First randomized, controlled study finds ultra-processed diet leads to weight gain
Studies throughout the world have shown associations between processed food and negative outcomes. Eating processed food may make you gain weight. It seems obvious, right? Surprisingly though, most scientific findings around this topic have only relied on self-reporting and are simply associations.
In mid-May, Dr. Kevin D. Hall, a senior investigator with the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) published a study on the first randomized, controlled trial at the NIH Clinical Center. The study found that people who ate processed food ate more calories and gained more weight than when they consumed a whole food diet with foods that were unprocessed.
The small-scale study, led by Hall, admitted 20 healthy volunteers for one continuous month. In random order, for two weeks on each diet, healthy volunteers ate either meals made up of ultra-processed foods or meals of minimally processed foods.
Foods were considered "ultra-processed" if they had ingredients predominantly found in industrial food manufacturing, such as hydrogenated oils, high-fructose corn syrup, flavoring agents and emulsifiers. They included foods such as potato chips, sugar-sweetened drinks, processed meats and French fries.
For the unprocessed foods, the team used the NOVA classification. This system categorizes foods based on the extent and purpose of food processing, rather than in terms of nutrients. It included minimally processed food, especially plant foods like vegetables, fruits, nuts, whole grains, legumes, milk, eggs, fish and meat.
For example, an ultra-processed breakfast might consist of a bagel with cream cheese and turkey bacon, while the unprocessed breakfast was oatmeal with bananas, walnuts and skim milk.
Previous studies published across the country were observational — looking at large groups of people that showed associations between diets high in processed foods and health problems. But because none of these studies randomly assigned people to eat specific foods and then measured the results, scientists couldn't say for sure whether the processed foods were the culprit, or whether there were other causes.
"Though we examined a small group, results from this tightly controlled experiment showed a clear and consistent difference between the two diets," said Hall. "This is the first study to demonstrate causality — that ultra-processed foods cause people to eat too many calories and gain weight."
The NIDDK researchers worked closely with the Metabolic Clinical Research Unit, which is especially equipped to study issues involving diet and metabolism.
Dietitians in the Metabolic Kitchen prepared controlled meals and matched nutritional components in the processed and non-processed diets. Each diet contained the exact same quantity of calories, sugars, fiber, fat, salt and carbohydrates. The foods needed to be commercially available and listed in the U.S Food and Drug Administration nutrition database to provide accurate information about the nutritional content. Participants could eat as much or as little as they wanted.
This study was considerably shorter than most studies. It took less than a year to complete thanks to the amount of the staff support in the Nutrition Department and Metabolic Unit. In addition, the only parameters for inclusion were age range and a certain Body Mass Index. An even number of women and men were enrolled.
Since patients were unable to leave the unit for the entire month of their stay, researchers included exercise in order to keep their physical activity level constant. Participants were required to complete 60 minutes a day in three 20-minute bouts on a fixed low watt cycle odometer, which equals what a person going about their everyday life exerts. One of the details researchers wanted to explore was whether there were changes in insulin sensitivity. They wanted to keep a constant physical activity level throughout the study so they did not get confounds by having prolonged periods of no activity. They also did several measurements of how many calories the subjects were burning.
The basic question for the researchers was - do people eat more on a processed diet than on an unprocessed diet? The answer was a definite "yes." On the ultra-processed diet, people ate about 500 calories more per day, ate at a faster rate and gained weight. On average, participants gained 0.9 kilograms, or 2 pounds while they were on the ultra-processed diet and lost an equivalent amount on the unprocessed diet.
This is the first study that shows causality: ultra-processed food causes people to consume calories and gain weight. If you remove those ultra-process foods and give them the same calories, they report liking the food just as much, yet they lose weight, and they eat fewer calories.
Justin Butner, one of the study's volunteers, said that the experience has caused him to think of processed food differently. "Ultra-processed foods are so calorie-dense that feeling full meant I'd overeaten. Some days I would get through my meal in a few minutes without really noticing I was eating. It wasn't satisfying." In contrast, he found that unprocessed foods tend to be more filling and the process of eating more enjoyable.
While the study reinforces the benefits of unprocessed foods, researchers note that ultra-processed foods can be difficult to restrict. "We have to be mindful that it takes more time and more money to prepare less-processed foods," Hall said. "Just telling people to eat healthier may not be effective for some people without improved access to healthy foods." However, the take-away is, as much as you can, try to avoid ultra-processed food and choose more whole foods. You can still like your food and it will be easier to lose weight.
One of the remaining questions is what is causing the increased calorie intake? Researchers still do not know the cause. It was not the components: sugars, salt, carbohydrates and fat. Hall's team has some hypotheses, which will inform their next study.
"We need to figure out what specific aspect of the ultra-processed foods affected people's eating behavior and led them to gain weight," Hall said. "The next step is to design similar studies with a reformulated ultra-processed diet to see if the changes can make the effect on calorie intake and body weight disappear." As far as replicating the study elsewhere, Dr. Hall notes that unfortunately, there are fewer and fewer in-patient facilities where these types of studies can be done, where subjects can be isolated, measured and fed.
Support for the study primarily came from the NIDDK Division of Intramural Research.
- Debbie Accame