Guillaume Raineri, 42, a former HVAC technician from Gonesse, a small town near Paris, faced an unexpected challenge after moving to the United States. His new home, Bethesda, Maryland, offered food that was very different from what he was used to. Portions were larger, and meals had more salt and sugar.
Curious about how this diet affected him, Raineri joined a four-week study at the National Institutes of Health (NIH). The study aimed to understand why the American diet often leads to weight gain and chronic illnesses.
Controlled Study at the NIH Clinical Center
At the NIH Clinical Center—the world’s largest hospital focused on scientific research—Raineri stayed in a room with a hospital bed, a recliner, and an exercise bike. Each day, he received three large meals, each around 2,000 calories. He could eat as much as he wanted.
For the first week, he ate mostly minimally processed foods like salads, vegetables, and grilled chicken. He felt good during this time. But after the first week, the researchers switched his diet to ultra-processed foods, including chicken nuggets, fries, and peanut butter and jelly sandwiches. Raineri said these foods “just sat in my stomach” and caused heartburn, bloating, and irritability.
Throughout the study, Raineri took metabolic tests, including time in a chamber that measured how his body used energy. The researchers also monitored his eating speed and hormone levels tied to appetite. The results showed that on the ultra-processed diet, he and others ate about 500 extra calories per day, gained weight, and had higher insulin and blood sugar levels.
The Impact of Food Processing on Health
Kevin Hall, the lead researcher and a nutrition expert at NIH, said the study shows that it’s not just nutrients like salt, fat, or sugar that matter, but also how food is processed. Ultra-processed foods often contain ingredients that cannot be made at home. Hall said, “We’re starting to learn that processing really matters.”
This idea comes from Brazilian epidemiologist Carlos Monteiro, who divided foods into four groups based on processing. Group 1 includes unprocessed or minimally processed foods like nuts and vegetables. Group 4 includes ultra-processed foods with additives, preservatives, and artificial substances designed to improve taste, shelf life, and convenience.
Critics say this classification can be confusing because it groups very different foods together—for example, canned beans and gummy bears. However, many studies link ultra-processed foods to higher risks of high blood pressure, heart disease, certain cancers, and mental health problems like depression. These risks are often related to how energy-dense and highly palatable these foods are, due to their mix of fat, sugar, and salt.
Ultra-Processed Foods and the Gut Microbiome
The study also looked at how ultra-processed foods affect the gut microbiome, the community of bacteria in our digestive system. Katherine Maki, a microbiome researcher at NIH, explained that most Americans consume too little dietary fiber. Without fiber, good bacteria like Bacteroides thetaiotaomicron may start eating the mucus lining of the gut, which can cause inflammation.
Many “no-sugar-added” products contain artificial sweeteners that disrupt gut bacteria further. This can harm the body’s ability to manage sugar and reduce bacterial diversity. The rise of Western diets worldwide may be linked to these changes and related health problems beyond digestion.
Complexities of Industrial Food Processing
A visit to Seviroli Foods, a pasta and sauce producer near Manhattan, showed how food processing can be complex. Franco LaRocca, the company’s chef and VP of research, said their products use enriched flour mixed with ingredients like guar gum and cornstarch. While the food is processed, Seviroli limits additives and avoids unhealthy oils.
Nutrition professor Marion Nestle of New York University clarified that industrial production alone does not make a food ultra-processed. “It has to be made to replace real food and usually loaded with additives,” she said.
Nestle pointed out that some foods look healthy but are ultra-processed, like whole-wheat breads made from highly processed flours. She also compared whole-fat yogurt, made simply from milk and cultures, to low-fat versions that contain emulsifiers and thickeners. This shows how processing affects health in subtle ways.
Ongoing Debate and Future Challenges
The NIH study has sparked debate among nutrition experts. Walter Willett, a Harvard professor critical of the ultra-processed food category, called some research “misleading” and warned against simple dietary rules.
Still, ultra-processed foods make up about two-thirds of the average American diet, raising concerns about their role in public health problems.
Some countries are exploring ways to regulate ultra-processed foods, such as marketing limits and taxes. But the relationship between diet, biology, and the food industry remains complex. Scientists, policymakers, and consumers face a difficult challenge in improving public health.
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