Ultra-processed foods like burgers, fries, and soda may be convenient and tasty. However, a new study reveals that diets heavy in these ultra-processed items can cause weight gain and hormonal changes even when calorie intake is kept the same, washingtonpost.com. In a controlled trial, young men eating an ultra-processed diet for just a few weeks gained more body fat and experienced shifts in reproductive hormones compared to when the same men ate unprocessed foods with identical calories and nutrients. These findings challenge the old notion that “a calorie is a calorie,” pointing to food processing itself as a culprit in metabolic health issues beyond just overeating.
What Are Ultra-Processed Foods?
“Ultra-processed” foods are industrial products made with ingredients and methods far removed from home cooking. In fact, over half (~55%) of the calories in Western diets come from ultra-processed foods – think chips, soft drinks, packaged snacks, fast-food meals, and sugary breakfast cereals. Unlike whole or minimally processed foods (like fresh fruits, vegetables, or unrefined grains), ultra-processed products are formulations packed with additives such as refined sugars, oils, salt, preservatives, flavor enhancers, and other synthetic ingredients. They tend to be high in refined starches and fats, low in fiber and micronutrients, but engineered to be hyper-palatable, cheap to produce, and have long shelf lives. This convenience comes at a cost: diets rich in ultra-processed foods have long been associated with higher risks of obesity, diabetes, heart disease, and other health problems, news-medical.net. Researchers have debated why – is it just because these foods make people overeat, or do the processing and additives themselves harm our bodies?
Same Calories, Different Outcomes: A Groundbreaking Study
To isolate the effect of food processing, scientists at the University of Copenhagen conducted a tightly controlled dietary intervention in humans. They recruited 43 healthy men (ages 20–35) and had each participant follow two different diets: one composed mostly of ultra-processed foods and one of unprocessed/minimally processed foods. sciencedaily.com. Each diet phase lasted three weeks and was separated by a three-month “washout” period during which the men returned to their normal eating habits. Importantly, both diets were designed to be nutritionally similar – each participant was given the same amount of calories, protein, carbs, and fat on the ultra-processed diet as on the unprocessed diet, tailored to their daily energy needs. (In fact, half the group received a weight-maintaining calorie level, and the other half received an extra 500 calories per day on both diets, to see if excess calories would amplify any effects.) Crucially, the volunteers were blinded – they weren’t told which diet they were on at a given time – to reduce bias.
Despite the equal calorie counts, the outcomes between the two diets were starkly different. On the ultra-processed diet, participants gained about 1 kilogram (~2.2 pounds) more body fat on average than they did on the unprocessed diet technologynetworks.com. In other words, when the same person ate an all-ultra-processed menu, he put on significantly more fat mass than when he ate a whole-foods diet, even though the calorie intake was identical in both conditions. This was true both for those on a normal-calorie plan and those on the higher-calorie plan – the ultra-processed nature of the food led to extra fat gain regardless of modest overeating or not. Notably, some markers of cardiovascular health (like blood cholesterol and other risk factors) also worsened during the ultra-processed diet period, suggesting broader metabolic downsides.
This finding is striking because it suggests not all calories are equal – the source and processing of those calories matter. In contrast, a few years ago a landmark NIH study had shown that people eating ultra-processed foods tend to eat more and gain weight because the foods are so palatable and easy to over-consume, cc.nih.gov. In that 2019 trial, volunteers on an ultra-processed diet ate about 500 extra calories per day ad libitum and gained about 0.9 kg in just two weeks, whereas the same people lost weight on an unprocessed diet. That earlier study demonstrated that ultra-processed foods can cause overeating and weight gain by encouraging people to consume more than they need. The new research goes a step further – by tightly controlling intake, it showed that even without overeating, ultra-processed foods still led to weight (fat) gain. This implies there is something metabolically harmful about the ultra-processing itself. As one researcher put it: Calorie intake from minimally processed vs. ultra-processed meals doesn’t have the same consequences on our bodies,”washingtonpost.com.
Hidden Pollutants and Hormonal Disruptions
Beyond extra fat, the ultra-processed diet had other worrying effects on the participants’ biology. The researchers discovered that when the men ate ultra-processed foods, their bodies accumulated significantly higher levels of a chemical pollutant called cxMINP. This substance is a type of phthalate – a chemical used in plastics (often in food packaging and processing equipment) to make them flexible. Phthalates are known endocrine disruptors, meaning they can interfere with hormones in the body. Indeed, in the study, the rise in cxMINP during the ultra-processed diet coincided with a drop in key reproductive hormones in the men. Testosterone and follicle-stimulating hormone (FSH) levels fell when the men were on the ultra-processed regimen. These hormones are crucial for sperm production and male fertility, so such changes raise red flags for reproductive health.
