A controlled-feeding study from South Dakota State University shows that older adults who ate less highly 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,” said Moul Dey, professor of health and nutrition sciences. “The degree of processing changes how the body handles the same nutrients. The quality of the diet depends not only on the nutrients but also on the ingredients and the level of processing taken together.”
For decades, the Dietary Guidelines for Americans urged balance and moderation, yet rates of obesity and other chronic diseases continued to rise. The Dietary Guidelines currently do not include clear guidance on ultra-processed foods, but this study shows that when diets meet the DGA’s nutrient goals by minimizing ultra-processed foods and ingredients, calorie intake is reduced and metabolic health is improved. The findings are the first to demonstrate that the DGA framework may provide stronger health benefits when the level of food processing is also taken into account.
Ultra-processed foods are industrial products produced 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 US adults’ daily calories and about 70 percent of the national food supply. Simply put, if it comes wrapped in plastic and lists ingredients you wouldn’t keep in your kitchen, it’s probably highly processed.
The meals in the study were designed and administered by the university’s human nutrition research group, prepared by a professional local chef, and eaten at home by clinical trial participants to reflect daily dietary patterns. Very few feeding trials have examined how highly processed foods affect the health of Americans. The first large study, conducted entirely within a research center, compared diets consisting almost entirely of highly processed foods with diets without any. This second trial tested a more realistic change, reducing highly processed foods from about half of daily calories to about 15 percent in nutritionally balanced menus for free-living seniors in the United States.
“Older adults often face metabolic challenges as appetite and energy needs shift,” said Dey, senior author and principal investigator of the study. “We saw that when the intake of ultra-processed food was reduced, total calories and markers of metabolic risk also fell.”
Saba Vaezi, a postdoctoral researcher in Dey’s lab and first author of the collaborative study, said the findings show that simple substitutions, rather than a restrictive diet, can make measurable differences. “Participants didn’t count calories or follow complicated weight loss guidelines,” he said.
Solid study plan
The study is among a handful of tightly controlled feeding trials in free-living older adults that:
- Try two low-to-no-process diets that align with the Dietary Guidelines for Americans. One involves a meat-based diet (lean pork) and the other a plant-based diet (lentils).
- Match diets for calories, protein, fat, carbohydrates, fiber and other key nutrients. a few well-known highly processed items were included sparingly 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 physical outcomes, with a subgroup followed for about a year after the intervention.
The elderly 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 based on meat, with pork as the main source of protein, and the other was plant-based, focusing on lentils, beans and peas. Both followed the dietary goals of the Dietary Guidelines for Americans.
An accompanying method paper published by Dey and her team at Current Developments in Nutrition confirmed strong participant compliance and described the complex operations that made this real-world feeding study possible.
Findings and implications
On average, participants spontaneously reduced their calorie intake and experienced approximately 10% loss in body fat and 13% loss in abdominal fat across both phases of the diet, along with a 23% improvement in insulin sensitivity and favorable changes in inflammatory markers and nutrient-sensing hormone levels. Daily caloric intake was reduced by about 400 calories per day, even without calorie restriction guidelines. These results suggest that replacing highly processed foods with minimally processed ones may improve metabolic efficiency and body composition in older adults, in the context of balanced diets aligned with US guidelines. The study also demonstrates that high nutrition quality and lower processing can be achieved in practical take-home meal plans.
The 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 results. At the one-year follow-up, when the participants’ ultra-processed food intake was gradually increased again, many of the metabolic improvements seen during the trial faded, suggesting that the benefits depended on continued reductions in ultra-processed foods. However, the consistency of effects across both dietary patterns underscores the central role of food processing in metabolic health.
“This study goes beyond the usual debate about whether plant-based or animal-based diets are better,” Dey said. “Both can promote health when foods are simply prepared and nutritionally balanced.”
