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Does intermittent fasting work? Research doesn't have a definite answer for its long-term effects

fasting empty plate fasting empty plate
Small, short-term research studies have found that intermittent fasting has weight loss benefits — though it may not be sustainable.
Chalermphon Kumchai/EyeEm/Getty Images
  • Intermittent fasting can work as a weight-loss tool, and short-term research has found that it can improve overall health for those with obesity. 
  • However, the long-term results are less clear, and intermittent fasting may be less sustainable than other calorie restriction diets. 
  • It also may not be safe for everyone. Teenagers, women who are pregnant or breastfeeding, people who take medication that can cause hypoglycemia, and those with a history of eating disorders should not try intermittent fasting. 
  • This article was reviewed by Melissa Rifkin, MS, RD, CDN, owner of Melissa Rifkin Nutrition LLC. 
  • This article is part of Insider's guide on How To Lose Weight.

Many cultures throughout history have used fasting as a tool to improve health. For example, ancient Greek athletes would fast to prepare their bodies for the Olympic games.

Intermittent fasting is a modern take on this practice and uses a controlled plan of fasting in limited time periods. 

There is some early research showing that it works well for dieters, but it can also be harmful to certain people. Here's what you should know before trying intermittent fasting.

How intermittent fasting works

Intermittent fasting involves following a schedule that switches between periods of eating and periods of fasting. Unlike most diets, it doesn't put limits on what you eat specifically but rather focuses on when you eat it.

There are many options for how to approach intermittent fasting. Your fasting schedule can involve fasting during certain hours of the day or certain days of the week.

Some of the most widely used fasting schedules are:

  • 16/8: Eating only during an eight-hour period, such as 10 a.m. to 6 p.m. and fasting for the rest of the day.
  • Eat-Stop-Eat: Fasting for 24 hours straight one or two days per week.
  • The 5:2 diet: Choose two days per week where you will eat only 500 to 600 calories per day.

Intermittent fasting can help with weight loss 

Research is limited on how well intermittent fasting works — especially in all its different forms. However, the small, short studies that have been conducted indicate that it can help people lose weight in just a matter of weeks. Whether participants kept the weight off long-term is unclear.

A very small 2009 study published in The American Journal of Clinical Nutrition found that 16 obese people who ate a very restricted diet three days per week lost an average of 12 pounds over 8 weeks. The dieters also saw improvements in their cholesterol and blood pressure.

Intermittent fasting has also led to weight loss for non-obese people. Another very small 2005 study published in The American Journal of Clinical Nutrition showed that the eight participants lost weight and burned body fat after fasting every other day for 3 weeks. However, they reported feeling incessant hunger the entire time, suggesting that this form of intermittent fasting may not be sustainable.

The amount of weight you lose while fasting intermittently depends on which version of the diet you choose. When fasting two days per week, you can expect to lose between 9 and 13 pounds in the first 2 to 3 months, says Leonie Heilbronn, an associate professor at Adelaide Medical School. If you fast three days per week, Heilbronn says you could lose 13 to 18 pounds. 

"Intermittent fasting can be a good alternate option to calorie restriction to improve health," Heilbronn says.

Intermittent fasting isn't safe for everyone

But according to Heilbronn, intermittent fasting is not for everyone. "People who take medication that can cause hypoglycemia (i.e. low insulin levels) shouldn't do this diet without talking with their doctors," she says. 

Heilbronn also recommends avoiding this diet while pregnant or breastfeeding. Experts recommend that people who are still growing, like teenagers, should not try intermittent fasting. It may also be dangerous for anyone with a history of eating disorders.

There have not been any long-term studies on intermittent fasting and the longest trial lasted for six months, Heilbronn says. After the study period, few people stayed with the diet at a follow-up six months later.

"Some people can struggle with not eating for prolonged periods," says Heilbronn. Ultimately, more research is needed to determine if this diet can work over the long term.

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More: Health Explainers Health Diet Weight Loss
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