A controlled feeding study out of South Dakota State University shows that older adults who ate fewer ultra-processed foods naturally consumed fewer calories, lost weight and abdominal fat, and showed improvements in insulin, nutrient-sensing hormones, and inflammation.
"Counting nutrients is not enough," Moul Dey, professor of health and nutritional sciences, said. "The degree of processing changes how the body handles those same nutrients. Diet quality depends not only on nutrients but also on 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 Dietary Guidelines currently include no clear directive on ultra-processed foods, but this study shows that when diets meet DGA nutrient goals while minimizing ultra-processed food and ingredients, calorie intake drops and metabolic health improves. The findings are the first to demonstrate that the DGA 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 percent of the national food supply. In simpler terms, if it 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. Very few feeding trials have examined how ultra-processed foods affect Americans' health. The first major study, conducted entirely inside a research center, compared diets made up of almost entirely ultra-processed foods to diets with none. This second trial tested a more realistic shift, reducing ultra-processed foods from about half of daily calories to about 15 percent within nutritionally balanced menus for free-living older adults in the United States.
"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 food intake went down, total calories and metabolic risk markers did too."
Saba Vaezi, a doctoral student 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.
Robust study design
The study is among a handful of rigorously controlled feeding trials in free-living older adults that:
- Tested two low–ultra-processed diets aligned with the Dietary Guidelines for Americans; one featuring a meat-based (lean pork) diet and the other a plant-based (lentil) diet.
- Matched the diets for calories, protein, fat, carbohydrate, fiber and other key nutrients; a few familiar ultra-processed items were included in moderation to support adherence.
- Prepared and served more than twelve thousand pre-portioned meals from scratch to study participants. The team measured daily food intake, metabolic, hormonal and body-composition outcomes, with a subset followed for about one year after the intervention.
Older adults completed an 18-week feeding study with two diet periods of eight weeks each, separated by a short break of at least two weeks. Every meal and snack was fully prepared and provided to eat at home. One diet was meat-based, featuring pork as the main protein source, and the other was plant-based, centered on lentils, beans and peas. Both followed the nutrient goals of the Dietary Guidelines for Americans.
A companion method paper published by Dey and her team in Current Developments in Nutrition confirmed strong participant adherence and described the complex operations that made this real-world feeding study possible.
Findings and implications
On average, participants spontaneously reduced calorie intake and experienced about 10% total body fat loss and 13% belly fat loss during both diet phases, along with a 23% improvement in insulin sensitivity and favorable changes in inflammatory markers and nutrient-sensing hormone levels. Daily calorie intake decreased by roughly 400 calories per day, even without instructions to restrict calories. These results suggest that replacing ultra processed food with minimally processed ones can enhance metabolic efficiency and body composition in older adults, within balanced, American Guidelines–aligned diets. The study also demonstrates that high dietary quality and lower processing can be achieved in practical, take-home meal programs.
Researchers note that the 18-week trial included a small sample of 36 participants who completed the study, and that larger studies are needed to confirm long-term outcomes. At the one-year follow-up, when participants' ultra-processed food intake gradually increased again, many of the metabolic improvements observed during the trial diminished, suggesting that benefits depend on sustained reductions in ultra-processed foods. 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 simply prepared and nutritionally balanced."
