People gain more weight on an ultra-processed diet than when eating a minimally processed diet, even when they eat the same number of calories, new research has found.
The study of 43 men aged aged 20 to 35 pitted nutritionally similar unprocessed and ultra-processed diets against each other and found men gained around one kilogram more of fat mass while on the ultra-processed diet compared to the unprocessed diet.
Researchers said their findings, published in Cell Metabolism, showed that total caloric intake is not the “sole determinant” of body weight gain, and that the “processed nature” of ultra-processed food itself harms reproductive and metabolic health – regardless of nutritional content.
They concluded calories from unprocessed or ultra-processed foods are “not equally stored or metabolised”, even when controlled for macronutrient load.
Their work also revealed a diet high in ultra-processed foods introduces higher levels of pollutants that are known to affect sperm quality.
Ultra processed foods (UPFs) contain substances not normally used in home cooking, such as preservatives, sweeteners and emulsifiers.
UPFs including ice cream, processed meats, crisps, mass-produced bread, some breakfast cereals, biscuits, and fizzy drinks, have already been linked to various health issues, including obesity, heart disease, cancer, and early death.
During the study, researchers from the University of Copenhagen put half of the men on an ultra-processed diet and half on an unprocessed one, before switching them with a three-week “washout” period in between. Both the unprocessed and ultra-processed diets had the same amount of calories, protein, carbs and fats.
Half of the men also received a high-calorie diet with an extra 500 daily calories, while half received the normal amount of calories for their size, age and physical activity levels.
They found participants gained around one kilogram more of fat mass on the ultra-processed diet, regardless of whether they were on the normal or excess calorie diet.
“Despite caloric matching of discordant diets, we observed differences in body weight accumulation between diets, which appear to reflect changes in fat mass. This uncoupling between total energy consumed and body weight suggests that total caloric intake is not the sole determinant of body weight gain,” the team said.
“By providing a fixed number of calories to study participants during both diets, we were able to determine that the processed nature of the food itself, independent of the caloric and macronutrient intake, impacts numerous health markers.”
Scientists also found a “worrying” increase in the level of phthalate cxMINP – a substance used in plastics and recognised to disrupt hormones – in men on the ultra-processed diet. They added men on this diet also saw decreases in their levels of testosterone and follicle-stimulating hormone, which are crucial for sperm production.
“We were shocked by how many body functions were disrupted by ultra-processed foods, even in healthy young men,” the study’s senior author Professor Romain Barrès said. “The long-term implications are alarming and highlight the need to revise nutritional guidelines to better protect against chronic disease.”
Lead author Jessica Preston added: “Our results prove that ultra-processed foods harm our reproductive and metabolic health, even if they’re not eaten in excess. This indicates that it is the processed nature of these foods that makes them harmful.”
