Why researchers say eating late at night is an especially bad idea

Welcome to Dear Julia, a column where readers submit everyday health questions. Which over-the-counter painkillers work best? Is it better to run or walk for exercise? Julia Belluz sifts through the research and consults with experts in the field to figure out how science can help us live happier and healthier lives.

Dear Julia: What does research say about eating meals late at night?

If your friends are anything like young people anywhere, there's a good chance that after a night out they stop for a snack on the way home. In Toronto, street kebabs or hot dogs were popular. New Yorkers seem to prefer pizza slices, while I noticed that Romans often opt for freshly baked cornetti (the Italian answer to croissants) and people in Tokyo like fried cutlets with rice and curry sauce.

These snacks are undoubtedly delicious — but also not a very good idea.

Researchers are finding that eating late at night seems to be particularly bad for your health, especially when it comes to weight control and diabetes.

To understand the research in humans, let's first look at the science on mice. For one 2009 paper published in the journal Obesity, researchers took mice, which are normally nocturnal, and split them into two groups. One group ate a high-fat diet during the night (as they normally would), and the second group were fed during the day (overturning their usual eating and sleeping habits).

The mice that ate during off-hours gained significantly more weight over time, even though all the mice ate about the same amount of food:

Obesity

This research suggested that unusual feeding times in mice increased the risk of weight gain.

"The same sort of the same thing happens in humans," explains Steven Shea, director of the Oregon Institute of Occupational Health Sciences. "When people eat more at night, they put on more weight."

For a 2013 study in the International Journal of Obesity, researchers followed more than 420 overweight and obese people who were undergoing a 20-week weight loss program. The researchers tracked the timing of meals, and found that those who ate their most major meal later in the day (after 3 pm) lost less weight than those who ate most of their calories before 3 pm.

People who ate most of their calories earlier in the day lost more weight than those who ate later.

"One of the most surprising findings in our study was that there were no significant differences in total energy intake or energy expenditure between late and early eaters despite the disparity in weight-loss magnitude," the researchers wrote.

Another randomized study, published in 2013, assigned a group of overweight and obese people to eat either a big breakfast or a big dinner (and then fewer calories at the opposite end of the day). Those who ate more earlier in the day lost more weight than those who ate big dinners.

The experimental findings were reflected in an observational study from 2011. This one followed 52 people for a week, tracking their sleep and food patterns. People who tended to sleep and wake later ate the bulk of their calories after 8 pm, more calories overall, and fewer servings of fruits and vegetables.

"Interestingly," the study authors noted, "even after controlling for sleep duration and timing, consuming calories in the evening was associated with a higher [body mass index]."

Besides weight loss, the other major finding is that late eating is associated with a higher risk of diabetes. Glucose tolerance is how well your body can take sugar from your bloodstream into your tissues. Impaired glucose tolerance is a risk factor for diabetes.

"What we have found is that if you eat an identical meal in the morning or evening, the glucose response is relatively impaired in the evening compared to the morning," explained Dr. Frank Scheer, a neuroscientist at Brigham and Women’s Hospital and an associate professor at Harvard Medical School. "In the evening hours, we can't cope with the glucose load as well."

Researchers don't know why this happens — but they have some good guesses

The researchers I spoke to all said that the science on timing of meals is still pretty preliminary and limited. Most of the research has been done on mice and humans. Many of the human studies are small or need to be repeated in different settings on different groups to make sure they hold up. And the researchers aren't sure why they're seeing these negative effects associated with late eating. But they've come up with a few potential theories.

First is that the circadian system — the body's internal clock, which is repeated in every single cell — seems to regulate how the body copes with food at different times of the day. "Your body is a very different machine when you’re awake or asleep, and it's not just because you’re awake or asleep," explained Scheer. "In the evening hours, primarily because of the influence of the circadian system, the body can't cope with the glucose load as well."

A second theory is that when you're eating at a time when you should be sleeping, it leads to what researchers call "internal desynchrony." This is when the different cellular clocks in your body are not synchronized, and this disruption may underlie negative metabolic consequences, like obesity and diabetes. (This research has only been done in animals so far.)

The other potential explanation is also related to how your body burns up energy at different times of the day. There's the "basal metabolic rate" — how you expend energy when you're resting. There's "physical activity–related energy expenditure," which is just what it sounds like. And there's "diet-induced thermogenesis" — the amount of energy your body uses in order to digest and absorb food. That last one accounts for about 10 percent of the body's overall energy expenditure.

Scheer, together with colleague Christopher Morris and others, looked at whether people expend more energy processing a meal later in the day versus earlier. They found that, independent of physical activity, the body uses less energy processing food later in the day.

"The amount of energy expended on food intake in the first two hours after the meal is only half if you eat it at 8 pm as compared to 8 am," Scheer explained.

Oregon's Shea also thinks there may be an evolutionary explanation for why we gain more weight from food at night. In his research, he's found that people seem to be hungrier at night compared with the morning. He guesses this could be carried over from our hunter-gatherer days, when humans didn't have ready access to food as soon as they woke. When they did finally find food later in the day, the body was designed to very efficiently store it overnight until the next feeding.

"Evolutionarily speaking, we have a situation where we're designed to be hungrier in the evening, and we lay down the food energy stores for the following day," Shea said. People who are trying to control their weight today are "victims of this system," he said. In other words, we're being led to eat at a time when we're most likely to store that food in a way that'll fatten us up.

This doesn't mean you need to gorge on breakfast

The researchers were hesitant to say how to apply this knowledge. And that's for good reason. The science on when to eat and how it affects diet is pretty complicated and new. The scientists did suggest that for people who are trying to lose weight, eating most calories earlier in the day and then consuming less at night is probably a good idea.

This doesn't mean you need to gorge on breakfast. The research on breakfast as a weight control tool is actually pretty lackluster. Overall, studies suggest breakfast doesn't deserve the health halo it's been given, and that eating a morning meal has little effect on weight loss.

This also doesn't mean you need to skip dinner. Remember, even people who lost more weight in studies ate dinner, albeit lighter suppers compared with their lunches and breakfasts.

For now we know one thing for sure: Calories are still just calories, but when you eat them may have little-appreciated implications for health.

Have a question? Use our submission form or ask @juliaoftoronto on Twitter.

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