Overweight adults who focused on eating minimally processed or whole foods lost nearly twice as much weight as those relying on ultra-processed diets, according to a clinical trial published this month in Nature Medicine. The difference was observed even when both diets met national health guidelines.
In the trial, 55 adults alternated between eight-week diets of either minimally processed foods (MPF) or ultra-processed foods (UPF). Both diets followed official government nutrition standards, provided all meals, and participants were allowed to eat as much as they wanted. The trial was conducted by researchers at University College London.
On average, participants lost 2.06 percent of their body weight while eating minimally processed foods, compared with 1.05 percent on the ultra-processed diet. If the diet continued for over a year, researchers estimate a 13 percent loss for men and 9 percent for women on the MPF diet, compared with 4 percent and 5 percent on the UPF diet.
In addition to weight loss, the minimally processed diet led to lower daily calorie intake, reduced fat mass and triglyceride levels, and better preservation of lean tissue.
Ultra-processed foods include packaged snacks, sweetened drinks, frozen meals, and processed breads. Such foods typically undergo multiple manufacturing steps and often contain added sugars, fats, salt, and chemical additives. According to the Centers for Disease Control and Prevention (CDC), these foods make up more than half of the average American diet.
“Ultra-processed foods tend to be hyperpalatable, energy-dense, low in dietary fiber, and contain little or no whole foods, while having high amounts of salt, sweeteners, and unhealthy fats,” states the CDC. The agency has linked diets high in UPFs with elevated risks of cardiovascular disease and early death.
U.S. dietary guidance, including the USDA's Dietary Guidelines for Americans, traditionally emphasizes nutrient metrics—limiting added sugars, saturated fats, and sodium—but does not explicitly address food processing levels. This trial provides experimental evidence suggesting that food processing matters.
In a similar study in 2019, the National Institutes of Health found that adults consuming ultra-processed diets ate an average of 500 more calories per day and gained weight compared with those on minimally processed diets, even though the meals were matched for sugar, fat, and fiber content.
For consumers, the findings may offer a practical takeaway. Replacing highly processed products with whole or minimally processed and whole food options like fruits, vegetables, lean meats, legumes, nuts, and eggs could support weight management and reduce risks for chronic disease. While occasional ultra-processed foods are not considered harmful, researchers say limiting them may provide a significant advantage for those trying to lose or maintain weight.