Ultra-Processed Foods Add Fat Without Extra Calories and Disrupt Hormones

Ultra-processed foods don’t just pack on pounds — they change the body in hidden ways.

In a tightly controlled study, young men gained more fat mass on a processed diet even when calorie counts were the same as unprocessed meals. Researchers also found worrying spikes in plastic-derived chemicals, along with drops in testosterone and other key fertility hormones.

Obesity, Diabetes, and Sperm Decline

Over the last 50 years, obesity and type 2 diabetes have climbed dramatically, while sperm quality has steadily declined. One factor that may be fueling these troubling shifts is the growing reliance on ultra-processed foods, which have been tied to numerous health problems. What scientists still debate is whether the harm comes from the industrial ingredients, the processing methods, or simply because these foods make people eat more than they need.

A new study provides fresh insight. Researchers found that people put on more weight when eating an ultra-processed diet compared to a diet of minimally processed foods, even though both contained the exact same number of calories. The human trial also revealed that ultra-processed meals exposed participants to higher levels of pollutants already linked to lower sperm quality. The work was published in the journal Cell Metabolism.

Proving the Hidden Harm

“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 Toxins

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.

Reference: “Effect of ultra-processed food consumption on male reproductive and metabolic health” by Jessica M. Preston, Jo Iversen, Antonia Hufnagel, Line Hjort, Jodie Taylor, Clara Sanchez, Victoria George, Ann N. Hansen, Lars Ängquist, Susan Hermann, Jeffrey M. Craig, Signe Torekov, Christian Lindh, Karin S. Hougaard, Marcelo A. Nóbrega, Stephen J. Simpson and Romain Barrès, 28 August 2025, Cell Metabolism.
DOI: 10.1016/j.cmet.2025.08.004

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