Poor health is clearly associated with them. Scientists are just now starting to piece together the why.
An alarming trend emerged in the mid-1990s, when Brazilian nutritional researcher Carlos Monteiro saw a dramatic increase in childhood obesity.

In order to comprehend the reason for this, he and his coworkers at the University of São Paulo tested data on Brazilian households’ food purchasing habits to determine if they had evolved over the past several years. Soda, sausages, instant noodles, packaged breads, cookies, and sugar were among the processed goods that individuals bought more often than basic essentials like rice and beans, as well as cooking oils and sugar.
According to Dr. Monteiro, the group redefined the term “ultraprocessed foods” (UPFs) and published it in the scientific literature to characterize the second group of foods. Eventually, they would find that UPFs caused both children and adults in Brazil to put on extra pounds.
Several other health issues, such as diabetes type 2, obesity, heart disease, gastrointestinal illnesses, depression, and premature death, have been linked to UPFs since then.
This is worrisome, according to specialists, because ultraprocessed foods are now prevalent in people’s diets all around the globe. They account for 67 percent of the calories consumed by children and teenagers in the United States, for example.
But many questions remain. What are ultraprocessed foods, exactly? And how solid is the evidence that they’re harmful? We asked experts to answer these and other topics.
What are ultraprocessed foods?
In order to examine foods depending on how they were processed, Dr. Monteiro and his colleagues established a food classification system called Nova, named after the Portuguese and Latin meanings for “new.” It has now been embraced by scholars across the world.
The Nova method organizes foods into four categories:
• Unprocessed or minimally processed foods, like fresh or frozen fruits and vegetables, beans, lentils, meat, chicken, fish, eggs, milk, plain yogurt, rice, pasta, maize meal, flour, coffee, tea and herbs and spices.
• Processed culinary items, such as cooking oils, butter, sugar, honey, vinegar and salt.
• Processed foods manufactured by mixing foods from Category 1 with the elements of Category 2 and preserving or changing them using relatively basic procedures including canning, bottling, fermentation and baking. This group includes freshly baked bread, most cheeses and canned vegetables, beans and seafood. These foods may contain preservatives that extend shelf life.
• Ultraprocessed foods created using industrial methods and substances you wouldn’t generally find in grocery shops — such high-fructose corn syrup, hydrogenated oils and concentrated proteins like soy isolate. They typically contain chemicals like flavorings, colorings or emulsifiers to make them appear more beautiful and tasty. Think sodas and energy drinks, chips, candies, flavored yogurts, margarine, chicken nuggets, hot dogs, sausages, lunch meats, boxed macaroni and cheese, baby formulas and most packaged breads, plant milks, meat substitutes and morning cereals.
“If you look at the ingredient list and you see things that you wouldn’t use in home cooking, then that’s probably an ultraprocessed food,” said Brenda Davy, a nutrition professor at Virginia Tech.

