., a professor of nutrition at the University of Chicago. “In the U.S.—and pretty much around the world—it was kind of ignored.”
Proponents of the practice claim that for humans, fasting is much more natural than eating three meals a day, plus snacks. As the theory goes, our hunter-gather ancestors spent much of their time in a fasted state, in part because food was relatively scarce (and wild boars can be hard to catch).
Fasting was so common, the human body actually evolved to function in a steady state of calorie-deprivation, according to a review published in The New England Journal of Medicine. Our cells rallied to the challenge by lowering inflammation levels, repairing DNA, and breaking down old cells and replacing them with new ones.
It’s thought that humans can still benefit from intermittent fasting—Varady’s research has shown that bouts of fasting can improve insulin resistance, lower blood pressure, and reduce inflammation.
But its real claim to fame is its usefulness as a weight-loss tool. Research shows that people who try intermittent fasting typically lose about 3 to 8 percent of their body weight— which, for a 200-pound person, is about 6 to 16 pounds.
Interested in trying it out for yourself? There are a few things you should know first.
The concept calls for going extended periods of time without eating, and people generally follow three common fasting schedules:
Alternate-day fasting entails switching from a ‘fast day’ to a ‘feast day.’ On fast days, you typically consume about 25 percent of your daily calorie needs. For example, you might eat one 700-calorie meal on Monday and resume your normal diet on Tuesday. Or you can break up those calories over the course of the day — say, by eating a 350-calorie lunch and a 350-calorie dinner.
Whole-day fasting is also known as the 5:2 fasting schedule, which means you eat roughly 500-calories two days a week. The other five days have no food restrictions.

Time-restricted fasting typically follows the 16:8 format, meaning you only eat for 8 hours throughout the day and fast for 16. For example, noon to 8 p.m. is a good guide as many people only eat lunch and dinner.
People typically lose twice as much weight when they do alternate fasting compared to time-restricted fasting, says Varady. The catch, though, is that people usually can’t do it for more than 6 months. “Eating 500 calories every other day is pretty tricky — it messes up their social eating schedules and is hard to stick to,” she says. “But people find it easier to incorporate time-restricted eating into their lifestyle.”
Not sure which one to try?
Everyone’s preferred fasting schedule will vary, but New York-based Alexandra Sowa, M.D., says it’s best to start slow with just a 12-hour fast. If that’s not unbearable, move onto a 16-hour fast several times a week.
“If you like it then see how you do extending it to a 24-hour fast,” says Sowa
There’s not a lot of evidence to show exactly what happens when we fast, says Nathalie Sessions, R.D., at Houston Methodist.
“There’s only been a few viable scientific studies that have been done with humans on intermittent fasting,” she tells Men’s Health.
Most research has been conducted in animals, and human bodies don’t function in the same way.
Many theorize that giving your body a break from eating helps your body repair damaged cells, a process known as autophagy, says Sowa. However, most of this research has been conducted in mice, so the science isn’t 100 percent clear.
However, there is evidence that fasting lowers insulin levels, according to Sowa. Healthy adults experienced a decline in insulin levels after fasting, according to a 2005 study published in The American Journal of Clinical Nutrition. Insulin, a hormone made by the pancreas, regulates blood sugar and other hormones. Too much insulin has been linked to obesity and health conditions like heart disease.
Much of these benefits, however, occur for a simple reason—people are eating less food, and therefore losing weight. “It’s the weight loss that improves cholesterol levels and glucose and blood pressure,” she says. In fact, a randomized clinical trial of obese adults showed that intermittent fasting didn’t result in any more weight loss or heart health benefits than standard calorie restriction, according to a 2017 paper published in JAMA Internal Medicine.
People who do alternate-day fasting usually lose about 10 to 30 pounds over a three-to-six-month period, says Varady. When they do time-restricted fasting, they tend to drop about 10 to 15 pounds.
Fasting turns out to be surprisingly simple in practice (if a little intimidating). People love it because you don’t need to count calories, buy certain foods, or even change up your existing diet, says Varady.
Which leads us to another point: intermittent fasting doesn’t guarantee weight loss. In theory, you’ll consume less calories because you’re eating fewer meals, but it’s entirely possible to eat a terrible diet and fast at the same time — especially if you gorge on a burger, fries, and shake once you can eat again.
“Our research shows that IF has no effect on diet quality, which is good and bad,” says Varady. “People don’t eat worse, but they definitely don’t eat better.”
Sowa recommends eating foods that are high in fiber and protein, like leafy green vegetables, eggs, and chicken. “In order to continue the benefits of the fast you want to choose foods that will not spike your insulin,” she says.
Very little, according to Sowa. Generally, you’ll want to stick to water or beverages that have virtually zero calories, like black coffee.
You may have seen people adding butter or MCT oil to coffee, and these are actually okay in small quantities because they don’t have protein or carbs, says Sowa.
If you just can’t stomach black coffee, she recommends adding a tablespoon of full-fat whipping cream to your coffee.
“That will allow most people to stay in a technically fasted state,” she says. Just don’t go overboard.
“If you’re drinking a ton of whole fat whipping cream or eating a lot of butter you’re not going to lose weight,” she says.
Aim for no more than two tablespoons per day.
Otherwise, research shows that intermittent fasting seems to be safe—trials of alternate-day fasting and time-restricted fasting don’t cause symptoms like constipation, diarrhea, nausea, irritability, fatigue, or dizziness.
In the past, people also worried that intermittent fasting would give someone an eating disorder. Varady’s research has shown that this isn’t the case, but doesn’t recommend IF for people who are at risk for an eating disorder.
You might not be very pleasant to talk to for the first few days, but don’t worry, you’ll eventually stop snapping at people. (Kidding. We think.)
“There’s a one-to-two-week adjustment period for your body to get used to the new eating pattern,” says Varady. During that time, people sometimes have headaches, but that’s usually because they’re dehydrated. “Once they increase their water intake, their headaches go away.”
One 2016 study found that young men who did time-restricted fasting for 2 months saw a slight decrease in testosterone levels, but this wasn’t enough to cause them any loss in lean muscle mass or strength.
And speaking of muscle mass, you can still exercise safely while you’re in a fasted state: “People actually have a boost of energy while fasting,” says Varady. “I think it’s an evolutionary thing where, if you don’t have food, your body will give you energy to go out into your environment and find food. People say that on their fast day, they have way more energy than on their feast day.”
Sessions says this plan probably works best for people who are regimented and says yo-yo dieters will likely have a hard time fasting. The best way to lose weight is by finding something you can stick to.
“The long-term key to weight loss is to establish a lifestyle that you can sustain,” says Sessions.
If you think this will work for you, it’s best to work with a doctor or dietitian who can ensure you’re getting enough nutrients.
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