Ultra-processed foods (UPFs) are everywhere. From the pre-packaged snacks lining supermarket aisles to fast food chains and sodas there’s almost no escaping them in modern society. In countries like the US or the UK, around half of the consumed calories come from UPFs, even though we know most of them really aren’t good for us. Now, a new study has even more bad news for men: UPFs seem to be bad for your fertility.

Not Just “calories in, calories out”

The world is undergoing a poorly understood male fertility crisis. This crisis, sometimes called “spermageddon”, has been taking off since the 1970s. Sperm counts in many countries are down by round 50% and the crisis seems to be accelerating. Several factors could be at play, including rising temperatures, changing lifestyles, and, of course, diet. Some studies have linked about a third of the declining sperm count to diet, and according to this new research, UPFs could play a role in that.

The research involved a tightly controlled, randomized crossover trial, a type of trial that assesses two or more separate interventions. In the study, 43 male participants were put on two different diets: an ultra-processed food diet and an unprocessed food diet. Both diets provided the same number of calories, so the team could isolate the impact of food processing itself on health, separate from overeating. The study measured a range of health indicators, including body weight, fat mass, cholesterol levels, hormone regulation, sperm quality, and more.

Conventional wisdom would suggest that if you eat the same amount of calories, you should keep the same body weight. That’s not what researchers found.

The ultra-processed food diet caused participants to gain more weight compared to the unprocessed food diet. Specifically, participants gained about 1.4 kg (3 lbs) more in fat mass on the ultra-processed diet than on the unprocessed one. This indicates that the body responds differently to UPFs, storing more fat even when total calorie intake is controlled. Unsurprisingly, the UPF group also had more “bad” LDL cholesterol and less “good” cholesterol.

But it wasn’t just body weight and cholesterol that were affected. Hormones, which govern everything from metabolism to fertility, were significantly altered by UPF consumption. Growth differentiation factor 15 (GDF-15), a key regulator of metabolism, was reduced on the ultra-processed diet. That could also explain why people in the UPF group got fatter. Likewise, follicle-stimulating hormone (FSH), which plays a vital role in sperm production, also dropped.

Ultraprocessed Foods and Sperm

These hormonal disruptions were tied to subtle yet telling declines in reproductive health. The decrease in sperm quality wasn’t dramatic, but it was clearly there. There was also a clear trend towards reduced motility, meaning sperm were less able to move efficiently, an essential factor for fertilization. This aligns with the broader trend observed globally.

The team also showed a lot of harmful pollutants accumulating in the body of the UPF group. Certain chemicals, including phthalates (a group of endocrine-disrupting compounds often used in food packaging) spiked. These chemicals are known to interfere with hormone function and may further explain the reproductive disruptions observed in the study.

“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.

Intriguingly, the study also found that the UPF diet led to significantly lower levels of lithium in participants’ blood and semen. Lithium is a naturally occurring element that plays a role in mood regulation, and altered levels could be linked to mental health. This finding was accompanied by a trend toward higher depression scores in men on the UPF diet, suggesting a possible, though not yet proven, link between UPF consumption, lithium levels, and mood.

Takeaways and Caveats

It’s important to acknowledge the study’s limitations, which the authors readily do. The three-week intervention period is relatively short and may capture acute responses rather than the effects of long-term consumption. The study also relied on participants’ self-reported adherence to the diets, which could introduce some bias. However, the strength of the randomized, crossover design and the consistency of the findings across numerous health markers provide powerful evidence.

UPFs aren’t necessarily bad, but they’re chemically and structurally different from real food, and our bodies know it. This study shows that even when consumed in moderation, these foods can push our metabolism toward disease, disrupt our delicate hormonal balance, and potentially compromise our reproductive fitness.

Journal Reference: Jessica M. Preston at al;. Effect of ultra-processed food consumption on male reproductive and metabolic health. Cell Metabolism, 2025; DOI: 10.1016/j.cmet.2025.08.004