The Science of Weight Loss

Fad Diet? Experts Take New View on Intermittent Fasting

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Going on a diet usually means that you moderately cut calories every single day, but some diets require you to drastically reduce calories just a few days of each week. Although this approach, known as intermittent fasting, was initially roundly decried by health experts as unhealthy, recent evidence shows it might not be so bad.

In fact, a growing body of research suggests that intermittent fasting works just as well as traditional dieting for people who want to lose weight, and that some people may even find it easier to stick with this fasting approach, because there are fewer days when self-discipline is needed. Some nutritionists who had previously advised against skipping meals now say they have changed their minds based on new research, and recommend intermittent fasting for some people who want to try it, Live Science has learned.

"We in the nutrition community always thought it was bad [to skip meals]," said Katherine Tallmadge, a registered dietitian and an op-ed contributor to Live Science. "But based on my experience and these studies combined, I think it's great," said Tallmadge, who now recommends a variation of intermittent fasting to a small minority of her clients. [ 4 Calorie-Cutting Tips That Won't Leave You Hungry]

However, experts caution that intermittent fasting isn't right for everyone, and there are some drawbacks to using this approach instead of a traditional diet.

For example, intermittent fasting is intended to be used only for the short term, so there's a risk that people could gain the weight back when they come off the diet, said Lauren Popeck, a registered dietitian at Orlando Health in Florida.

"Coming off one of these intermittent-fasting plans, the person would really need to have a maintenance plan, or a plan they're going to be following afterwards, so they don’t end up overeating and gaining all the weight back," Popeck said.

And there have also been few studies on the long-term effectiveness of intermittent fasting, Popeck added.

What's intermittent fasting?

Intermittent fasting involves significantly cutting calories only on some days, and eating regularly the rest of the time. Some people practice alternate-day fasting, where they fast every other day, while others try the "5:2 diet," which involves fasting just two days a week. On fasting days, people eat about one-fourth of what they would eat in a typical day, around 400 to 600 calories.

Recent studies suggest that the method is effective for weight loss. A review study published in April 2015 found that people on intermittent-fasting diets lost about 9 percent of their body weight over six months —about the same amount of weight loss seen in studies of traditional dieting. And about 80 percent of the participants were able to stick with the diet.

In another study, published in 2011, women who were overweight or obese were randomly assigned to either an intermittent-fasting diet, in which they ate 540 calories two days a week, or a regular diet, in which they ate 1,500 calories every day. After six months, women in both groups lost a similar amount of weight, about 11 to 13 lbs. (5 to 6 kilograms).

Although the intermittent-fasting diets that were used in these studies involved cutting calories quite drastically on fasting days, Tallmadge said that, anecdotally, she has seen results in her clients who haven't cut calories so severely. If clients want to try intermittent fasting, Tallmadge advises them to fast in the evenings twice a week, meaning they skip dinner, or eat something small like yogurt or fruit. Overall, this means they consume about two-thirds of their usual calorie intake on fasting days, or about 1,300 calories instead of their typical 2,000, Tallmadge said.

Who should try it?

Intermittent fasting may work well for people who are used to skipping meals, or feel like they are too busy to eat, Popeck said. On the other hand, this approach might not work for people who like to snack every few hours. People who are diabetic also would not be advised to try this diet, because going long periods without eating could cause their blood sugar to drop too low, Popeck said.

Although it seems logical that people would overeat on the days when they don't fast, they often don't. The fasting slows their metabolic rate, which lowers appetite, Popeck said.

Still, there are some people who have a tendency to binge on nonfasting days, so this diet would not work for them, Popeck said.

Tallmadge stressed that people should be careful to eat nutrient-dense foods on their fasting days, meaning foods that have lots of nutrients relative to their number of calories, like fruits, vegetables and whole grains.

And people should still have an eating plan for the other five days that allows them to eat about the same number of calories they need to maintain their weight. But if they slip up on these days, the intermittent-fasting diet is a little more forgiving than traditional dieting.

"It's not as detrimental if you have a little slip," because people make up for it on their fasting days, and can still end up losing weight, Tallmadge said.

Follow Rachael Rettner @RachaelRettner. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.

Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.