Intermittent fasting works for weight loss—but not much else

Time-restricted eaters consume on average between 200 and 500 fewer calories a day, some studies suggest. Illustration: Daisy Korpics/WSJ; iStock (2)
Time-restricted eaters consume on average between 200 and 500 fewer calories a day, some studies suggest. Illustration: Daisy Korpics/WSJ; iStock (2)

Summary

The recent research on fasting isn’t as promising as scientists had once hoped.

Intermittent fasting probably isn’t the health hack you hoped it would be.

More studies suggest the tactic can help you lose weight, but likely isn’t a silver bullet for other health improvements like lowering your inflammation levels or lengthening your lifespan. And some evidence suggests fasting can make it harder to build and retain muscle.

“People were hoping it was this magical thing that did amazing things for them," says Krista Varady, a professor of nutrition at the University of Illinois Chicago who has been studying intermittent fasting for 20 years. “All it does is help people eat less."

The popularity of intermittent fasting exploded starting in the late 2010s. The most popular type is time-restricted eating, which involves eating meals within a limited time frame each day. It’s the easiest for most people to stick to, researchers say, and can be as simple as skipping breakfast in the mornings.

Early research in animals suggested it could reduce the risk of developing heart disease and certain types of cancer, and potentially even extend lifespan. But limited research in humans and lukewarm results have muted some hopes.

Fasting for weight loss

Time-restricted eaters consume on average between 200 and 500 fewer calories a day, some studies suggest. Most research participants lose some weight—from less than 1% to roughly 8%.

A six-month clinical trial whose results were published last year compared people who followed a type of intermittent-fasting with people who ate normally. People in the fasting group lost roughly 8% of their body weight on average. But aside from a small improvement in the body’s processing of glucose, the fasters didn’t show benefits in health and longevity markers, such as inflammation levels.

At the same time, some doctors and intermittent fasters themselves say skipping meals makes it difficult to eat enough protein or maintain their muscle mass. Time-restricted eaters lost roughly twice as much lean mass—the body’s nonfat weight—as fat mass compared with a control group, according to ancillary findings in a study a few years ago. Other research hasn’t reached the same conclusion.

Health experts say intermittent fasting can be a helpful weight-loss tool for some, but for others, the hunger during fasting periods will lead to extra snacking during their eating periods. And when it comes to overall health, what people eat is still more important than when they eat, they say.

Second thoughts

Several big names in health and wellness have reined in or abandoned their fasting regimens in recent years, in part citing concerns around protein and muscle mass. Some used to do prolonged fasts, which some researchers believe can stimulate a cellular recycling process called autophagy that declines with age.

Dr. Peter Attia, a physician, author and podcast host, used to fast for three days a month, and up to 10 days every three months, he said in an interview last year with The Wall Street Journal. Attia, 51, stopped and is now skeptical of the benefits of prolonged fasts because of lean mass loss and a lack of scientific understanding about how to measure the benefits.

Dr. Brad Stanfield, a 32-year-old New Zealand-based primary care physician who runs a YouTube channel on preventive medicine, stopped daily time-restricted eating after reviewing studies whose findings suggested a lack of benefit aside from calorie restriction. He also stopped prolonged fasts after noticing muscle loss.

Jake Boone found that fasting put him in a bad mood. Photo: Cinthia Pedraza
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Jake Boone found that fasting put him in a bad mood. Photo: Cinthia Pedraza

On social media, Jake Boone, a former competitive cyclist who lives in Austin, Texas, often saw health influencers tout the purported mental and physical health benefits of fasting. Last year, after cutting back on exercise and gaining some weight while his wife was pregnant, he decided to give it a try and started skipping breakfasts.

In his first month, Boone, 31, lost roughly 6 pounds and exercise started feeling easier and more efficient. But then he started feeling weaker and more irritable, and got several injuries in quick succession. The fixation he developed during his six-month stint with intermittent fasting felt like an eating disorder.

“I was getting really snappy and a little bit angry with people," he says, adding that he gave up the practice after his wife talked to him about his bad mood and concerning eating habits. “I eat breakfast now and I am a better person for it."

Emerging research

The science on intermittent fasting is still developing. Much of it began with studies of caloric restriction in animals. Most studies in humans are relatively small, short-term, and often limited to specific groups of people, like those with diabetes or obesity.

One study from the National Institute on Aging published several years ago found that fasting helped mice live 11% to 14% longer, even when their calories weren’t restricted. That helped fuel the hope that fasting could have longevity benefits even for people who didn’t restrict calories. But what’s true for animals might not hold true in humans, cautions Rafael de Cabo, a senior investigator at the NIA who led the mice study.

Some scientists are still optimistic that fasting could have benefits beyond weight loss. Courtney Peterson, a researcher at the University of Alabama at Birmingham who studies time-restricted eating, ran a study following prediabetic men who ate the same amount of food on regular schedules versus intermittent-fasting schedules. The fasting group didn’t lose more weight than the control group, but still had more improvements in the body’s processing of glucose and blood pressure.

However, the fasting men in that trial ate earlier in the day and stopped by 3 p.m.

There are probably more benefits to keeping an earlier eating window rather than later, she says. “We’re testing the old adage of eating breakfast like a king, lunch like a prince, and dinner like a pauper."

Write to Alex Janin at alex.janin@wsj.com

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