You might think you know a processed meal when you see one, but here’s some food for thought: nearly everything you can eat at the supermarket has undergone some kind of processing—such as washing, blanching, canning, drying or pasteurizing. In other words, if there is any change from the way the food began to the way it ends up on a shelf, it counts as processed.

But then there are ultraprocessed foods. Both frozen chopped spinach and canned sausages are processed, but the latter has undergone much more processing than the former. Ultraprocessed foods undergo an industrial process to move from farm to table. This often includes steps such as hydrogenation, which produces semisolid oils, and hydrolysis, which enhances flavors. These foods also have a variety of additives that help bind the ingredients together, increase their shelf life or make them more palatable.

According to some estimates, nearly 60 percent of the daily calories U.S. adults consume are from ultraprocessed foods. It’s worse for kids and teenagers, whose diet is almost 70 percent ultraprocessed.

But a growing number of studies have linked higher consumption of ultraprocessed foods to a long list of health effects, and scientists are only just beginning to understand why.

WHAT ARE ULTRAPROCESSED FOODS?

Chicken nuggets, chips and hotdogs are considered ultraprocessed, but so are things such as fruit yogurts, mass-produced bread and even some canned foods.

As a rule of thumb, these are any foods that cannot be made in an ordinary kitchen—in other words, they contain an ingredient that is not typically found in homes or one that has undergone an industrial process that a home cook would not be able to replicate.

“A whole lot of things that you could never imagine can be done [to food],” says Barry Popkin, a professor of nutrition at the University of North Carolina at Chapel Hill. “You can’t tell simply by the ingredients.” For example, he says, “it’ll be flour, but you really don’t know that wheat flour has been decomposed in such complex ways and then put back together.”

Researchers commonly use a four-part scale known as NOVA to categorize foods based on the extent of industrial processing they involve. The categories are unprocessed or minimally processed foods (which include vegetables and eggs); processed culinary ingredients (those that are usually added to food and rarely eaten alone, such as oils, butter and sugar), processed foods (those that are made from a combination of the first two groups, such as homemade bread) and ultraprocessed foods (those made with industrially modified raw ingredients and additives).

When NOVA first came about in 2009, it offered a new way of looking at food beyond its nutritional value. Take fortified breads or protein-rich cookies, for example: Compared with their unfortified equivalents, they would be considered relatively nutritious. But through the lens of NOVA, both are ultraprocessed.

Other researchers, such as Julie Hess, a nutritionist at the U.S. Department of Agriculture’s Agricultural Research Service and former vice president of scientific affairs at the National Dairy Council, contend that NOVA is not the best or most consistent way to identify an ultraprocessed food. She argues that not all ultraprocessed foods are the same, in terms of nutrition. “When we say ultraprocessed food, are we going to include things like canned beans? Are we including canned oranges and dried peaches?” Hess says. “That question of nutrient density isn’t currently reflected in the NOVA categorization system.”

Popkin is proposing another way to identify foods as ultraprocessed in a forthcoming paper. He says that having just one of 12 types of additives—including specific flavors, emulsifiers, foams, thickening agents and glazing agents—as an ingredient is a feature of all ultraprocessed foods. The presence of artificial coloring and flavorings would already be a telltale sign for about 97 percent of these foods, he says.

ARE ULTRAPROCESSED FOODS BAD FOR YOUR HEALTH?

Many people believe that eating ultraprocessed food will make them gain weight or cause a host of other health issues, and some evidence backs this up. Research has tied ultraprocessed food consumption to a slew of health conditions, including obesity, type 2 diabetes, some cancers, cardiovascular disease, and even mild depression and anxiety, but a clear mechanism for harm hasn’t been identified.

A landmark paper in 2019 was the first to show a cause-and-effect link between ultraprocessed foods and weight gain. A group of 20 healthy volunteers was confined to a ward at the National Institutes of Health Clinical Center in Bethesda, Md., where the participants were randomly assigned to receive a diet of either ultraprocessed or minimally processed food for two weeks and then were switched to the other diet for the next two weeks. For example, a person receiving the ultraprocessed diet would start their day with foods such as packaged cereal and a blueberry muffin or croissants and turkey sausages. Someone on the minimally processed diet would instead get Greek yogurt and fruit or a fresh omelet and sweet potato hash.

On average, people in the ultraprocessed diet group consumed about 500 calories more per day, compared with those in the minimally processed diet group. Participants in the former group also ate faster and gained about two pounds after two weeks. On the minimally processed diet, participants ate less and lost about the same amount of weight as they gained on the processed diet. In both settings, participants were given access to about double the number of calories they needed and were told to eat as much as they wanted.

Kevin Hall, the study’s principal investigator and a clinical researcher at the National Institute of Diabetes and Digestive and Kidney Diseases, says he designed the investigation because he thought the NOVA classification system—which doesn’t account for the nutrients contained in different foods—was “nonsense.” He says he matched the foods in both diets to have the same total amount of nutrients, including fat, carbohydrates and fiber, “because I thought the nutrients were going to drive the effects,” Hall says. “And I was wrong.”

