Intermittent Fasting May Not Be a Useful Approach to Weight Loss, Study Suggests

  • Only eating meals during a certain period of time throughout the day—also called time-restricted eating—was not linked to weight loss over time, a new study found.
  • Rather than time-restricted eating, researchers found that the biggest indicator of weight loss over time was the size and frequency of meals a person ate each day.
  • There are still many questions to be answered about when fasting might be helpful, but experts say the best way to approach weight loss is sticking with small, sustainable changes that fit with each individual’s lifestyle and preferences.
man standing in kitchen drinking glass of water

Getty Images/Maryna Terletska

Time-restricted eating patterns—sometimes known as intermittent fasting—may not be as beneficial for weight loss as once previously thought, new research suggests.

The claim comes from a new study published last week in the Journal of the American Heart Association, which found that the biggest indicator of a person’s weight change over time was the frequency and size of meals they ate, rather than the timing of meals.

“More small meals [were] associated with weight loss over time. And more large or medium meals—or just more meals in general—[was] associated with weight gain over time,” study co-author Wendy Bennett, MD, MPH, associate professor of medicine at Johns Hopkins University School of Medicine, told Health. “And so that makes us think that timing doesn’t matter as much in terms of preventing weight gain or even promoting weight loss.”

Here’s what experts had to say about the study’s findings, which questions still remain about the benefits of some fasting, and why practicing time-restricted eating is likely unhelpful for most people in the long run.

Tracking Meals and Eating Periods Over Time

For the study, Dr. Bennett and her colleagues set out to determine if the intervals during which people were eating meals had anything to do with their weight changes over time. Researchers were particularly interested in the time between a person's first and last meal of the day.

Over the course of six months, 547 participants—the majority of whom were white women with an average age of 51—used an app to track their sleeping habits, when they ate, and the approximate size of the meal or snack they ate. The average body-mass index (BMI) for participants was 30.8, which is considered obese by BMI standards.

BMI is a biased and outdated metric that uses your weight and height to make assumptions about body fat, and by extension, your health. This metric is flawed in many ways and does not factor in your body composition, ethnicity, sex, race, and age. Despite its flaws, the medical community still uses BMI because it’s an inexpensive and quick way to analyze health data.

Researchers then accessed their health data and weight for up to 10 years prior to the study—for most participants, researchers had access to about six years of data. Most participants said their eating habits had remained consistent, Dr. Bennett said, which allowed researchers to assume that those habits recorded over the course of the study were fairly similar to what was typical for each participant.

On average, 11.5 hours passed between the study participants’ first and last meal of the day. But even as this number changed, researchers didn’t see any difference in participants’ weight change. Rather, the frequency and size of the meals a person ate were better indicators of whether they would lose or gain weight over time.

Specifically, the total daily number of large meals (meals with more than 1,000 calories) and medium meals (meals with 500–1,000 calories) were associated with increased weight over the six-year follow-up period. Fewer small meals (meals with less than 500 calories) each day were associated with decreased weight.

The study could not determine whether certain types of intermittent fasting—such as 16:8, which consists of fasting for 16 hours and eating for eight within the course of a 24-hour day—have any impact on weight over time, Dr. Bennett explained.

But restricting the time that a person eats—for example, only eating between 10 a.m. and 6 p.m.—may not, at least for the general population, have a significant effect on weight loss.

Measuring Eating Habits Can Be Difficult

The study had a large sample size and was more rigorous than many other similar ones that have looked into this topic, Dr. Bennett said.

Even still, it can be challenging to glean meaning from this kind of data, Alice Lichtenstein, DSc, Gershoff professor of nutrition science and policy at the Friedman School of Nutrition Science and Policy at Tufts University, told Health.

“This is one of many studies,” she said. “This is what they found, but to conclude for everyone that it’s unlikely to be beneficial, is probably a bit of an overreach.”

There are other studies that have actually demonstrated some benefits to time-restricted eating, or intermittent fasting. Specifically, one study found that mice had a lower chance of metabolic diseases when they ate for just 8 hours during the day. Another review said 12 small studies showed similar weight loss results between people who did time-restricted eating and others who restricted their calories.

But it’s difficult to pinpoint exactly what’s causing the dissonance. Time-restricted eating could just be a tool that helps people eat less frequently.

“It’s really challenging to draw conclusions about fasting,” she said. “We don’t know if fasting is beneficial, or if it’s just that when you’re eating, you’re eating in a shorter period of time, and therefore you’re eating less.”

To truly understand what is going on and determine why just eating less has a bigger impact on weight than when you eat, a more controlled, rigorous study would have to be conducted, she added. It would need to be one where people eat the same amount of calories over a period of time, though some practice time-restricting eating and others don’t, she said.

In an observational study like this though, certain limitations will affect what people can take from the data. For one, Dr. Bennett and the study authors had to assume that the participants’ eating habits during the six-month study were consistent with their habits for the past six or so years.

The size of the meals that study participants ate was also self-reported, Lichtenstein added, which could make things confusing. Participants estimated the size of their meal based on a drop-down menu that listed small meals as less than 500 calories, large meals as over 1,000 calories, and medium as in-between.

“I’ve always had a hard time defining a meal. Because for some people, a meal is a slice of pizza and a Coke. And for other people, it’s half a pizza. But then for some people, that slice of pizza is a snack,” Lichtenstein said.

Because of these limitations, it’s also not possible to conclude any sort of explanation for why the study found the results that it did, Dr. Bennett emphasized.

“We were able to get free-living people telling us when they eat and then looking at whether it was associated with weight trajectories. But they weren’t told to eat a certain way,” Dr. Bennett said. “It wasn’t a rigorous randomized design where we can really figure out a causal association.”

Eating Advice Is Not One-Size-Fits-All

Because so many questions still remain when it comes to weight loss and recommended eating habits, it’s important to remember that every person’s body and relationship with food is different, Lichtenstein said.

“If one is going to give guidance to an individual in terms of how to maintain healthy body weight, you have to understand that there are different triggers for different people, and there are different life circumstances for different people,” she said.

For some people, intermittent fasting may be a helpful tool, while others may want to experiment with smaller plates or portion sizes, or something else altogether, Lichtenstein added.

“If you don’t work within somebody’s preferences, habits, and lifestyle, and go to something artificial,” she said, “then it’s not going to be sustainable.”

If a person wants to lose weight, they should consider first consulting with their healthcare provider. Commercial weight loss programs are available, Dr. Bennett said, but the experts agreed that the best way to lose weight is to work in small, gradual changes that fit with each individual person’s preferences and routines. If someone tends to spread out their meals and snacking throughout the day, adopting a strict intermittent fasting diet will probably be really challenging and they may not be able to commit to it long term.

Dr. Bennett also recommended that people check in with their weight every once in a while to watch for any dramatic changes, or she suggested using tracking—either with a pen and paper, or with an app—to help a person get a better idea of how many calories they’re consuming each day, and if there are areas where they could practice cutting down gradually. Some of these tracking apps, however, have been associated with disordered eating, so they’re not a good choice for all people.

Making sure weight loss isn’t mentally, emotionally, or physically damaging—and is sustainable—likely means different programs and techniques for different people.

“You really have to understand what people are consuming, what their triggers are, and on an individualized basis, figure out where they can cut back,” said Lichtenstein. “[It’s about] making small changes that are going to be sustained over a lifetime.”

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