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Intermittent fasting has taken off in popularity in recent years as an alternative to more traditional weight loss advice, including counting calories, which can be cumbersome and hard to sustain for some people.
Intermittent fasting can take different forms. One approach — called time-restricted eating — limits when people eat to a specific window of time, often around six to eight hours.
Some research suggests this can be successful for weight loss in the short term because people end up eating less, but it has become less clear how well it works over a longer stretch of time.
A study published Monday may have an answer.
“We really wanted to see if people could lose weight with this over a year. Can they maintain the weight loss?” says Krista Varady, a professor of nutrition at the University of Illinois Chicago, who has studied intermittent fasting for the past two decades and led a new study.
Varady’s research finds that intermittent fasting can indeed help people lose weight and keep it off over the course of a year, with effects similar to tracking calories. The results of the clinical trial were published in the Annals of Internal Medicine.
The amount of weight loss wasn’t dramatic — equivalent to about 5% of body weight — but the findings are encouraging to researchers in the field, in part because they underscore that people could keep this habit up over a long stretch of time.
“That is pretty exciting,” says Courtney Peterson, a professor of nutrition at the University of Alabama at Birmingham, who wasn’t involved in the research. “This study has the most compelling results suggesting that people can stick with it, that it’s not a fad diet in the sense that people can do it for three months and they fall off the wagon for a year.”
“Natural calorie restriction”
Varady’s team recruited 90 adults with obesity (meaning a body mass index over 30) from the Chicago area and randomly assigned them to one of three groups: One group could eat only between noon and 8 pm, another had to count calories and cut daily energy intake by 25%, and the third group didn’t make any changes to their eating.
After six months of weight loss, the participants had a “weight maintenance phase.” That was accomplished by extending the eating window from eight hours to 10 hours in the intermittent fasting group and bumping up the caloric intake in the calorie restriction group.
Varady says they designed the study that way because “most people with diets will lose weight for about six months and after that it usually plateaus.”
The study found that those who did time-restricted eating lost, on average, about 10 pounds more than those in the control group, while those who counted their calories lost about 12 pounds more. The difference between the two groups wasn’t statistically significant.
“The key take-away is that you can basically achieve the same amount of energy restriction by counting time instead of counting calories,” says Varady.
Previous research on intermittent fasting found that when people limit eating to an eight-hour window and deliberately restrict calories, they achieve a similar weight loss over the course of a year as people who only restrict calories but don’t limit eating to a specific time window .
What’s different about the new study is that people in the intermittent fasting group weren’t instructed to watch their calories, but they ended up reducing their daily intake anyway, by around 400 calories — the same amount as the calorie-counting group.
The results suggest that time-restricted eating can lead to a kind of “natural calorie restriction,” Varady says. She says it may be in large part a result of people having less time to eat, particularly in the after-dinner hours.
“People usually eat within a 12-to-14-hour window, so all we’re doing is cutting out around six hours,” she says. “Mainly we’re cutting out, I think, after-dinner snacks.”
Peterson says that placing limits on when you eat can have an “anti-snacking effect” that can make you avoid mindless eating later in the night. She says data from her lab also shows that intermittent fasting can affect hormones and help regulate appetite.
The study didn’t find any meaningful difference in cardiovascular and metabolic health between the two weight loss groups. Research suggests that eating earlier in the day can be beneficial to metabolic health, but Varady says they chose noon to 8 pm because this mirrors how people tend to go about time-restricted eating in the real world.
“From a feasibility perspective, I just don’t know anyone who is going to stop eating by 4 pm every day,” she says. “If you can do that or if it fits into your lifestyle, then, sure, go ahead.”
Support and counseling may make weight loss more sustainable
Another feature of the study was that both weight loss groups had regular counseling with dietitians in which they learned about healthy food choices and learned cognitive behavioral strategies to prevent themselves from regaining weight.
This kind of “intensive support” is significant, says Dr. Adam Gilden. “Most people who are doing this are not doing it with any type of dietary or behavioral support. They’re doing it on their own,” says Gilden, an associate professor of medicine at the University of Colorado Anschutz Medical Campus, who authored an editorial that was published alongside the new study.
For this reason, he says his patients often tell him they are not successful when they try time-restricted eating.
Gilden cites the results of another trial that found that time-restricted eating didn’t lead to meaningful weight loss over the course of 12 weeks. In that study, he points out, there was no dietary counseling or support.
In the new study, where participants got that support, “time-restricted eating is about the same in effectiveness as traditional caloric restriction,” he says. But he’s skeptical that these techniques will yield the same results in the real world without support.
In the study, those who did time-restricted eating or calorie counting had “moderately high adherence” throughout the course of the year-long study.
But Peterson says previous research suggests that the legwork involved with calorie counting — what tends to be standard advice for people when they are counseled about weight loss — makes it hard to sustain. People need to be educated about portion sizes and how many calories are in different foods and then track and log meals.
“It can be a big pain for a lot of folks,” she says.
Peterson says this study’s comparison of time-restricted eating and standard calorie counting suggests “for a lot less effort, you can cut your calories by the same amount.”
The implication of this research is not that intermittent fasting is somehow an “excuse to change your diet for the worse,” says Dorothy Sears, a professor of nutrition at Arizona State University’s College of Health Solutions and executive director of clinical and community translational science.
“We are designed to most optimally process nutrients during the day,” Sears says. “So let’s just start by having people eat during the day and avoid eating at night, which in itself is associated with negative health outcomes.”
There’s no need to “arm-wrestle” about whether calorie counting is better or worse, she says, “but we do need to test whether time-restricted eating is as effective, and this study is showing, yes, it’s effective.”