Ultra-processed foods harm men’s health. They increase weight, disrupt hormones, decrease testosterone, and introduce harmful substances linked to declining sperm quality. They contain industrial and synthetic ingredients. This may be why over the past 50 years, sperm quality has plummeted.

[u/mvea]

https://cbmr.ku.dk/news/2025/not-all-calories-are-equal-ultra-processed-foods-harm-mens-health/

[u/Ateist]

Link to the actual study https://www.cell.com/cell-metabolism/fulltext/S1550-4131(25)00360-2

Relevant part on study limitations:

Due to the study design, the estimation of energy intake relied on participants' adherence to the nutritional intervention and the accuracy of their reporting. Although adherence was assessed through detailed daily questionnaires and questionnaires at the end of each treatment period, we cannot rule out potential bias in the reporting of actual caloric intake. This may affect our ability to determine whether the effects of the UPF diet are dependent on caloric intake. However, the similar effects observed on body composition markers in both the adequate and excess caloric study arms suggest that the detrimental effects of the ultra-processed diet compared with the unprocessed diet are not solely attributable to caloric intake but extend across both study arms. Another limitation of our study is the relatively short duration of the experimental diets, which may not reveal stable effects of chronic diet consumption; 3-week diet interventions may have induced acute responses that may normalize with time if diets were prolonged. Although the ultra-processed diet unlikely triggered an adaptive response due to the similar amounts of UPFs in the experimental diet compared with the habitual diets of participants, the unprocessed diet represented an important shift compared with their habitual diets and may have induced acute responses, as suggested by the increased levels of inflammatory markers after the diet period. As we did not measure inflammatory markers in a time-course fashion, we cannot determine whether elevated markers of inflammation are normalizing at the completion of the unprocessed diet, as would be expected after an acute response.

So their unprocessed diet was a stress that was different from the normal diet of participants and caused inflamation, which might have contributed to weight gain difference.