Ultra-processed foods: a hidden threat to health and fertility

A landmark study shows ultra-processed foods don’t just fuel obesity—they disrupt hormones, harm sperm quality, and raise cardiovascular risks, even when calorie intake is controlled.

We’ve long known that ultra-processed foods are bad for us. They’ve been linked to weight gain, hormonal disruptions, and metabolic disease. But new research reveals something even more unsettling: these foods may also carry toxic substances that damage sperm, with consequences for male fertility.

The study, published in Cell Metabolism, offers fresh scientific confirmation of what nutrition experts have warned for years—while also shedding light on mechanisms we hadn’t fully understood.

What exactly are ultra-processed foods?

To understand the debate, it helps to look at the NOVA classification system, which sorts foods by how much they’ve been altered. Group 1 includes fresh or minimally processed foods like vegetables and uncut meat. Group 4 is at the opposite end of the spectrum: ultra-processed foods, which are industrial creations loaded with synthetic and highly refined ingredients.

Manufacturers rely on techniques such as molding, extrusion, and pre-frying to make these foods. The goal isn’t nutrition but efficiency—longer shelf life, lower costs, and flavors engineered to keep us hooked.

“An ultra-processed food is any product that contains at least one ingredient that doesn’t exist in nature or in a traditional pantry,” explain experts from the French consumer magazine 60 Millions de Consommateurs. They call these ingredients markers of ultra-processing (MUTs).

MUTs often show up as additives, but the list is wider: refined oils, artificial flavors, glucose syrup, even supposedly “natural” extracts. And the foods they end up in are everywhere: sugary breakfast cereals, hot dogs, flavored yogurts, frozen breaded fish, cordon bleu cutlets, soft drinks, even stir-fried vegetables sold pre-seasoned.

The health risks we already knew

Over the last decade, study after study has tied high consumption of ultra-processed foods to serious health concerns. One large-scale analysis, published in The Lancet in 2023, followed 200,000 people for over 30 years. It found that those eating the most ultra-processed foods faced a 16% higher risk of coronary heart disease compared with those who ate the least.

And the risks aren’t limited to the heart. Researchers have linked these foods to obesity, type 2 diabetes, and even early death.

The new research

©Cell Metabolism

The latest study, led by Romain Barrès and colleagues, went further. It showed that ultra-processed foods cause weight gain, hormonal imbalances, and exposure to harmful chemicals, regardless of calorie intake. In other words, it isn’t just overeating—it’s the processing itself that harms health.

The backdrop is striking: in the last 50 years, rates of obesity and type 2 diabetes have surged, while sperm quality has plummeted. Ultra-processed foods may be a hidden driver of both trends.

To test this, scientists designed a tightly controlled trial. They recruited 43 men between the ages of 20 and 35. Each man followed two diets—one minimally processed, the other ultra-processed—for three weeks each, with a three-month break in between. The diets were carefully matched in calories, protein, fat, and carbs. Some participants consumed an extra 500 calories a day (about 2,100 kilojoules) to test the impact of overeating.

Half the men started with the processed diet, half with the unprocessed one. None were told which they were on.

What they found

“Our results show that ultra-processed foods damage reproductive and metabolic health, even when not consumed in excess,” explained co-author Jessica Preston.

On average, participants gained about 2.2 pounds (1 kilogram) of body fat during the ultra-processed phase, compared with virtually no gain on the unprocessed diet—despite eating the same number of calories. Their cardiovascular markers also worsened.

More disturbingly, lab tests revealed that the men on the ultra-processed diet absorbed higher levels of contaminants known to impair sperm quality.

One culprit stood out: the phthalate cxMINP, a chemical used in plastics that disrupts hormones. Men eating ultra-processed foods showed reduced testosterone and follicle-stimulating hormone (FSH)—two key regulators of sperm production.

“We were shocked by how many bodily functions were disrupted by ultra-processed foods, even in young, healthy men,” said Barrès. “The long-term implications are alarming and underscore the urgent need to rethink nutritional guidelines.”

Sources: University of Copenhagen / Cell Metabolism