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A controlled feeding study led by Moul Dey, professor in the School of Health and Human Sciences at �ձ�av��Ƶ, shows that older adults who ate fewer ultra-processed foods naturally consumed fewer calories and lost weight and abdominal fat. They also showed improvements in insulin, nutrient-sensing hormones and inflammation.

“Counting nutrients is not enough,” Dey said. “The degree of processing changes how the body handles those same nutrients. Diet quality depends on both the ingredients and the level of processing considered together.”

For decades, the Dietary Guidelines for Americans have urged balance and moderation, yet rates of obesity and other chronic diseases have continued to rise. The guidelines currently include no clear directive on ultra-processed foods. But this study shows that when meals meet Dietary Guidelines for Americans nutrient goals while minimizing ultra-processed food and ingredients, calorie intake spontaneously drops and metabolic health improves.

The findings are the first to demonstrate that the Dietary Guidelines for Americans framework can deliver stronger health benefits when the level of food processing is also considered.

Ultra-processed foods are industrial products made by reconstructing parts of whole foods with synthetic additives such as flavors, colors, preservatives and emulsifiers. They dominate modern diets, providing more than half of U.S. adults’ daily calories and about 70% of the national food supply.

In simpler terms, if food comes wrapped in plastic and lists ingredients you wouldn’t keep in your kitchen, it’s probably ultra-processed.

Meals in the study were designed and administered by the university’s human nutrition research team, prepared by a professional local chef, and eaten at home by clinical trial participants to reflect everyday eating patterns. More than 12,000 preportioned meals were served to free-living older adults in the community who took part in the study. Data was collected on how much of the provided food was consumed each day.

“Older adults often face metabolic challenges as appetite and energy needs shift,” said Dey, the senior author and principal investigator of the study. “We saw that when ultra-processed foods went down, total calories and harmful metabolic markers did, too.” This is the first feeding trial to examine how reducing ultra-processed foods affects the health of older Americans. The trial reported a realistic reduction from about half of daily calories from ultra-processed foods to about 15% among individuals who were generally overweight and at risk for age-related health decline.

Saba Vaezi, a doctoral researcher in Dey’s laboratory and first author of the collaborative study, said the findings show that simple substitutions, rather than restrictive dieting, can make measurable differences. “Participants did not count calories or follow complicated weight-loss instructions,” she said.

Participants completed an 18-week all-food-provided feeding study with two diet periods of eight weeks each, separated by a short break of at least two weeks. One diet was meat-protein based, featuring lean pork as the main protein source, and the other was plant-protein based, centered on lentils. Both followed the nutrient goals of the Dietary Guidelines for Americans.

On average and across both diets, participants spontaneously reduced calorie intake and experienced 10% body fat and 13% belly fat loss during both diet phases, alongside 23% improved insulin sensitivity and lower inflammation and altered nutrient-sensing hormone levels. The results suggest that even without formal calorie restriction, reducing food processing level can improve metabolic efficiency and body composition in aging adults.

“The work demonstrates that high dietary quality and lower processing can be achieved in practical, take-home meal programs,” Dey said.

The results also suggest a simple strategy for everyday life: when grocery shopping, choosing more whole or minimally processed foods and fewer shelf-stable products with long ingredient lists may help prevent overeating and improve metabolism.

The trial had a smaller sample size of 36 participants who completed the study, which is typical for complex feeding clinical trials. Still, the consistency of effects across both diet patterns underscores the central role of food processing in metabolic health.

“This study moves past the usual debate over whether plant-based or animal-based diets are better,” Dey said. “Both can be health-promoting when foods are minimally processed and nutritionally balanced.”

This research was supported by the National Pork Checkoff and the U.S. Department of Agriculture. Authors on the study include Vaezi, Jessica Freeling, Bruna de Vargas, Lee Weidauer, Marni Shoemaker, Wade Sanders and Dey. The can be viewed online.