All-you-can-eat: Young adults and ultra-processed foods

Young Americans are gaining weight.

An analysis published in The Lancet predicts that one in three Americans age 15 to 24 will meet the criteria for obesity by 2050, putting their health at risk.

While genetics, inactivity, and many other factors are at play, diet features prominently.

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.

Researchers at Virginia Tech wanted to investigate the effects on 18- to 25-year-olds of a diet high in ultra-processed food and a diet without ultra-processed food. After two weeks on each diet, would this age group eat differently at an all-you-can-eat meal?

While the results of the controlled feeding study found no overall increase in calories or grams consumed at a post-diet buffet-style breakfast, participants age 18 to 21 ate more calories after following the diet high in ultra-processed food. The same was not true for those age 22 to 25. The findings, slated for publication Nov. 19 in Obesity, suggest adolescents may be more vulnerable to 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

The team recruited 27 men and women age 18 to 25 whose weight had been stable for at least six months. For two weeks, participants followed one of two diets that consisted of breakfast eaten in the lab, with remaining meals prepared in a metabolic kitchen. One diet derived 81 percent of calories from ultra-processed foods. The other had none.

Researchers tightly matched the diets’ nutrient profiles, only feeding people the calories they needed to maintain their weight and measuring intake at a single buffet meal after each tightly controlled diet.

“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.”

Researchers used the NOVA classification system — “nova” means new in Portuguese — which categorizes foods based on their level of processing. Nutrition researchers at the University of São Paulo in Brazil developed the scale while studying the country’s sharp increase in obesity.

Unprocessed or minimally processed foods include fresh fruit, legumes, or plain yogurt. Processed culinary ingredients, such as cooking oils, butter, and salt, make up another category. Processed foods — cheese, canned vegetables, or freshly baked breads — combine the two through simple methods. Ultra-processed foods, such as soft drinks, flavored yogurt, and most pre-packed meals and snacks, are made through industrial processing and contain additives rarely used in a home kitchen.

Each participant served as their own control in this crossover study. They would eat one diet for two weeks, return to their normal eating patterns for four weeks, then switch.

Following each two‑week period, participants ate freely from a breakfast buffet that included offerings with and without ultra-processed food. They arrived fasting and were taken to a private room where they were given a tray with roughly 1,800 calories of food — four times the calorie content of a standard American breakfast. They were given 30 minutes to eat as little or as much as they liked.

To analyze eating in the absence of hunger, participants were provided a tray of snacks immediately following breakfast. They were given 15 minutes to take one bite of each snack and rate its pleasantness and familiarity. After sampling and rating each, they could either eat more or relax for the remainder of the period.

What they found

In the group as a whole, the diet they had been assigned did not affect total calories or grams consumed at the buffet, nor did it alter the proportion of ultra-processed foods selected. The findings were also unaffected by participants’ sex or body mass index (BMI), a measure of body fat.

But researchers saw something significant when they analyzed participants by age. The 18- to 21-year-olds, but not the 22- to 25-year-olds, ate more calories after the diet high in ultra-processed foods. In addition, the younger group was also more likely to eat in the absence of hunger.

“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.”

Unlike previous clinical trials in adults that provided continuous access to ultra-processed foods and saw higher daily intake and weight gain, this study matched calories and energy density and assessed food 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 study’s short duration and focus on one meal may not capture the constant food availability many people experience.

Davy suggests that future studies could extend the intervention period, include younger participants, or examine continuous access that more closely captures a real-world environment. This study also looked at a modest sample size; replicating the study with a larger cohort could provide another look at difference based on age subgroups.

By incorporating neuroimaging and biomarkers, scientists could also find mechanisms that link exposure to ultra-processed food to changes in eating behavior across development, an area DiFeliceantonio and Davy are actively examining.

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