The Nova method notably doesn’t classify foods based on components like fat, fiber, vitamins or minerals. It’s “agnostic to nutrition,” said Maya Vadiveloo, an associate professor of nutrition at the University of Rhode Island.
That has led to discussion among nutrition experts about whether it’s effective for characterizing the healthfulness of a product, partially as many UPFs — like whole grain breads, flavored yogurts and infant formulae — can deliver significant nutrients, Dr. Vadiveloo said.
Are ultraprocessed meals harmful?
Most information linking UPFs to poor health is based on observational studies, in which researchers interview people about their diets and then study their health over several years. In a major analysis of papers that was published in 2024, experts reported that drinking UPFs was associated with 32 health problems, with the most strong evidence for heart disease-related fatalities, Type 2 diabetes and common mental health concerns like 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, an associate professor of nutrition at the Harvard T.H. Chan School of Public Health. She said that the constancy of the association between UPFs and health difficulties enhanced her confidence that there was a true problem with the foods.
But the observational studies also have limits, said Lauren O’Connor, a nutrition expert and epidemiologist who formerly worked at the Department of Agriculture and the National Institutes of Health. It’s true that there is an association between certain foods and chronic diseases, she said, but that doesn’t prove that UPFs actually cause poor health.
Dr. O’Connor questioned whether it’s beneficial to group such “starkly different” items — like Twinkies and breakfast cereals — into one category. Certain forms of ultraprocessed meals, such sodas and processed meats, are more clearly detrimental than others. UPFs such flavored yogurts and whole grain breads, on the other hand, have been related with a lower risk of developing Type 2 diabetes.
Clinical experiments are needed to test if UPFs directly cause health concerns, Dr. O’Connor stated. Only one such study, which was tiny and had certain limitations, has been done, she noted.
In that study, published in 2019, 20 adults with a range of body sizes lived in a research hospital at the National Institutes of Health for four weeks. For two weeks, they ate largely unprocessed or barely processed foods, then for another two weeks, they ate mainly UPFs. The diets provided identical amounts of calories and nutrients, and the participants may eat as much as they desired at each meal.
During their two weeks on the ultraprocessed diet, individuals gained an average of two pounds and consumed around 500 calories more per day than they did on the unprocessed diet. During their time on the raw diet, they shed roughly two pounds.
That discovery might help explain the link between UPFs, obesity and other metabolic problems, said Kevin Hall, a nutrition and metabolism researcher at the National Institutes of Health, who conducted the investigation. But the study needs to be duplicated, which Dr. Hall is in the midst of doing now.
Why may UPFs be harmful?
There are numerous “strong opinions” regarding why ultraprocessed foods are unhealthy, Dr. Hall noted. “But there’s actually not a lot of rigorous science” on what those mechanisms are, he added.
Because UPFs are typically inexpensive, easy and available, they’re definitely displacing healthy foods from our diets, Dr. Hall added.

But he and other scientists suspect that the foods could be having more direct consequences on health. They can be simple to overeat — maybe because they include hard-to-resist mixtures of carbohydrates, sugars, fats and salt, are high-calorie and easy to chew. It’s also possible that ensuing blood sugar spikes may damage arteries or ramp up inflammation, or that specific dietary additives or chemicals may interfere with hormones, induce a “leaky” bowel or upset the gut microbiota.
Researchers, including Dr. Hall and Dr. Davy, are beginning to conduct small clinical trials that will test some of these possibilities. Such research may help identify the most dangerous UPFs and possibly indicate how they may be made healthier, Dr. Hall said.
But most studies assume there are multiple ways the meals are causing harm. “Rarely in nutrition is there a single factor that fully explains the relationship between foods and some health outcome,” Dr. Vadiveloo said.
What should we do about ultraprocessed foods?
In 2014, Dr. Monteiro helped create new dietary guidelines for Brazil that recommended individuals to avoid ultraprocessed foods.
Other countries like Mexico, Israel and Canada have also explicitly suggested avoiding or reducing UPFs or “highly processed foods.” The U.S. dietary guidelines contain no such advise, but an advisory council is now looking into the information on how UPFs may effect weight growth, which could influence the 2025 standards.
It’s tough to know what to do about UPFs in the United States, where so much food is already ultraprocessed and those with lower incomes can be more dependent on them, Dr. Hall said.
“At the end of the day, they are an important source of food, and food is food,” Dr. Mattei noted. “We really cannot vilify them,” she remarked.
While research continues, expert perspectives differ on how people should approach UPFs. Dr. Monteiro stated that the safest way is to avoid them altogether — to change flavored yogurt for plain yogurt with fruit, for example, or to buy a fresh loaf from a local bakery instead of boxed bread, if you can afford to do so.
Dr. Vadiveloo proposed a more moderate technique, focusing on limiting UPFs that don’t contain important nutrients, including soda and cookies. She also advocated eating more fruits, vegetables, entire grains (ultraprocessed or not), legumes, nuts and seeds.
Cook at home as often as you can, using minimally processed foods, Dr. Davy advised. “We can’t really say a whole lot beyond that at this point.