People who choose not to eat for 12 hours a day, aka those who fast, claim it gives you better sleep and abs. Are these people onto something — or are they just annoying?
Intermittent fasting is a diet strategy that involves alternating periods of eating and extended fasting (meaning no food or very low calorie consumption). One thing not intermittent is the arguing over it.
“There’s quite a bit of debate in our research community,” said Courtney Peterson, an assistant professor in the Department of Nutrition Sciences at the University of Alabama at Birmingham who studies time-restricted feeding, a form of intermittent fasting. “Could you get the same benefits by just cutting your calories by the same amount?”
There are four popular fasting approaches: time-restricted feeding, periodic fasting, alternate day fasting and the 5:2 diet.
Time-restricted feeding, sometimes called daily intermittent fasting, is perhaps the easiest and most popular method. Eating is restricted to certain time periods each day — say, 11 in the morning to 7 at night. Periodic fasting will feel most familiar: No food or drinks with calories for 24-hour periods. Alternate day fasting requires severe calorie reduction every other day. Lastly, the 5:2 method was popularized by author Kate Harrison’s book “The 5:2 Diet” and requires fasting on two nonconsecutive days a week.
As for fasting’s effectiveness, if you are obese or overweight, fasting can be an effective weight loss method, Peterson said. (The caveat, as with any diet, is “if you stick to it.”)
“But it is no more effective than a diet that restricts your daily calories,” she added. “We know this because there were no additional weight loss or cardiovascular benefits of fasting two days per week over an ordinary calorie restriction diet in a study of 150 obese adults over the course of 50 weeks.”
But back to that “if you stick to it” part. In a study of 100 randomized obese and overweight adults published in 2017, the dropout rate was higher with those who were fasting — 38%, compared to 29% for calorie restrictors and 26% for those who kept eating as they normally did.
“Some people really struggle with having to monitor their intake and constantly record food in an app every day,” said Krista Varady, professor of nutrition at the University of Illinois at Chicago and senior author of the study. “So the takeaway of the study was, if daily calorie restriction doesn’t work for you, maybe alternate day fasting would be a little easier.”
There is some new evidence that shows different forms of fasting are not equal — in part because some are easier than others but also because some forms of fasting better match our body’s natural circadian rhythm, thus lowering insulin levels, increasing fat-burning hormones and decreasing appetite. Basically, because our metabolism has evolved to digest food during the day and rest at night, changing the timing of meals to earlier in the day might be beneficial.