Why might eating TV dinners and packaged snacks lower your testosterone? One likely explanation is that chemicals like phthalates leaching from packaging (or introduced during manufacturing) get into the food and then into our bodies. The lead scientist noted that the accumulation of phthalates in the body could be one reason why the ultra-processed diet caused weight gain and hormonal changes. In other words, it’s possible that these contaminants disrupt metabolism and endocrine function, contributing to fat storage and lower sex-hormone levels. (There may also be other factors at play – ultra-processed foods could alter gut bacteria, glycemic responses, or the thermic effect of food (the calories burned digesting food) in ways that promote fat gain. Those aspects are still under investigation.)
The implication of this finding is especially notable given trends in public health. Over the past few decades, sperm counts in men have been declining worldwide, and some experts suspect environmental chemicals and poor diet quality as contributors. This study draws a direct line between an ultra-processed diet and factors linked to fertility – suggesting that a diet of donuts and hot dogs isn’t just bad for your waistline, but could also threaten reproductive health. As nutrition professor Marion Nestle (who was not involved in the research) remarked, the results add to “overwhelming evidence that diets based on ultra-processed foods are not good for us”washingtonpost.com – and the big surprise here is the potential impact on male fertility, an area not typically associated with food processing.
Study Limitations and Future Questions
Like all studies, this research has its caveats. Here are some important points to keep in mind:
- Small, specific sample: The experiment involved only 43 young men (ages 20–35) as participants. It did not include any women or older adults, so we don’t know if females or other age groups would experience the same effects. Further studies on a more diverse population are needed to confirm generalizability.
- Short-term intervention: Each diet was followed for just 3 weeks in this trial, which is relatively short. While the changes observed in that time were significant, the long-term effects of eating an ultra-processed diet (over years or a lifetime) remain unclear. Would the fat gain and hormone disruptions worsen over time? Could the body adapt? These questions require longer studies to answer.
- Unclear mechanisms: Why ultra-processed foods cause fat gain and hormonal disruption independent of calories is not fully understood. The study identified phthalate exposure as a possible contributor, but that may be only part of the picture. Ultra-processed foods differ in many ways (additive content, fiber structure, how they affect gut microbes, etc.), and more research is needed to pinpoint the biological mechanisms involved. Scientists will continue exploring how these foods alter metabolism beyond just their basic nutrient content.
Despite these limitations, the study was well-controlled and lends strong support to the idea that the quality of calories matters. The authors deliberately kept calories and macros equal between diets – yet still saw negative changes on the ultra-processed diet. This strengthens confidence that it’s the “processed nature” of the foods driving the harm, as the lead author emphasized in news-medical.net.
Conclusion: Rethinking “You Are What You Eat”
This research provides powerful evidence that all calories are not created equal – 2000 calories of fast-food fare can affect your body very differently than 2000 calories of fresh, whole foods. Ultra-processed diets appear to inherently promote fat accumulation and disrupt hormones, even in the absence of overeating. These findings underscore that maintaining a healthy weight and hormonal balance is not just about counting calories, but also about choosing high-quality, unprocessed foods whenever possible. If you’re aiming to improve your health or waistline, cutting back on ultra-processed products (and replacing them with natural, whole foods) could give you an edge beyond what any calorie math alone would predict.
Importantly, this study also shines a light on a potential link between diet and reproductive health. For men especially, regularly eating ultra-processed foods might carry unseen risks – like lower testosterone or impaired fertility – in addition to more belly fat. Public health experts are taking note: the rise of ultra-processed food consumption has paralleled surges in obesity and diabetes and declines in sperm quality, washingtonpost.com. Addressing this may require more than just telling people to eat less; it calls for broader changes in food policy and education. The researchers behind this study even suggest that nutritional guidelines may need updating to warn people about the unique dangers of ultra-processed diets.
In summary, while convenience foods are tempting in our busy lives, their hidden costs to our metabolism and hormones are becoming increasingly clear. For better health, the old advice holds true: focus on whole, minimally processed foods – your body will thank you. As science continues to unravel the chemical and physiological effects of ultra-processing, one message is already loud and clear: eating real food is about more than just the calories.
