Why ultra-processed diets make you gain fat even without extra calories

An international team of scientists has now discovered that people gain more weight on an ultra-processed diet compared to a minimally processed diet, even when they eat the same number of calories. The study in humans also revealed a diet high in ultra-processed foods introduces higher levels of pollutants that are known to affect sperm quality. The findings were published in the journal Cell Metabolism.

"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," says Jessica Preston, lead author of the study, who carried out the research during her PhD at the University of Copenhagen's NNF Center for Basic Metabolic Research (CBMR).

Same calories, different outcomes

To get the best possible data, the scientists compared the health impact of unprocessed and ultra-processed diets on the same person. They recruited 43 men aged 20 to 35, who spent three weeks on each of the two diets, with three months 'washout' in between. Half started on the ultra-processed and half started on the unprocessed diet. 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 were not told which diet they were on. Both the unprocessed and ultra-processed diets had the same amount of calories, protein, carbs and fats.

Men gained around 1 kg more of fat mass while on the ultra-processed diet compared to the unprocessed diet, regardless of whether they were on the normal or excess calorie diet. Several other markers of cardiovascular health were also affected.

Ultra-processed foods polluted with endocrine disruptors

The scientists also discovered a worrying increase in the level of the hormone-disrupting phthalate cxMINP, a substance used in plastics, in men on the ultra-processed diet. 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 long-term implications are alarming and highlight the need to revise nutritional guidelines to better protect against chronic disease." says the study's senior author Professor Romain Barrès from the University of Copenhagen's NNF Center for Basic Metabolic Research, and the Université Côte d'Azur.