“Somehow during lockdown I managed to lose weight instead of gain it,” wrote freelance journalist Dana McMahan on Today.com in September 2020. “How on earth did that happen? I stumbled into intermittent fasting.”
“Intermittent fasting is an umbrella term for three different types of diets,” explains Krista Varady, professor of nutrition at the University of Illinois at Chicago.
■ Alternate-day fasting. “People restrict their intake, often to about 500 calories a day, and they alternate those days with ‘feast’ days on which they can eat whatever they want.”
■ 5:2 diet. “This is a spinoff of alternate-day fasting where you have two ‘fast’ days and five ‘feast’ days per week.”
■ Time-restricted eating. People limit themselves to eating only between certain hours, with nothing but calorie-free drinks at other times. “You can apply that window to any part of the day,” says Varady. “A popular version is the 16:8 diet, where people fast for 16 hours and eat during an 8-hour window.”
Most studies show that people lose no more weight when they try alternate-day fasting or the 5:2 diet than when they simply eat fewer calories every day.
In other words, while fasting may be a simple strategy, it isn’t magic.
“The reason people lose weight is because they’re eating fewer calories,” explains Varady.
In her studies on alternate-day fasting, “you’d think that people would go crazy on their ‘feast’ days, but they don’t. They eat about 10 percent more calories than they normally would. So the net result is that they have an average calorie deficit of about 600 to 700 calories per day.”
But for many, eating so little food is a struggle. “A lot of people drop out of the studies because it’s really hard to adjust to,” Varady notes.
Instead, she is now looking at time-restricted eating, which “short-term findings suggest is a lot easier to stick to.”
In a recent study, researchers randomly assigned 116 adults with overweight or obesity to eat three meals at any time of day or to eat only between noon and 8 p.m. After 12 weeks, the time-restricted eaters had lost no more weight than the control group. (Each lost, on average, about two pounds.)
And in a subgroup of the time-restricted eaters whose body composition was measured, 65 percent of the weight they lost was muscle.
Those results give Varady pause. “The researchers didn’t collect diet records,” she notes. Without knowing what people ate, it’s hard to explain why the time-restricted volunteers lost so much more muscle than fat.
In a new study, Varady tested whether a shorter eating window would boost weight loss.
She randomly assigned 58 people with obesity to one of three daily eating patterns: eat their usual diet; eat only between 1 p.m. and 7 p.m.; or eat only between 3 p.m. and 7 p.m.
(Why those time frames? “People really don’t want to skip dinner,” says Varady.)
After eight weeks, only people in the two time-restricted groups had lost weight—about seven pounds each.
“You don’t have to torture yourself and eat within a small, four-hour window when a six-hour window produces the same weight loss,” says Varady.
But the research on time-restricted eating for weight loss is limited, she adds. “It’s so popular, but there are only a handful of studies.”
Varady is hoping to run a year-long study comparing time-restricted eating to daily calorie cutting. Stay tuned.
Bottom Line: “If you want to lose weight, find something that works for you long term,” says Varady. “I’m not pushing intermittent fasting, but it’s simple and it probably works better than cutting calories for some people.”
The information in this post first appeared in the December 2020 issue of Nutrition Action Healthletter.
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