Processed Foods Are Bad for Weight Loss, Study Shows

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You've probably heard the message that fresh, whole foods are healthier for you than processed, packaged ones. But some processed foods are worse for you than others. Studies have linked diets that include a lot of foods considered “ultra-processed” to an increased risk of cancer and early death. And experts think that our increased consumption of these foods over the past 50 years has something to do with the rising obesity rates over the same period.

A new study published in the journal Cell Metabolism shows that eating an ultra-processed diet vs. a whole-food one (rich in fruits, vegetables, whole grains, lean meats) increases calorie consumption and leads to weight gain.

The study is notable in that it is the first randomized control trial—the kind of study thought to be the best for assessing cause and effect relationships—to compare these two types of diets. In addition, the researchers designed the two diets so that the calories, fat, fiber, sugars, protein, and other nutrients would closely match.

“The purpose of our study was to try to answer the question of whether it’s the nutrient composition of the foods or something about the processing of them that’s leading people to overeat,” says the lead study author, Kevin Hall, Ph.D., a senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases. “I thought if we matched for those nutrients, there would be very little difference in calorie intake. I was wrong.”

Processed vs. Ultra-Processed

“Ultra-processed” has no set definition, either among scientists or by a regulatory agency like the Food and Drug Administration. For the purposes of this study, the researchers used a food classification system developed by scientists in Brazil, called NOVA.

This system groups foods into four categories, from unprocessed or minimally processed to ultra-processed. Ultra-processed foods are those with long lists of ingredients that include things like added sugar, salt, dyes, flavorings, emulsifiers, and high-fructose corn syrup.

“They are designed to be tasty, plus they are cheap, convenient, and easy to make,” Hall says.

“We know that ultra-processed foods contain lots of refined carbohydrates, added sugars, fat, and salt,” says Qi Sun, M.D., Sc.D., an assistant professor of nutrition at the Harvard T.H. Chan School of Public Health. “That combination is highly palatable, and it can promote overeating.”

What the Study Found

Twenty people spent one month living at the National Institutes of Health (NIH) Metabolic Clinical Research Unit, where they were served three meals plus snacks throughout the day and allowed to eat as much or as little as they wanted.

For two weeks, one group of participants ate a diet made up almost entirely of ultra-processed foods, and the other group ate a mostly unprocessed, whole-foods diet. Then they switched.

Both diets were rated by the study subjects as equally palatable. During the two weeks on the ultra-processed diet, subjects ate an average of 508 more calories per day than they did during the two weeks on the whole-foods diet. Not surprising—given the calorie difference—they gained an average of 2 pounds on the ultra-processed diet and lost an average of 2 pounds on the whole-foods one.

Why We Overeat Ultra-Processed Foods

There are many theories about why we tend to consume more calories when we eat ultra-processed foods. It could simply be because they’re cheap, tasty, and require little or no work to go from package to plate.

But findings from the new study suggest it mighty be more than that.

“One interesting result, that warrants further study, is that we saw changes in some of the hormones involved in appetite regulation,” says Hall. While eating the whole-foods diet, levels of the appetite-suppressing hormone PYY increased significantly and levels of the hunger hormone, ghrelin, decreased. “There could be something about how the gut processes whole foods that supports these beneficial hormone changes.” 

In addition, people in the study ate the ultra-processed foods faster than they did the unprocessed ones, to the tune of about 17 more calories per minute.

It could be that many ultra-processed foods included in the study (such as a Spam sandwich on white bread and Chef Boyardee ravioli) require less effort to eat than the unprocessed foods subjects were given (such as a spinach salad with chicken breast, diced apples, bulgur, and sunflower seeds). “It can take a lot more time to chew and break down whole foods compared with highly refined processed foods,” says Sun. “And when you eat food more quickly, your brain doesn’t get the signal that you’re full fast enough.”

What the Study Doesn't Tell Us

This was a relatively small study that lasted only a month. And while the carefully controlled environment meant the researchers were able to measure exactly what the subjects ate, it hardly replicated a real-life diet. “Very few people probably eat all one extreme or the other in terms of processed and unprocessed foods,” says Hall.

And although the researchers tried to match the two diets’ nutrient profiles as closely as possible, they couldn’t do it with food alone. In order to increase the fiber content of the ultra-processed diet, they had to include juice and lemonade supplemented with fiber. The primary beverage for the unprocessed diet was plain water.

“Your body doesn’t respond to liquid calories the same way it does solid food,” says Hall. “You won’t feel as full or satiated from drinking your calories, so you will tend to consume more.” So we don’t know whether people would have eaten even more (or less) of the ultra-processed foods if the fiber-supplemented drinks weren’t included.

The Bottom Line: Eat More Whole Foods

Even if all the reasons why are still unclear, this study does indicate that a diet of ultra-processed foods leads to weight gain. “The best advice is to limit your intake of ultra-processed foods as much as possible,” says Sun. But he also cautions against getting too caught up in whether it’s better to eat a diet that’s low-carb, high-protein, keto, paleo, etc. Instead, he urges people to focus on the quality of the food they eat. “What all healthy diets have in common is they emphasize eating whole, unprocessed foods,” says Sun, “like fruits, vegetables, and whole grains."

It should be fairly easy to recognize (and ideally avoid) most ultra-processed foods. Start by looking at the ingredients on the package and eliminate anything with a long list of things that sound more like chemicals than food.

Here are some specific examples from the two diets in the study. (If you're reading this article on your smartphone, rotate it to landscape mode to view the table below better.)

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