But experts who study intermittent fasting cautioned against misinterpreting the results and sticking with the approach if it works for you.
First, the findings: A randomized controlled trial found participants who fasted on alternate days lost less weight than those who simply ate less every day — even when both groups ended up restricting the same amount of calories overall, according to the study published in the journal Science Translational Medicine.
People in the fasting group also lost more muscle mass and less body fat than the traditional dieters, likely because they moved less throughout the day, researchers found. Reducing physical activity may be the body’s subconscious response to fasting, they noted.
The findings suggest intermittent fasting is “no magic bullet” and there’s nothing special about it compared to a standard diet, said lead author James Betts, professor of metabolic physiology at the University of Bath and co-director of the school’s Centre for Nutrition, Exercise & Metabolism, in a statement.
Still, it’s important to note that both eating approaches led to weight loss, he said.
“Our study was not designed to work out which diet is best, but rather to understand fasting better,” Betts told TODAY.
“Anyone who finds fasting appealing does not therefore need to switch to another diet, but should consider what effects fasting may and may not have.”
The study involved 36 “lean healthy” adults who were first monitored for a month to gauge their typical diet and exercise habits. They ate about 2,000 to 2,500 calories per day at baseline and were then randomly divided into three groups:
Group 1 represented a traditional diet. Participants ate 25% fewer calories every day than their baseline diet.
Group 2 represented alternate day fasting. Participants ate nothing one day, then consumed 150% of their baseline diet the next.
Group 3 also represented alternate day fasting, but without a net calorie deficit. Participants ate nothing one day, then consumed 200% of their baseline diet the next.
The first two groups reduced the same amount of calories overall, but in different ways. The third group didn’t reduce overall calories at all since participants ate twice as much on their “feast” days.
After three weeks of these regimens, the people doing a traditional diet lost about 4.2 pounds — most of it fat. Those doing alternate fasting lost 3.5 pounds — half of it fat and half muscle mass. The people in group 3 didn’t lose a significant amount of weight. Researchers found no evidence of fasting-specific effects on metabolic or heart health.
The loss of muscle mass wouldn’t be immediately noticeable or harmful for most people, “but preserving lean tissue is important for long-term functionality and health, especially as we get older,” Betts said. Staying active is important to keep muscle mass, as is consuming enough energy, particularly protein, he added.
'Nobody does alternate day fasting like this'
Critics pointed out the study involved lean adults who don't lose weight very easily in the first place. They also noted that the study didn’t reflect real-life conditions because the fasting participants were instructed to eat too much on their “feast days.”
“Nobody does alternate day fasting like this,” said Krista Varady, a professor of nutrition at the University of Illinois, Chicago, who has been studying alternate day fasting for 15 years.
“Nobody force feeds themselves 150% of their needs… No one is ever going to eat like 4,000 calories every other day and then fast. It's a little weird.”
Previous studies have shown alternate day fasting works because people don't binge on their “feast days” in real life and eat only about 10% more than usual, so they can't make up for the lack of calories from their fasting days and lose weight, Varady noted.
Most people wouldn’t eat 50% more than usual if left to their own devices, Betts agreed, noting the study participants still lost weight even when prescribed to consume those extra calories.
Intermittent fasting is a great option for weight loss, but it’s not better than calorie restriction — both produce similar benefits, so you just have to figure out which works better for you, Varady advised.
“The popularity of intermittent fasting is that it requires no knowledge about nutrition and no need to count calories — people only need know how to read their watch,” Betts added.
There are different types of intermittent fasting. Some people like the alternate day approach — eating nothing or very little one day, then normally the next and repeating that pattern — because it lets them have a “day off” from dieting. Any weight loss and health gains will be more evident if the fasting is alternated with healthy eating and combined with an active lifestyle, Betts said.
The less strict 5:2 plan calls for two non-consecutive fasting days a week. Time restricted feeding, or 16:8, calls for people to consume all their calories during an eight-hour window and fast for the rest of the day.
Alternate day fasting and the 5:2 plan generally result in about twice as much weight loss as time restricted feeding, Varady said.
“I'd say definitely keep on doing (fasting) if it works for you,” she added. “I wouldn't be put off by these findings because they don't represent what people actually do during these diets.”