Does eating ultraprocessed food affect weight gain? It’s complicated

A new diet study is highly controlled, but still has conflicting results

ultraprocessed dinner

NOT HOMEMADE  Participants in a new nutrition study ate more calories when on a diet consisting of ultraprocessed foods, including this meal of precooked steak, canned corn and chocolate milk.

K. Hall et al/Cell Metabolism 2019

Nutrition advice can be confusing. Studies that bolster the health benefits of a food or nutrient seem inevitably to be followed by other work undercutting the good news.

One reason for the muddle is that nutrition studies sometimes depend on people’s self-reporting of past meals. And because people may forget or even lie about what they’ve been consuming, that data can be flawed, creating conflicting reports about what’s healthy and what’s not, research has shown.

But even if people had a photographic memory of all of their meals, that alone wouldn’t provide enough information. How bodies react to and process food can vary widely from person to person and be dependent on genes, the microbes that live inside the gut, a person’s current health, what the food contains or even how it was made (SN: 1/9/16, p. 8).

“The problem is that nutrition research is rocket science,” says David Ludwig, a pediatric endocrinologist at Boston Children’s Hospital. “There are potentially thousands of different nutrients and factors in food that could influence our biology or our senses as we eat. Those can interact in unpredictable and complicated ways.”

Given the complexity that comes with researching diet, one approach is to study people in a controlled environment, so that researchers know exactly what the participants are eating. A study that tied eating highly processed foods to weight gain, published online May 16 in Cell Metabolism, did just that. Here’s what the researchers learned — and what they still can’t answer.

‘Ultraprocessed’ vs. whole foods

Ultraprocessed foods include packaged snacks, premade canned or frozen dishes and soft drinks. They often contain various additives, such as flavors and colors, to enhance their palatability, and have ingredients processed from industrial farming staples like corn, soy or wheat, all melded together as if in a chemistry experiment. Whole foods, in contrast, are those that are in their original state or are minimally changed.

Previous research has suggested that people who eat ultraprocessed foods are at higher risk for obesity or cancer. Why isn’t clear. It may have something to do with the fact that ultraprocessed foods usually contain more sugar, carbohydrates, salt and fat than whole foods.

So physiologist Kevin Hall and colleagues conducted a highly controlled experiment in which 20 people, 10 men and 10 women, lived on-site at the National Institute of Health’s campus in Bethesda, Md. Each person stayed for four weeks.

The researchers randomly selected the study participants to receive either a diet of whole foods for two weeks or a diet made of ultraprocessed foods for two weeks. Each person got the alternate diet for the next two weeks. The team designed the meals such that, no matter which diet, each meal offered the same amount of calories, sugar, fat, fiber and other nutrients. Participants could eat as much or as little as they wanted for up to an hour.

“I was skeptical that we would see any difference in how many calories people ate, which was the primary interest of our study,” says Hall, of the National Institute of Diabetes and Digestive and Kidney Diseases. But the team did. When people were on the ultraprocessed diets, they ate about 500 kilocalories more per day than when they ate whole foods, and gained on average about a kilogram (roughly two pounds).

That finding suggests that there may be something about ultraprocessed foods not related to added sugar or fat that can lead to overeating, Hall says. People on the ultraprocessed diets ate more quickly, for example, so perhaps that speed disrupted the molecular signaling that tells a person “stop eating, because you’re full.”

Unanswered questions

Though everyone in the study ate from precisely the same menu, it turned out that there was still a lot of variation in people’s individual responses to the two diets. Nine people ate many more calories, up to 1,500 kilocalories per day more, while on the ultraprocessed diet compared with the whole foods diet. Eleven people gained more weight on the ultraprocessed diet, as much as six kilograms, while a few saw no weight change between the diets.

“We don’t know what drove those differences,” Hall says, although the researchers could tell it wasn’t related to a person’s body size or sex.

And the study didn’t find an answer for how ultraprocessed foods may cause some to overeat. Hall and his colleagues are planning another study with a similar design but with tinkered meals to try to figure that out.

There’s also a limit to how generalizable the results are to a wider population. For example, the study didn’t include people with diabetes or heart disease, Hall says. “It’s likely and possible that there might be differences in these different groups of individuals.”

Such controlled settings, while needed in dietary research, also remove people from how they function in real life, says Ludwig, who wasn’t involved in the new study. That artificial environment “affects eating behavior in many ways: There’s social isolation, stress, boredom and the fact that the foods are prepared in a laboratory.” While these types of studies “are interesting and helpful, they’re not the whole story,” he says.

Weighty issues

The stakes of nutrition research are very clear, even if nutrition advice sometimes isn’t. Nearly 40 percent of U.S. adults are obese, according to 2015–2016 figures from the U.S. Centers for Disease Control and Prevention. Excess weight increases the risk of type 2 diabetes, heart disease, stroke, kidney disease and other disorders.

The growth of the industrialized food system parallels the rise in obesity and related health problems in the country.  Ultraprocessed foods are the majority of calories eaten in the United States, comprising nearly 58 percent of energy taken in, according to a study published in 2016 in BMJ Open (based on data from self-reported, 24-hour diet recall).

To the extent that a person can lay off the ultraprocessed foods, Hall says his study suggests “that might help with managing your body weight.” But it’s not the last word, as obesity is complex, too. It’s a disorder marked by a host of factors — genetic, metabolic, hormonal, psychological, behavioral, environmental, economic and societal, Hall says.

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