Does it matter if two diets have the same calories and nutrients? A new study suggests it might, at least for young men eating ultraprocessed foods.
Researchers at the University of Copenhagen tested the effects of ultraprocessed and unprocessed diets in 43 men ages 20 to 35. Each participant spent three weeks on one diet, took a three-month break, then switched to the other. Both diets were matched for calories, protein, carbohydrates and fat.
Despite being nutritionally identical on paper, the ultraprocessed diet led to an average gain of about 1 kilogram (2.2 pounds) of fat mass, along with changes in markers of cardiovascular health. The researchers also found higher levels of a plastic-derived chemical called phthalate cxMINP, known to disrupt hormones, in men on the ultraprocessed diet. Levels of testosterone and follicle-stimulating hormone, both important for sperm production, were also reduced.
“We were shocked by how many body functions were disrupted by ultraprocessed foods, even in healthy young men,” said study senior author Romain Barrès of the University of Copenhagen and the Université Côte d’Azur.
Experts caution, however, that not all ultraprocessed foods are the same. The category covers a wide range, from packaged snacks to nutrient-rich products like fortified plant-based proteins. Some may introduce contaminants during processing, while others can provide important nutrition. This study highlights that diet quality and food sources matter beyond calorie counts, but more research is needed to understand which foods drive these effects and why.
This work, which was published in Cell Metabolism, was supported by the Novo Nordisk Foundation, the Lundbeck Foundation, the Danish Council for Independent Research and other nonprofit sources. One author reported honoraria and consulting relationships with Merck, Ferring and Novo Nordisk; all other authors declare no conflicts of interest.