Ultra-processed foods impaired metabolism and reproductive health in men, even without excess calories. Digest
Modern diets are often dominated by ultra-processed foods (UPFs), yet scientists still debate whether their harm arises mainly from their tendency to drive excess calorie intake or also from other aspects of industrial processing. To test this, researchers at the University of Copenhagen conducted a tightly controlled crossover study in healthy men.
The study enrolled 43 men aged 20 to 35, each randomized to two 3-week diet phases with a 12-week break between them. Both meal plans were matched for calories and macronutrients, but differed sharply in composition: the ultra-processed version supplied about 77% of calories from UPFs, while the minimally processed plan contained less than 1%. One study arm received adequate calories based on estimated energy expenditure and the other received 500 calories per day in excess.
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Results showed that processing level, not just calories, shaped health outcomes:
- Men gained about 1.3 to 1.4 kilograms more (mostly from increased fat mass) on the ultra-processed diet than on the minimally processed diet, in both the adequate- and excess-calorie groups.
Adequate-calorie group:
- Total cholesterol and the LDL-to-HDL ratio were higher on the ultra-processed diet than on the minimally processed diet — both signs of worse lipid balance.
- The signaling molecule interleukin-4 (IL-4) increased on the ultra-processed diet. IL-4 is a cytokine with a dual role in immunity, promoting both allergic responses and supporting anti-inflammatory and tissue-repair processes.
Excess-calorie group:
- Diastolic blood pressure was higher on the ultra-processed diet than on the minimally processed diet.
- Growth differentiation factor-15 (GDF-15) decreased on the ultra-processed diet. GDF-15 is a mitochondrial stress signal that promotes energy expenditure and appetite suppression.
- Follicle-stimulating hormone (a hormone essential for sperm development) was lower on the ultra-processed diet, yet sperm concentration did not change.
Because calories and macronutrients were matched and estimated intake was similar within arms, the findings indicate effects attributable to processing level itself. The researchers suggest several explanations, including that the processing may increase the usable energy of foods or influence hormones that regulate appetite and metabolism. Packaging chemicals and additives might also contribute through subtle endocrine effects.
The study was brief, so some effects may not have had time to fully manifest, and it also depended on participants strictly adhering to the prescribed diets. Additionally, the findings published so far are not the main outcome the study was originally designed to test, but rather exploratory health measures the researchers chose to examine as well — so they should be viewed as early signals rather than firm conclusions. However, the pattern suggests that reducing heavily processed foods could benefit both metabolic and reproductive health. In this clip, I explain how ultra-processed foods impact appetite, inflammation, and long-term disease risk.