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A New Study Says Eating Late Slows Metabolism and Makes Fat Stick — But Is This True?

The research provides great evidence, but it's not the end of the story.

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Does eating late at night make you gain weight? Scientists have flip-flopped on the answer for a long time. Those in favor of early meals argue that eating late is not in line with your natural circadian rhythm; early eaters also say your metabolism slows down in the evening, so your body doesn’t burn as many calories from a late-night meal.

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By contrast, some experts think your dinner time makes no difference. A calorie is a calorie, they argue, and calories don’t “count more” at night. So, what’s the truth? The short answer: We don’t know for certain, but eating earlier seems to be the healthier habit. Still, it’s not likely that eating late is the primary reason you’re gaining weight (or can’t lose it).

In a new study published in the esteemed scientific journal Cell Metabolism, researchers from the Brigham and Women’s Hospital in Boston explored this topic in depth. They state that their trial was possibly the “most controlled and most comprehensive” out of all similar studies that investigate late eating. It was a randomized controlled trial — the gold standard in the scientific community — but just five women participated in the testing (11 other participants were men). We’ll get into the nitty-gritty below.

Here’s how they set up the study.

The researchers recruited 16 participants with an average age of 37 (five women, 11 men, and four people were over age 40). The average BMI was 28.7 (two participants had BMIs over 30 and were considered obese). For two to three weeks before the trial, all participants had to complete the same amount of walking and sleep the same number of hours. In the last three days before the trial, they all had to eat the same prepared meals at the exact same time.

On the day of the trial, participants were called into a lab. Each person stayed in a lab suite; everything was controlled down to the room temperature (73 degrees Fahrenheit). Then, each person in the study was randomly placed into one of two groups. In one group, participants ate prepared meals at 8 a.m., 12 p.m., and 4 p.m. In the other, participants ate the same meals a little over four hours later, at 12 p.m., 4 p.m., and 8 p.m. After a few days, the groups swapped — the early eaters ate late, and the late eaters ate early. To measure hunger hormones, the researchers took frequent blood samples. They also took biopsies of body fat tissue to determine how the body stores fat in these conditions. (Only seven participants consented to biopsies. Two were women.)

The study supports the theory that eating late contributes to weight gain.

Here’s what the researchers learned: Eating late doubled the odds of feeling hungry. It also dramatically influenced the hormones leptin and ghrelin, hormones that decrease and increase appetite, respectively. Appetite-reducing leptin decreased over 24 hours in people when they ate late, while appetite-boosting ghrelin increased. Furthermore, late eaters burned fewer calories, even though they completed the same amount of physical activity as early eaters.

As for the biopsies of fat tissue, researchers found that late eating altered the gene expressions of fat cells in late eaters. (Gene expression is when a cell activates certain genes.) Specifically, late eating increased fat cell creation and decreased fat cell breakdown.

“In this study, we asked, ‘Does the time that we eat matter when everything else is kept consistent?'” first author Nina Vujovic, PhD, said in a press release. “And we found that eating four hours later makes a significant difference for our hunger levels, the way we burn calories after we eat, and the way we store fat.”

The study had limitations, and there are conflicting theories.

While this study was tightly controlled, it was not perfect. It used only five women, because the study authors did not accept women who were menstruating. (Menstruating can naturally increase a person’s appetite-reducing leptin levels, which would have messed with the study findings.) Also, four of the participants were pre-diabetic, and diabetes makes it harder to regulate a person’s metabolism. The study authors argue that pre-diabetes didn’t significantly alter the findings, but it’s something to think about.

Also, many experts are still doubtful that eating late causes weight gain. They say true weight gain depends on how many calories you eat and burn each day — not the time of day that you eat. In addition, they argue that late eaters usually make unhealthy food choices, which is the real cause of weight gain.

What’s the bottom line?

This study provides great evidence that eating late may contribute to weight gain — but we still don’t know it for certain. Regardless of whether your meal schedule makes it harder to lose body fat, most medical professionals arrive at the same answer: Eating earlier is healthier. Indeed, many agree that you’ll sleep better if you stop eating about three hours before bedtime. You’ll also be less likely to get heartburn.

However, if you didn’t eat enough at dinner one night and find yourself hungry at 10 p.m., having a healthy snack is okay. As long as you’re getting enough physical activity during the day, a late-night bite once in a while is nothing to worry about.

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