Why ultra-processed foods make teens eat more when they aren’t hungry

An analysis published in The Lancet predicts that by 2050, about one in three Americans between 15 and 24 years old will meet the criteria for obesity, putting them at higher risk for serious health problems.

Many influences contribute to this trend, including genetics and low levels of physical activity, but diet plays a central role.

Ultra-processed foods -- which make up 55 to 65 percent of what young adults eat in the U.S. -- have been associated with metabolic syndrome, poor cardiovascular health, and other conditions in adolescents.

Ultra-processed foods and adolescent vulnerability

Researchers at Virginia Tech set out to examine how eating patterns high in ultra-processed foods affect young adults age 18 to 25. They compared two types of diets, one that was rich in ultra-processed foods and another that contained no ultra-processed foods at all. After two weeks on each diet, they tested whether participants would eat differently when faced with an all-you-can-eat meal.

When the researchers looked at everyone in the study together, they did not see an overall increase in calories or grams of food consumed at a buffet-style breakfast after the different diets. However, a different picture emerged when they focused on age. Participants between 18 and 21 years old ate more calories at the breakfast after the ultra-processed diet, while those age 22 to 25 did not show this increase. The results, scheduled for publication Nov. 19 in Obesity, suggest that adolescents and very young adults may be more susceptible to the effects of ultra-processed foods.

"Although this was short-term trial, if this increase in caloric intake persists over time, this could lead to weight gain in these young people," said Brenda Davy, a senior author on the paper and professor in Virginia Tech's Department of Human Nutrition, Foods, and Exercise.

"The younger age group took in more calories from ultra-processed foods, even when they weren't hungry," said neuroscientist and co-author Alex DiFeliceantonio, an assistant professor with Virginia Tech's Fralin Biomedical Research Institute at VTC who investigates the mechanisms of food choice.

Understanding this age group is important because adolescence and young adulthood represent an important developmental window. As people gain independence, eating habits take shape and obesity risk begins to rise.

What they did: controlled diets in young adults

The team recruited 27 men and women between 18 and 25 years old whose weight had remained stable for at least six months. For two weeks, each participant followed one of two eating plans that included breakfast served in the lab, with the rest of their meals prepared in a metabolic kitchen. One diet provided 81 percent of total calories from ultra-processed foods. The other diet contained no ultra-processed foods at all.

Researchers carefully matched the nutrient content of the two diets. Participants were given only the number of calories needed to maintain their weight, and the team measured how much they ate at a single buffet meal after each tightly controlled diet period.

"We very rigorously designed these diets to be matched on 22 characteristics, including macronutrients, fiber, added sugar, energy density, and also many vitamins and minerals," Davy said. "Previous studies had not matched diets to this extent."

How foods were classified with the NOVA system

Researchers used the NOVA classification system -- "nova" means new in Portuguese -- which groups foods by how heavily they are processed. Nutrition experts at the University of São Paulo in Brazil created this system while investigating a rapid rise in obesity in their country.

Unprocessed or minimally processed foods include items such as fresh fruit, legumes, or plain yogurt. Processed culinary ingredients, including cooking oils, butter, and salt, form another category. Processed foods -- cheese, canned vegetables, or freshly baked breads -- combine these ingredients through relatively simple methods. Ultra-processed foods, such as soft drinks, flavored yogurt, and most pre-packed meals and snacks, are produced through industrial processing and contain additives that are rarely used in home kitchens.

Each participant acted as their own comparison in this crossover study. They followed one of the diets for two weeks, returned to their usual eating habits for four weeks, and then switched to the other diet.

Buffet breakfast and eating without hunger

After each two-week diet period, participants were invited to eat freely from a breakfast buffet that included both ultra-processed and non-ultra-processed options. They arrived in a fasting state and were escorted to a private room, where they received a tray with about 1,800 calories of food -- four times the calorie content of a standard American breakfast. They had 30 minutes to eat as much or as little as they wanted.

To study eating in the absence of hunger, participants then received a tray of snacks immediately after breakfast. For 15 minutes, they were asked to take one bite of each snack and rate how pleasant and familiar it was. After tasting and rating all the items, they could choose to keep eating or simply rest for the remainder of the session.

What they found: younger participants ate more

In the full group of participants, the type of diet they had just followed did not change the total calories or total grams of food eaten at the buffet. The proportion of ultra-processed foods selected also remained similar. These results did not differ by sex or by body mass index (BMI), which is a standard measure of body fat.

The age breakdown, however, revealed an important difference. The 18- to 21-year-olds, but not the 22- to 25-year-olds, consumed more calories after the period on the ultra-processed diet. The younger participants were also more likely to continue eating when they were no longer hungry.

"Our adolescent participants had just consumed more in the buffet meal after the ultra-processed diet. Then, given the opportunity to snack when not hungry, they ate more yet again," said DiFeliceantonio, who is also an assistant professor in the Department of Human Nutrition, Foods, and Exercise. "Snacking when not hungry is an important predictor of later weight gain in young people, and it seems ultra-processed food exposure increases this tendency in adolescents."

Isolating the effect of food processing

Earlier clinical trials in adults that offered continuous access to ultra-processed foods found that people ate more each day and gained weight over time. In contrast, the Virginia Tech study kept daily calories and energy density the same between diets and evaluated intake at one buffet-style meal.

"This is important, because it helps isolate the effect of food processing on energy intake," DiFeliceantonio said. "In the previous trial people ate more each day, which meant they gained weight each day, which meant their energy needs also increased. Here, since everyone was weight stable, we can see the effect of processing alone."

The researchers note that the brief length of the study and its focus on a single meal may not fully reflect how people encounter food in everyday life, where eating opportunities are nearly constant.

Future research on ultra-processed foods and youth

Davy suggests that future research could lengthen the intervention period, include younger participants, or provide continuous access to foods to more closely mirror real-world conditions. This study also included a modest number of participants, so repeating it with a larger group could give a clearer picture of how age affects responses to ultra-processed diets.

By adding tools such as brain imaging and biomarkers, scientists may be able to uncover the biological pathways that link exposure to ultra-processed foods with changes in eating behavior across development. This is an active area of study for DiFeliceantonio and Davy.

This research was supported by a grant from the National Institutes of Health.