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Intermittent fasting wasn't associated with weight loss over 6 years, a new study found

The research suggests that consuming fewer, smaller meals may be more effective for weight loss than restricting eating to a narrow time window.

When it comes to losing weight, how much food you eat likely matters more than the timing of your meals, according to a study published Wednesday in the Journal of the American Heart Association.

Researchers at Johns Hopkins University asked 547 people to record the size of their meals and the timing of when they ate in a mobile app daily for six months. The scientists then looked at how much the participants weighed over the course of around six years — five-plus years before they began logging their meals and roughly six months after — using electronic health records.

The study separated the recorded meals into three size categories: a small meal had fewer than 500 calories, medium meals ranged from 500 to 1,000 calories and large meals consisted of more than 1,000. On the whole, the results showed, the participants who ate the most large and medium meals gained weight over six years, whereas those who ate fewer, smaller meals lost weight.

That’s consistent with the long-standing and well-understood rule that eating fewer calories contributes to weight loss.

The researchers did not find a link between weight change and the practice of limiting food intake to a specific time window — often referred to as intermittent fasting. Nor did they find an association between weight change and the timing of a person's first meal after waking up or last meal or snack before bed.

"This study shows that changing your timing of eating is not going to prevent slow weight gain over many, many years — and that probably the most effective strategy is by really monitoring how much you eat, and by eating fewer large meals and more small meals," said Dr. Wendy Bennett, an author of the study and associate professor at Johns Hopkins Medicine.

The study included people of various weights, including those who were overweight or had severe obesity. The observed weight changes were small overall, though: People who ate an extra daily meal saw less than 1 pound of additional weight gain per year, on average, relative to people who did not eat that extra meal.

"The effect is so small, I wouldn’t tell anyone to change what they’re doing," said Courtney Peterson, an associate professor of nutrition sciences at the University of Alabama at Birmingham, who wasn’t involved in the research.

Bennett, however, said her study provides evidence that restricting meal size can be effective for weight loss, even after adjusting for people’s baseline weights. (People who weigh more tend to have an easier time gaining or shedding pounds.)

She also noted that the average person gains 1 or 2 pounds per year, which can amount to significant weight gain over time. Eating fewer large meals and more small meals, then, could "prevent that slow creep of weight gain," Bennett said.

But Peterson said she doesn't see the study as "a slam dunk" when it comes to determining the best weight loss strategy.

Other research has found that the timing of a person’s first meal of the day can matter: A study published in October found that eating earlier in the day may contribute to weight loss, perhaps because it helps people burn calories or feel fuller throughout the day.

On average, the participants in Bennett's study ate their meals during an 11.5-hour window, with their first meal less than two hours after waking up and their last around four hours before bed.

To better test whether intermittent fasting can help with weight loss, Peterson said, researchers have to directly compare people who limit their food intake to a specific window to those who do not in a controlled trial.

Prior studies with that type of design have produced mixed results. Some research suggests that fasting every other day, or restricting calories on two days per week, could help people with obesity lose weight. But other studies have found that restricting eating to certain time periods does not reduce body weight any more than restricting daily calorie intake.

"Time-restricted eating can be really helpful, I think, when it helps people restrict their calories," Bennett said. "We already know that caloric restriction is the most effective strategy for weight loss."

Peterson also emphasized that the nutritional quality of a person's diet influences whether they gain or lose weight. Consuming too much highly processed food like hot dogs, chips or soda can contribute to weight gain, whereas diets that rely on vegetables and whole grains may assist with weight loss.

"Some of our best data in humans suggests probably diet quality matters more than meal timing," Peterson said.