For decades, obesity and type 2 diabetes have been on the rise, while men’s reproductive health has been declining, with sperm quality dropping worldwide.
Many scientists believe that one of the key drivers behind these trends is the growing popularity of ultra-processed foods—items like packaged snacks, sugary drinks, instant noodles, and processed meats.
These foods have long been linked to poor health, but until now, it wasn’t clear whether the problem was the ingredients themselves, the way they’re processed, or the fact that they make people overeat.
A new international study has provided important answers.
Published in Cell Metabolism, the research shows that even when people eat the same number of calories, ultra-processed foods cause more weight gain and harm reproductive health compared to minimally processed diets. In other words, not all calories are created equal.
To get the clearest picture, scientists designed a study where each participant followed both diets at different times. Forty-three healthy men between the ages of 20 and 35 were recruited. Each man spent three weeks on a minimally processed diet and three weeks on an ultra-processed diet, with a three-month break in between to reset their systems.
The meals were carefully prepared to have the same number of calories, proteins, carbohydrates, and fats, so the only difference was the level of processing. Some men were also given an extra 500 calories a day to test the effect of overeating, but most ate the normal recommended amount for their body size and activity.
The results were striking. On average, men gained about one kilogram more fat while on the ultra-processed diet compared to the unprocessed one, even when their calorie intake was the same. Several markers of heart health were also worse when men consumed processed foods.
But perhaps most concerning was the effect on reproductive health. The men on the ultra-processed diet had higher levels of phthalates in their bodies—chemical pollutants commonly found in plastics that can disrupt hormones. At the same time, their levels of testosterone and follicle-stimulating hormone, both essential for sperm production, went down.
“We were shocked by how many body functions were disrupted by ultra-processed foods, even in healthy young men,” said Professor Romain Barrès, senior author of the study. Lead author Jessica Preston added that their results prove it’s the processed nature of the food itself, not just overeating, that makes it harmful.
The findings raise big questions about current nutrition guidelines, which often focus on calories rather than the level of food processing. If ultra-processed foods can damage metabolic and reproductive health even when eaten in moderation, scientists warn that long-term consequences could be serious.
The study adds to growing calls for a shift in dietary advice—one that recognizes that the quality of calories matters just as much as the quantity.
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