Junk Food Linked To Weight Gain, Fertility Changes In Men In Just 3 Weeks

Researchers show that what you’re eating may matter more than total calories consumed when it comes to weight gain.

COPENHAGEN — A study reveals that men eating ultra-processed foods for three weeks experienced significantly more weight gain and fertility-related concerns compared to those eating whole foods, even when consuming similar calories. The research suggests that total calorie intake isn’t necessarily what determines weight gain and health outcomes.

New research involving 43 healthy men has shown that food processing itself, independent of calorie content, significantly impacts weight gain, fertility, and metabolic health. When comparing the two diets, men showed 3.1 pounds more weight gain in the adequate calorie group and 2.9 pounds more weight gain in the excess calorie group on ultra-processed foods versus whole foods.

Published in Cell Metabolism, the Danish study also revealed harmful effects on male fertility, with decreased levels of follicle-stimulating hormone and trends toward reduced sperm motility.

Controlled Study Shows Processing Matters Beyond Calories

Previous studies showed that people naturally consume 500-800 more calories daily when given unlimited access to ultra-processed foods. However, researchers wanted to determine whether these foods cause harm due to overconsumption or due to the processing itself.

The research team provided participants with precisely measured portions matched for total calories and macronutrients. Half the men received adequate calories while the other half received 500 extra calories daily. Both groups ate each diet type for three weeks with a 12-week break between.

Even with similar calorie intake, men eating ultra-processed foods consistently gained more weight and fat mass than those eating whole foods. The ultra-processed diet contained 77% processed foods, similar to typical American consumption patterns. The whole food diet contained just 1% processed foods and 66% unprocessed ingredients.

The study notes that weight differences were “primarily driven by a weight loss observed in response to the unprocessed dietary intervention,” meaning the unprocessed diet actually led to weight loss while the processed diet led to weight gain.

Is Ultra-Processed Junk Food Wrecking Male Fertility?

The fertility effects occur amid a broader global trend. As the study notes, “concurrent with the rise in UPF consumption, semen quality has declined globally, with sperm count falling by approximately 60% since the 1970s.” Ultra-processed foods now account for over half of American diets.

Men eating processed foods showed decreased follicle-stimulating hormone levels in the excess calorie group. FSH stimulates sperm production, and lower levels can indicate fertility problems. Total sperm motility also showed a trend toward decrease in the excess calorie arm, though this finding did not reach statistical significance.

The study also detected differences in chemical accumulation between the two diets. Men on the ultra-processed food diet showed decreased levels of lithium, a mineral that may support mental health, in their blood and semen. They also showed trends toward increased levels of a plastic-related chemical called a phthalate in their blood.

“We were shocked by how many body functions were disrupted by ultra-processed foods, even in healthy young men,” says the study’s senior author Professor Romain Barrès in a statement. “The long-term implications are alarming and highlight the need to revise nutritional guidelines to better protect against chronic disease.”

Metabolic Changes Beyond Simple Calorie Math

Ultra-processed foods appear to alter normal metabolism through mechanisms beyond simple energy balance. The study found that levels of a hormone called growth/differentiation factor 15 decreased in men consuming ultra-processed foods in the excess calorie group. This hormone helps regulate how the body uses energy.

The researchers observed that cholesterol levels and the ratio of bad to good cholesterol increased in response to the ultra-processed diet, but only in the adequate calorie group. Blood pressure increased, but only in the excess calorie group.

Fat mass increased by approximately 2.2 pounds in the adequate calorie group and 2.1 pounds in the excess calorie group when comparing ultra-processed to unprocessed diets. These changes occurred within just three weeks.

“Our results prove that ultra-processed foods harm our reproductive and metabolic health, even if they’re not eaten in excess. This indicates that it is the processed nature of these foods that makes them harmful,” says Jessica Preston, lead author of the study, who carried out the research during her PhD at the University of Copenhagen’s NNF Center for Basic Metabolic Research (CBMR).

Public Health Concerns for Modern Diets

Ultra-processed foods have become dominant in Western diets. The study notes that UPFs are “now accounting for over 50% of total energy intake in the UK, Australia, Canada, and the USA.” These products undergo extensive industrial processing and contain ingredients rarely used in home cooking.

Despite being matched for macronutrients, the ultra-processed diet contained “elevated levels of saturated fat, cholesterol, refined grains, added sugars, and dairy products and lower amounts of fiber” compared to the unprocessed diet.

Study authors conclude that their findings demonstrate “that consumption of ultra-processed food is detrimental for cardiometabolic and reproductive outcomes, regardless of excessive caloric intake.” They suggest that “moving dietary patterns away from UPF and toward less-processed alternatives may promote cardiometabolic and mental health, along with amelioration of male reproductive fitness.”

That age-old advice about eating real food isn’t just grandmotherly wisdom. For men thinking about starting a family or simply wanting to maintain their health, the choice between processed convenience and whole foods might matter more than anyone realized. Sometimes the simplest changes make the biggest difference.

---

Disclaimer: This article summarizes a 3-week randomized crossover trial in 43 healthy men (ages 20–35) and is not medical advice. Results may not apply to everyone. Fertility signals were limited (FSH fell only with extra calories; the sperm-motility dip was not statistically significant). Calories/macros were matched, but menus differed (e.g., fiber, added sugars), which can influence outcomes. If you’re concerned about how your diet influences your fertility, talk with a healthcare professional about personal nutrition choices.