Americans Are Eating Later, and That May Contribute to Weight Troubles

Our bodies didn’t evolve to handle midnight pizzas

Can you resist the temptation of a midnight snack? (sextoacto/iStock)
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Unable to shed those extra pounds? The problem might not only be what you're eating, but when you're eating it. A study of American dining habits suggests that much of our eating is out of sync with the ways our bodies evolved to handle food.

Volunteers used a smartphone photo app to report everything they ate and drank, including the times of day they indulged. The results show that for many people, the traditional “three square meals” a day have been replaced by late-morning pastries, mid-afternoon snacks or late-night pizzas.

Getting back on schedule could have important benefits for weight loss and other health concerns—and the same mobile technology used in the study might even be able to help.  

Scientists at the Salk Institute for Biological Studies analyzed the daily food and beverage habits of 150 participants over a three-week period. They found that the majority of those people spread out their eating across 15 hours or longer each day, consuming less than 25 percent of their calories before noon and more than 33 percent of them after 6:00 p.m.

The results are an example of how available electricity has altered human behavior in unprecedented ways, says co-author Shubhroz Gill, formerly with the Salk Institute and now at the Broad Institute of MIT and Harvard. The effect has been extensively studied in terms of how nighttime illumination effects sleep.

“But we never knew how this effects diet,” he says. “What we show is that not only do people stay up longer, they also eat late into the night.”

It's also been known that increasingly convenient access to food thanks to refrigeration and fast food restaurants has led to the consumption of excess calories.

“Our ancestors spent some calories trying to get calories, now we don't do that anymore. If you want a chicken sandwich, you just go and pick it up," he says. "But the main point for our paper is that not only are we consuming excess calories, we are consuming them later in the day. We don't have a way to measure this, but it's almost certain that our ancestors weren't staying up until 1 a.m. and consuming a lot of calories.”

This shift in meal times is a problem, Gill explains, because our metabolism goes through a series of peaks and valleys throughout the day. During millennia of evolution, these circadian rhythms evolved on a schedule meant to align our internal clocks with that of the environment around us. Natural light long guided these cycles of day and night—and now electricity has changed the game.

“We are not supposed to be consuming food at night, that's how our bodies have evolved, but now we are sort of forcing our bodies to have food when they aren't supposed to be.” Eating off schedule even creates a kind of “metabolic jet lag,” Gill adds, which can throw other circadian rhythms off schedule.

As they report today in the journal Cell Metabolism, Gill and his co-author Satchidananda Panda also explored whether their phone app could help people who want to try and eat on a more consistent time frame, and the impacts of doing so on weight loss. 

Gill and Panda asked a smaller group of eight overweight people from their initial study to adopt restricted eating hours. “We told them to pick their own 8 to 11 hours, but to be consistent every day for 16 weeks,” Gill says. “That includes weekends, when our larger study had showed that many people woke up later and thus ate later as well.”

The small group lost an average of 8 pounds in the 16 weeks, and they kept it off. “These people were extremely happy to do this longer on their own,” Gill says. “After a year, the group returned and their weight loss, on average, remained about the same.”

It's not clear exactly how the group lost weight. They weren't asked to change the types or amounts of foods they ate, but they may have consumed fewer calories simply by eating during a shorter time frame. The group also reported improved sleep, which could have been a factor, Gill says. 

“We don't know the mechanism, but we do know that at least in this very small group it seemed to work,” Gill notes.

The application of smartphone technology also worked extremely well in several ways. Because participants simply photographed everything they ate and drank, scientists gained some interesting context on how Americans are eating.

“I look at it as a window into human behavior,” Gill says. “We see people eating at their computers, we see people eating in their beds, we see people at drive-throughs—it's really the full gamut.”

The phones also helped to eliminate the self-deception that can plague dieters and dietitians alike.

“You can lie on a food diary when you write down what you're eating,” Gill says. “But when you have to take a picture of it, we're getting a much higher quality look at exactly what people are consuming.” 

The study is merely a first step and had limitations—volunteers were all healthy 21- to 55-year-olds from the San Diego area who hadn't recently managed their diets for weight loss or other goals. But scaling it up to include larger and more diverse groups should be as easy as the distribution of smartphone technology, Gill notes.

The app is currently available for those willing to contribute their data to an ongoing Salk Institute study—though only healthy individuals should participate, Gill stresses.

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