It's Not Just Salt, Sugar, Fat: Study Finds Ultra-Processed Foods Drive Weight Gain

Originally heard on All Things Considered

Over the past 70 years, ultra-processed foods have come to dominate the U.S. diet. These are foods made from cheap industrial ingredients and engineered to be super-tasty and generally high in fat, sugar and salt.

The rise of ultra-processed foods has coincided with growing rates of obesity, leading many to suspect that they've played a big role in our growing waistlines. But is it something about the highly processed nature of these foods itself that drives people to overeat? A new study finds the answer is yes.

The study, conducted by researchers at the National Institutes of Health, is the first randomized, controlled trial to show that eating a diet made up of ultra-processed foods actually drives people to overeat and gain weight compared with a diet made up of whole or minimally processed foods. Study participants on the ultra-processed diet ate an average of 508 calories more per day and ended up gaining an average of 2 pounds over a two-week period. People on the unprocessed diet, meanwhile, ended up losing about 2 pounds on average over a two-week period.

"The difference in weight gain for one [group] and weight loss for the other during these two periods is phenomenal. We haven't seen anything like this," says Barry Popkin, a nutrition professor at the University of North Carolina who has studied the role of ultra-processed foods in the American diet but was not involved in the current research.

Dariush Mozaffarian, dean of Tufts University's Friedman School of Nutrition Science and Policy, agrees that the findings are striking. He says what was so impressive was that the NIH researchers documented this weight gain even though each meal offered on the two different diets contained the same total amount of calories, fats, protein, sugar, salt, carbohydrates and fiber. Study participants were allowed to eat as much or as little as they wanted but ended up eating way more of the ultra-processed meals, even though they didn't rate those meals as being tastier than the unprocessed meals.

"These are landmark findings that the processing of the foods makes a huge difference in how much a person eats," says Mozaffarian. That's important, because the majority of foods now sold in the U.S. — and increasingly, around the globe — are ultra-processed.

And ultra-processed foods include more than just the obvious suspects, like chips, candy, packaged desserts and ready-to-eat meals. The category also includes foods that some consumers might find surprising, including Honey Nut Cheerios and other breakfast cereals, packaged white bread, jarred sauces, yogurt with added fruit, and frozen sausages and other reconstituted meat products. Popkin says ultra-processed foods usually contain a long list of ingredients, many of them made in labs. So, for example, instead of seeing "apples" listed on a food label, you might get additives that re-create the scent of that fruit. These are foods designed to be convenient and low cost and require little preparation.

The new research, which appears in the journal Cell Metabolism, was led by Kevin Hall, a senior scientist at the National Institute of Diabetes and Digestive and Kidney Diseases. Hall says he was surprised by his findings, because many people have suspected that it is the high salt, sugar and fat content in ultra-processed foods that drives people to gain weight. But "when you match the diets for all of those nutrients, something about the ultra-processed foods still drives this big effect on calorie intake," Hall says.

To conduct the study, Hall and his colleagues recruited 20 healthy, stable-weight adults — 10 men and 10 women — to live in an NIH facility for a four-week period. All their meals were provided for them.

Participants were randomly assigned to one of two diets for two-week stretches: One group was fed an unprocessed diet full of whole or minimally processed foods like stir-fried beef with vegetables, basmati rice and orange slices. The other group ate an ultra-processed diet of meals like chicken salad made with canned chicken, jarred mayonnaise and relish on white bread, served with canned peaches in heavy syrup. When the two weeks were up, the groups were then assigned to the opposite diet plan.

Even though the study was small, it was also highly controlled. Researchers knew exactly how many macronutrients and calories participants were eating — and burning, because they took detailed metabolic measurements. The scientists tracked other health markers too, including blood glucose levels and even hormone levels. Hall notes that this makes these kinds of studies extremely difficult and expensive to carry out. But the study design also makes the findings that much more significant, Popkin and Mozaffarian both say.

"Putting people in a controlled setting and giving them their food lets you really understand biologically what's going on, and the differences are striking," says Mozaffarian.

For one thing, previous studies have linked an ultra-processed diet to weight gain and poor health outcomes, like an increased risk for several cancers and early death from all causes. But these studies were all observational, which means they couldn't prove that ultra-processed foods caused these outcomes, only that they were correlated. 

Hall says the new study wasn't designed to see what exactly it is about ultra-processed foods that drives overeating, but the findings do suggest some mechanisms.

"One thing that was kind of intriguing was that some of the hormones that are involved in food intake regulation were quite different between the two diets as compared to baseline," Hall says.

For example, when the participants were eating the unprocessed diet, they had higher levels of an appetite-suppressing hormone called PYY, which is secreted by the gut, and lower levels of ghrelin, a hunger hormone, which might explain why they ate fewer calories. On the ultra-processed diet, these hormonal changes flipped, so participants had lower levels of the appetite-suppressing hormone and higher levels of the hunger hormone.

Another interesting finding: Both groups ate about the same amount of protein, but those on the ultra-processed diet ate a lot more carbs and fat. There is a concept, called the protein leverage hypothesis, that suggests that people will eat until they've met their protein needs. Hall says that this seems to be the case in this study and it partially explains the difference in calorie consumption they found. Even though the meals were matched for calories and nutrients, including protein, the ultra-processed meals were more calorie dense per bite. In part, that's because ultra-processed foods tend to be low in fiber, so researchers had to add fiber to the beverages served as part of these meals to match the fiber content of the unprocessed diet. That means participants on the ultra-processed diet might have had to munch through more carbs and fat to hit their protein needs.

And one last finding of note: People ate much faster — both in terms of grams per minute and calories per minute — on the ultra-processed diet. Hall says it might be that, because the ultra-processed foods tended to be softer and easier to chew, people devoured them more quickly, so they didn't give their gastrointestinal tracts enough time to signal to their brains that they were full and ended up overeating.

Hall says his findings have implications for the diet wars — vegan versus low-carb or low-fat diets. "They all have something in common. ... Proponents of healthy versions of those diets suggest that people cut out ultra-processed foods." He says that this elimination might account for at least part of the success that people have on these diets.

Popkin says the take-home message for consumers is, "We should try to eat as much real food as we can. That can be plant food. It can be animal food. It can be [unprocessed] beef, pork, chicken, fish or vegetables and fruits. And one has to be very careful once one begins to go into other kinds of food."

But Popkin says the findings also present a challenge for the global food industry: how to preserve the convenience, abundance and low cost of food without sacrificing health. "Let's see if they can produce ultra-processed food that's healthy and that won't be so seductive and won't make us eat so much extra," he says. "But they haven't yet."