Hall et al./Cell Metabolism
For the past 70 years, ultra-processed foods have dominated the US diet. These are foods that are made from cheap industrial ingredients and made to be particularly tasty, and generally high in fat, sugar and salt are a big part of our growing waist. But is it something of the highly processed nature of these foods that makes people overeat? A new study finds the answer: Yes.
The study, which was conducted by researchers at the National Institutes of Health, is the first randomized, controlled trial that shows that eating a diet made from ultra-processed foods actually drives people to overeating and weight gain as compared to a diet that consists of whole or minimally processed foods. Study participants on the ultra-processed diet ate an average of 508 calories per day, averaging 2 pounds over a two-week period. Meanwhile, people who took the unprocessed diet lost an average of 2 pounds over a two-week period.
"The difference between the weight gain for one [group] and the weight loss for the other during these two periods is phenomenal." We have not seen anything like that, "says Barry Popkin, a nutritional professor at the University of North Carolina who has studied the role of ultra-processed foods in the American diet, but was not involved in the current research.
Dariush Mozaffarian, Dean of Tufts University's Friedman School of Nutrition Science and Policy, agrees that the results are striking. He says that was so impressive that the NIH researchers documented this weight gain, although each meal offered on the two different diets contained the same total amount of calories, fats, protein, sugar, salt, carbohydrates and fiber. The study participants were allowed to eat as much or as little as they wanted, but in the end they ate more of the ultra-processed meals, though they did not rate them as tastier than the unprocessed ones.
"These are groundbreaking insights that the processing of food makes a big difference in how much a person eats," says Mozaffarian. This is important as most foods currently sold in the US and increasingly worldwide are being ultra-processed.
Ultra-processed foods include more than just the obvious suspects like French fries, sweets, packaged desserts and ready meals. The category also includes foods that may surprise some consumers, including honey-nut cheerios and other breakfast cereals, packaged white bread, sauces, frozen sausages and other reconstituted meat products, and fruit-flavored yoghurt. According to Popkin, ultra-processed foods usually contain a long list of ingredients, many of which are made in laboratories. For example, instead of seeing "apples" on a food label, you may receive additives that restore the fragrance of that fruit. These are foods that are convenient and inexpensive and require little preparation.
The new study, which was published in the journal Cell Metabolism was led by Kevin Hall, a senior scientist at the National Institute of Science, for diabetes and digestive and kidney disease. Hall says he was surprised by his results because many people have suggested that it's the high salt, sugar and fat content in ultra-processed foods that drives people to gain weight. "If you adjust the diets for all these nutrients, then some of the ultra-processed foods still have that big effect on calorie intake," says Hall.
To complete the study, Hall and his colleagues recruited 20 healthy, stable foods Adults with obesity – 10 men and 10 women – are expected to live in a NIH facility for four weeks. All meals were provided for her. Participants were randomly assigned to one of two diets for two weeks: One group was fed an unprocessed diet with whole or minimally processed foods such as roast beef with vegetables and basmati rice and orange slices. The other group ate an ultra-processed diet with meals such as chicken salad made from chicken preserves, mayonnaise and white bread, served with peach preserves in heavy syrup. At the end of the two weeks, the groups were then assigned to the opposite diet plan.
Although the study was small, it was also heavily controlled. The researchers knew exactly how many macronutrients and calories the participants consumed – and burned because they performed detailed metabolic measurements. The scientists also tracked other health features, including blood sugar levels and even hormone levels. Hall notes that this makes the execution of such studies extremely difficult and expensive. The study design also makes the results all the more meaningful, as Popkin and Mozaffarian say.
"By putting people in a controlled environment and giving them their food, one can truly understand biologically what is going on and the differences are striking." says Mozaffarian.
First, previous studies have linked an ultra-processed diet to weight gain and poor health outcomes, such as an increased risk of multiple cancers and early death for all reasons. However, all of these studies were observational studies, meaning that they could not prove that ultra-processed foods caused these results, only that they correlated.
Hall said the new study was not designed to see exactly what ultra-processed foods lead to overeating, but the results suggest some mechanisms.
"One thing That was somehow fascinating was that some of the hormones involved in regulating food intake differed significantly between the two diets compared to baseline," says Hall.
For example, when participants ate the unprocessed diet, they ate a higher level of appetite-suppressing hormone called PYY, which is secreted by the gut, and a lower level of ghrelin, a starvation hormone, which might explain why they burn fewer calories ate. In the ultra-processed diet, these hormonal changes were reversed so that participants had a lower level of appetite-lowering hormone and a higher level of starvation hormone.
Another interesting finding: Both groups ate about the same amount of protein, but these on the ultra-processed diet ate a lot more carbs and fat. There is a concept called the protein-lever hypothesis, which suggests that people eat until they meet their protein needs. Hall says that this seems to be the case in this study and partly explains the difference in calorie consumption they have found. Although meals were matched to calories and nutrients, including protein, the ultra-processed meals per bite were higher in calories. This is partly because ultra-processed foods tend to be low in fiber. Therefore, researchers had to add fiber to the drinks that are served as part of these meals to reach the fiber content of the unprocessed food. This means that the participants in the ultra-processed diet may have had to consume more carbs and fat to meet their protein needs.
And one final finding: People ate a lot faster – both in grams per minute and in calories per minute – on the ultra-processed diet. Hall says people may have consumed them faster because the ultra-processed foods were softer and easier to chew so they did not give their gastrointestinal tract enough time to signal their brain that they were full and done were overeating.
Hall says his findings impact on the food wars – vegan versus low-carbohydrate or low-fat diets. "They all have something in common … Advocates of healthy versions of these diets suggest people cut out ultra-processed foods." He says the elimination could be at least part of the success of these diets.
Popkin says that the takeaway message for consumers is, "We should try to eat as much real food as we can." It can be plant food, it can be animal food. It can be beef, pork, chicken, fish or vegetables and fruit, and you have to be very careful when you start looking at other types of food. "
But Popkin says The results are global Food industry challenged: How can convenience, abundance and low food costs be maintained without compromising health? "Let's see if they can produce ultra-processed foods that are healthy and not so seductive and will not make us eat so much more," he says. "But they do not have it yet."