Intermittent fasting is a trendy weight loss strategy. But a new study found that a popular form of intermittent fasting called time-restricted eating produced minimal weight loss and one potential downside: muscle loss.
The new research, published in JAMA Internal Medicine, is one of the most rigorous studies to examine time-restricted eating, which involves fasting for 12 or more hours a day. Many followers of the diet, which has been popularized in best-selling diet books and touted by celebrities, routinely skip breakfast and eat all their meals between roughly noon and 8 p.m., resulting in a daily 16-hour fast.
Research over the years has suggested that the practice spurs weight loss and improves metabolic health, although much of the data has come from animal experiments or small studies of relatively short duration in humans. Experts say the diet works because it allows people the freedom to eat what they want so long as they do it in a narrow window of time, which leads them to consume fewer calories over all.
But the new research found that overweight adults who were assigned to routinely fast for 16 hours daily, eating all their meals between noon and 8 p.m., popularly known as the 16:8 diet, gained almost no benefit from it. Over the course of the three-month study, they lost an average of just 2 to 3 and a half pounds — only slightly more than a control group — and most of the weight they shed was not body fat but “lean mass,” which includes muscle.
While it is normal to lose some muscle during weight loss, the fasting group lost more than expected. That is concerning because muscle provides many health benefits: It protects against falls and disability as people age, and it is linked to lower mortality. It also increases metabolism and can help prevent weight that is lost during dieting from returning later on. The researchers speculated that one reason for the muscle loss may have been that the fasting diet led people to consume less protein.
The new findings were surprising to the study’s senior author, Dr. Ethan Weiss, a cardiologist at the University of California, San Francisco. Weiss had been practicing time-restricted eating since 2014, eating all of his meals between noon and 8 p.m. on a daily basis. But when he analyzed the data and saw the results of his study, he stopped his daily fasts and began eating breakfast again.
“My bias was that this works and I’m doing it myself, and so I was shocked by the results,” he said.
But some experts cautioned that the study was too short for a weight loss trial. They said it was likely that the fasting group would have showed greater weight loss had the study been longer and included more participants. They also pointed out that previous research has shown that people do better when they consume the bulk of their calories relatively early in the day, which is when our bodies are better able to metabolize food, rather than skipping breakfast and eating most of your food in the afternoon and evening, which goes against our biological clocks.
Fasting became popular for health reasons after small studies suggested it promotes longevity and a wide range of metabolic benefits, such as improved cholesterol profiles and reductions in insulin resistance.
Studies have found, for example, that overweight adults lose more weight and have greater improvements in their cardiovascular risk factors when they eat a large breakfast, a modest lunch and a light dinner, compared to when they eat a small breakfast and a big dinner.
“It could be that the benefits of time-restricted eating are smaller than we thought, or that you just get better results when you eat earlier in the day,” said Courtney Peterson, a researcher at the University of Alabama at Birmingham who studies intermittent fasting and who was not involved in the new study. “The jury is still out.”
Many cultures around the world practice fasting for religious or spiritual reasons. But fasting became popular for health reasons after small studies suggested it promotes longevity and a wide range of metabolic benefits, such as improved cholesterol profiles and reductions in insulin resistance. Some of the other common forms of intermittent fasting are alternate day fasting, in which followers eat no more than 500 calories every other day, and the 5:2 diet, which entails eating normally for five days a week and fasting for two.
Many people, however, have trouble going an entire day with little or no food. Krista Varady, a professor of nutrition at the University of Illinois, Chicago, has found in her research that people lose weight more slowly with time-restricted eating than other forms of fasting but that it is generally the easiest form of fasting to adopt. People tend to eat 300 to 500 calories fewer per day when they restrict themselves to an eight-hour window, said Varady, who was not involved in the new research.
“The best part is there are no limitations during the window,” she said. “There is no carb or calorie counting, and people don’t have to switch out all the food in their pantries.”
Varady said she is planning to start a yearlong study of time-restricted eating in the near future. “I find it fascinating that this diet has become so popular and there are so few studies,” she said.
Weiss got interested in time-restricted eating after learning about research that showed that mice provided round-the-clock access to high-fat, high-sugar foods got fat and sick, while mice that ate the same foods in an eight-hour daily window were protected against obesity and metabolic disease. In his own experience, Weiss found that skipping breakfast and eating between noon and 8 p.m. was not very difficult.
He wanted to see whether the diet could be a simple prescription for people to lose weight, so for the current study, he and a colleague, Dylan Lowe, designed an experiment: They recruited 116 overweight and obese adults and split them into groups. One group, which served as controls, was told to eat three structured meals daily. The other group was assigned to eat all their food between noon and 8 p.m., with only noncaloric beverages like tea and black coffee permitted outside that window.
The researchers chose noon to 8 p.m. because it would be more socially acceptable for participants to skip a meal in the morning rather than in the evening, when they might be sitting down to dinner with family or friends. “We wanted to make this something that was simple enough that people could actually do,” Weiss said.
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The researchers gave the participants digital scales and tracked their weight for 12 weeks. They also had 50 of the participants, from both the fasting and three-meals-a-day groups, visit their lab four times during the study so they could measure things like their fat and muscle mass, blood sugar levels and energy expenditure.
In the end, the fasting group lost an average of two pounds, only half a pound more than the controls. When the researchers looked at participants who had visited their lab for extensive testing, they found that the fasting group had little or no improvement in most of their metabolic markers, though they did tend to lose more weight, a little over 3 and a half pounds. That was roughly 2 and a half pounds more than the control group, a difference that fell narrowly short of being statistically significant. But 65% of the weight that the fasting group lost was from lean mass — more than double what is considered normal for weight loss.
Weiss said it is possible that the fasting group lost an unusual amount of muscle because skipping breakfast each day caused their overall protein intake to fall. But that could potentially be avoided: Other studies have found that people can maintain muscle while fasting by doing resistance training and consuming more protein during their eating windows. Weiss said the findings need to be explored further, but for now he remains skeptical of time-restricted eating.
“This was a short study, but it was enough of a study that to me it calls into question whether this works — and if it does work, then the magnitude of the benefit is very small,” he said.