Hess, who was not involved in the study, notes some limitations. There were “very, very different” foods in the two groups, she says—in other words, the study didn’t match the diets for quality. Hall says the two diets used different foods because it would have been “very difficult to make homemade versions of many popular ultraprocessed foods while also maintaining precise control over their nutrition content.” Hess’s own lab designed a diet in which 90 percent of the calories were from ultraprocessed foods, and it still met most national guidelines for nutrients—calling into question how useful NOVA is for determining the healthfulness of a food when existing dietary guidelines are used as a benchmark.

Others say findings such as Hall’s study suggest that processing may change how a food affects our body, independent of the nutrients that food contains. “It goes to show how much the [U.S. dietary] guidelines are focused on nutrients,” says Filippa Juul, a nutritional epidemiologist at the New York University School of Global Public Health. “You could have any food and just tune up the nutrients; it doesn’t mean the food is necessarily healthy … or has the same activity as nutrients that are in [unprocessed] foods.”

Studies have also suggested a link between higher consumption of ultraprocessed foods and a profound change in the composition of gut microbes. And an altered gut microbiome has been linked to mental health conditions.

The negative effects of these foods might also be a result of what they lack: fiber. The act of industrially processing a food can lower its fiber content, which can make one less satiated after eating it. Fiber also feeds bacteria in the gut, and the absence of this nutrient may explain the link between diet, depression and gut health, too.

“There are probably some subcategories [of ultraprocessed foods] that are perfectly fine—maybe even really good for you—and others that are particularly damaging,” Hall says. “I just don’t think we know which ones [are which].” Part of the problem with ultraprocessed foods is that they’re often packed with calories yet leave us craving more.

WHY DO WE LIKE ULTRAPROCESSED FOODS SO MUCH?

Scientists still don’t know for sure why humans gravitate toward ultraprocessed foods. One hypothesis, according to Hall, is that we might not be able to resist their combination of ingredients. Think about the last time you ate just one chip out of a bag—it’s almost impossible not to eat more.

In a 2021 study Hall attempted to compare a low-carbohydrate diet with a high-carbohydrate one to examine the effect on energy intake. When people were presented with meals that were high in both fat and sugar, fat and salt or carbohydrates and salt, people tended to eat more calories, he says. “These are so-called hyperpalatable foods,” Hall adds.

Such foods essentially have artificially enhanced palatability that exceeds the palatability any ingredient could produce on its own—in other words, they have a combination of fat, salt or sugar “that would never exist in nature,” Juul says. Previous research has shown that foods combining fat and carbohydrates were better at activating the brain’s reward system than foods with just one of those ingredients. The ultraprocessed meals in Hall’s study also had more calories per bite than the minimally processed diet.

Some researchers hypothesize that certain foods are addictive. People don’t lose control over eating bananas, but with ultraprocessed foods, they show all the hallmarks of addiction, says Ashley Gearhardt, a professor of psychology and a nutritionist at the University of Michigan. Addictive drugs activate the striatal dopamine system—the brain’s pleasure center—by creating a dopamine spike followed by a rapid crash. “It’s like a quick hit that isn’t sustaining,” Gearhardt says. Ultraprocessed foods mimic nicotine and ethanol in the magnitude of that effect in the brain.

“That makes sense because the reward system of the brain was really shaped by the need to get calories,” Gearhardt says. The addictive agent in food could be one of many things, she says—taste, smell, sugar, fat and additives are all potential culprits. Studies in animals have shown that stopping the consumption of ultraprocessed foods—much like other addictive substances—elicits withdrawal symptoms such as anxiety and agitation.

SHOULD ULTRAPROCESSED FOOD BE REGULATED?

There are people who want to do away with ultraprocessed foods altogether and others who say there are not enough data to warrant any action, according to Hall. “It’s not realistic to say, ‘Well, we’re just going to cut out 50 percent of the food,’” he says. “Who’s going to make everybody’s meals?” Ultraprocessed foods are a lot cheaper and more convenient than less processed ones, Hall says. In his study, the minimally processed meals cost 40 percent more to buy and took the chefs longer to prepare.

Spending hours hunched over a kitchen bench to churn butter is not the answer. But reducing consumption of ultraprocessed foods doesn’t mean we have to make everything from scratch.

“There’s an enormous number of things you can do,” says Popkin, who eats unprocessed foods apart from an occasional iced tea sweetened with the sugar substitute Splenda. “There’s a hell of a lot of packaged real food out there.” He suggests looking for minimally processed options that make cooking faster, such as a salad mix or chopped vegetables.

We have to do our best to make healthy choices, Gearhardt says, but everything is stacked against us. As a food scientist herself, she leaves the grocery store befuddled. “It’s easy to say we should just tell the individual to do better while everything in the environment is set up for the industry to profit,” she says.

In an ideal world, we would focus on making healthy alternatives convenient and affordable and reducing marketing to kids, Gearhardt says. “We need to take some courageous action and have some common sense that this food environment is not good for anybody,” she adds.