How bad are ultra-processed foods really?

In the mid-1990s, Carlos Monteiro, a nutritional epidemiologist in Brazil, noticed something alarming: obesity rates among children in his country were rising rapidly.

To understand why, he and his colleagues at the University of São Paulo examined data on the food buying patterns of Brazilian households to see if they had changed in recent years. The researchers found that people bought less sugar, salt, cooking oils and staples such as rice and beans, and more processed foods such as soft drinks, sausages, instant noodles, packaged breads and biscuits.

To describe this second category of foods, the team created a new term: ultra-processed foods, or UPF, which they would later link to weight gain in children and adults in Brazil.

Since then, scientists have found associations between UPFs and a range of health conditions, including heart disease, type 2 diabetes, obesity, gastrointestinal disease and depression, as well as earlier death.

That’s worrying, experts say, as ultra-processed foods have become an important part of people’s diets around the world. They represent 67 percent of the calories consumed by children and adolescents in the United States, for example.

But many questions remain. What exactly are ultra-processed foods? And how strong is the evidence that they are harmful? We asked the experts to answer these and other questions.

What are ultra-processed foods?

In order to study foods based on how they were processed, Dr. Monteiro and his colleagues developed a food classification system called Nova, named after the Portuguese and Latin words for new. Since then, it has been adopted by researchers around the world.

The Nova system classifies foods into four categories:

• Unprocessed or minimally processed foodssuch as fresh or frozen fruits and vegetables, beans, lentils, meat, poultry, fish, eggs, milk, plain yogurt, rice, pasta, cornmeal, flour, coffee, tea, and herbs and spices.

• Processed culinary ingredientssuch as cooking oils, butter, sugar, honey, vinegar and salt.

• Processed foods it is made by combining Category 1 foods with Category 2 ingredients and preserving or modifying them using relatively simple methods such as canning, bottling, fermentation and cooking. This group includes freshly baked bread, most cheeses and canned vegetables, beans and fish. These foods may contain preservatives that extend shelf life.

• Ultra-processed foods made with industrial methods and ingredients you wouldn’t normally find in grocery stores, including high-fructose corn syrup, hydrogenated oils, and protein concentrates like soy isolate. They often contain additives such as flavorings, colorings or emulsifiers to make them appear more attractive and palatable. Think soft drinks and energy drinks, potato chips, candy, flavored yogurts, margarine, chicken nuggets, hot dogs, deli meats, lunch meats, boxed macaroni and cheese, infant formula and most packaged breads, plant-based milks, milk replacers meat and cereals for breakfast.

If you look at the ingredient list and see things you wouldn’t use in home cooking, it’s probably an ultra-processed food, said Brenda Davy, a professor of nutrition at Virginia Tech.

The Nova system does not classify foods based on nutrients such as fat, fiber, vitamins or minerals. It’s nutrition agnostic, said Maya Vadiveloo, associate professor of nutrition at the University of Rhode Island.

That’s led to a debate among nutrition experts about whether it’s useful in describing a food’s healthfulness, in part because many UPFs like whole-wheat bread, flavored yogurts and infant formulas can provide valuable nutrients, said Dr. . Vadiveloo

Are ultra-processed foods harmful?

Most of the research linking UPFs to poor health is based on observational studies, in which researchers ask people about their diet and then track their health over many years. In a large review of studies that was published in 2024, scientists reported that consuming UPF was associated with 32 health problems, with the most compelling evidence for deaths related to heart disease, type 2 diabetes and common problems of mental health such as anxiety and depression.

Such studies are valuable because they can look at large groups of people, the 2024 review included results from nearly 10 million over the many years it can take for chronic health conditions to develop, said Josiemer Mattei, associate professor of nutrition at Harvard TH. Chan School of Public Health. He added that the consistency of the link between UPFs and health problems increased his confidence that there was a real problem with the food.

But observational studies also have limitations, said Lauren OConnor, a nutrition scientist and epidemiologist who formerly worked at the Department of Agriculture and the National Institutes of Health. It’s true that there is a correlation between these foods and chronic disease, he said, but that doesn’t mean UPFs directly cause poor health.

Dr. OConnor wondered if it’s useful to lump foods as different as Twinkies and breakfast cereal into one category. Some types of ultra-processed foods, such as soft drinks and processed meats, are more clearly harmful than others. UPFs like flavored yogurts and whole grain breads, on the other hand, have been associated with a reduced risk of developing type 2 diabetes.

Clinical trials are needed to test whether UPFs directly cause health problems, Dr. O’Connor said. Only one such study has been done, and it was small and had some limitations, he said.

In this study, published in 2019, 20 adults of various body sizes lived in a research hospital at the National Institutes of Health for four weeks. For two weeks they ate mostly unprocessed or minimally processed foods, and for another two weeks, they ate mostly UPF. The diets had similar amounts of calories and nutrients, and participants could eat as much as they wanted at each meal.

During their two weeks on the ultra-processed diet, the participants gained an average of two pounds and consumed about 500 more calories per day than they did on the unprocessed diet. During their time on the unprocessed diet, they lost about two pounds.

This finding could help explain the link between UPF, obesity and other metabolic conditions, said Kevin Hall, a nutrition and metabolism researcher at the National Institutes of Health, who led the trial. But the study needs to be replicated, which Dr. Hall is in the process of doing now.

Why can UPFs be harmful?

There are many strong opinions about why ultra-processed foods are unhealthy, said Dr. Hall. But there really isn’t a lot of rigorous science about what those mechanisms are, he added.

Because UPFs tend to be cheap, convenient and accessible, they’re likely displacing healthier foods from our diets, said Dr. Hall.

But he and other scientists think food could have more direct effects on health. It can be easy to overeat because they contain hard-to-resist combinations of carbohydrates, sugars, fat and salt, are high in calories and easy to chew. It’s also possible that the resulting blood sugar spikes can damage arteries or increase inflammation, or that certain food additives or chemicals can interfere with hormones, cause leaky gut, or alter the gut microbiome.

Researchers, including Drs Hall and Davy, are starting to conduct small clinical trials that will test some of these theories. Such studies can help identify the most harmful UPFs and even suggest how to make them healthier, said Dr. Hall.

But most researchers think there are several ways in which foods cause harm. Rarely in nutrition is there a single factor that fully explains the relationship between food and some health outcome, Dr. Vadiveloo said.

What should we do with ultra-processed foods?

In 2014, Dr. Monteiro helped write new dietary guidelines for Brazil that advised people to avoid ultra-processed foods.

Other countries such as Mexico, Israel and Canada have also explicitly recommended avoiding or limiting UPF or highly processed foods. The U.S. Dietary Guidelines do not contain such advice, but an advisory committee is investigating evidence on how UPFs can affect weight gain, which could influence the 2025 guidelines.

It’s hard to know what to do with UPFs in the United States, where so many foods are already ultra-processed and lower-income people may be especially dependent on them, said Dr. Hall.

After all, they are an important source of food, and food is food, Dr. Mattei added. We can’t really vilify them, he said.

While research continues, expert opinions differ on how people should approach UPFs. Dr. Monteiro said it’s safest to avoid them altogether, swapping flavored yogurt for plain yogurt with fruit, for example, or buying fresh bread from a local bakery instead of packaged bread, if you can afford it.

Dr. Vadiveloo suggested a more moderate strategy, focused on limiting UPFs that don’t provide valuable nutrients, such as sodas and crackers. He also recommended eating more fruits, vegetables, whole grains (ultra-processed or not), legumes, nuts and seeds.

Cook at home as much as you can, using minimally processed foods, Dr Davy said. We can’t really say anything beyond that at this point.