In a fascinating first-of-its-kind study, researchers have discovered diets composed of ultra-processed foods lead to overeating and weight gain when compared to diets composed of unprocessed foods. Both diets contained the same amounts of nutrients such as sugars and fats, leading to a hypothesis that something else in the ultra-processed food engenders overeating and weight gain.

The experiment took 20 healthy adults and administered them one of two specifically controlled meal plans. For two weeks the subjects ate either a diet consisting of unprocessed foods such as fruits, vegetables and nuts, or a diet of ultra-processed foods such as bread, sausages and baked goods. Importantly both diets were matched for calories, energy density, macronutrients, sugar, sodium, and fiber. The subjects were directed to eat as much as they felt they wanted.

"I was surprised by the findings from this study," explains Kevin Hall, lead author on the new study, "because I thought that if we matched the two diets for components like sugars, fat, carbohydrates, protein, and sodium, there wouldn't be anything magical about the ultra-processed food that would cause people to eat more. But we found that, in fact, people ate many more calories on the ultra-processed diet, and this caused them to gain weight and body fat."

After two weeks on the diets, the ultra-processed group had gained around two pounds of body weight and ate on average 500 calories more every day compared to the unprocessed group. It has previously been assumed that the high sugar, fat and salt content in processed foods is what drives a person to overeat and subsequently gain weight, but this study offers an interesting counterpoint to that hypothesis. The implication of the study is that there is some other unexplained mechanism at play causing a person to eat more when consuming ultra-processed foods.

"There may be something about the textural or sensory properties of the food that made them eat more quickly," hypothesizes Hall. "If you're eating very quickly, perhaps you're not giving your gastrointestinal tract enough time to signal to your brain that you're full. When this happens, you might easily overeat."

Blood measurements from each group revealed another intriguing datapoint. In those subjects eating the unprocessed diet increased levels of a hormone called PYY were detected. This hormone is known to act as an appetite suppressant. The unprocessed diet also resulted in decreases in ghrelin, a satiety hormone.

Perhaps the biggest limitation of the study comes in its somewhat arbitrary distinction between unprocessed foods and ultra-processed foods. Gunter Kuhnle, a nutrition expert from the University of Reading, suggests while the study is interesting and well-designed, it is difficult to find much meaning in the results due to the vague distinction between the two food categories.

"'Processed food' has become a catch-all phrase to describe 'unhealthy' food, when most foods are processed – and processing is usually important for palatability, safety and preservation," says Kuhnle, who did not work on this new study. "According to the NOVA classification, which has been used by the authors to categorize foods, chilling, freezing or packaging is already a processing step – and for example butter, cheese or bread are processed foods. The 'ultraprocessed' category is somewhat arbitrary, as it categorizes foods not by the actual processing steps used, but rather the intended outcomes of these processes."

The researchers behind this new study seem acutely aware of this limitation. Future work hopes to better home in on specific types of processed food and how they can possibly trigger overeating or body weight changes.

"We need to figure out what specific aspect of the ultra-processed foods affected people's eating behavior and led them to gain weight," says Hall. "The next step is to design similar studies with a reformulated ultra-processed diet to see if the changes can make the diet effect on calorie intake and body weight disappear."

The new research was published in the journal Cell Metabolism.

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