Dive Brief:

  • Eating ultra-processed foods high in fat, sugar and salt causes overeating and weight gain when contrasted with a diet of whole foods or minimally processed ones, according to a new study from researchers at the National Institutes of Health. The study, published in the journal Cell Metabolism, is the first randomized, controlled trial to show these findings.
  • The study admitted 10 male and 10 female adults to the NIH Clinical Center’s Metabolic Clinical Research Unit for 28 days, where they were randomly given either an ultra-processed or unprocessed diet for two weeks, followed by two weeks of an alternate diet. They were told to eat as much or as little as they wished.
  • Participants who ate the ultra-processed diet consumed an average of 508 more calories daily and gained 2 pounds on average over the two-week period. Those on the minimally processed diet lost about 2 pounds on average over that same time. 

Dive Insight:

These study results could have a significant impact on manufacturers of ultra-processed foods. It’s the first randomized, controlled trial of its kind to show that eating two different diets — with the same total amount of calories, fats, protein, sugar, salt, carbs and fiber — can still lead to weight gain because participants ate more ultra-processed foods. 

Processed foods have already gained a bad rap with consumers in recent years. Shoppers have grown more wary of unpronounceable ingredients and more studies have shown the potential negative impact of ultra-processed foods. Besides the connection between ultra-processed foods and weight gain, previous studies have linked them with cancer and early death. In addition, eating more fresh produce and whole grains has been linked to a lower risk of cancer.

Many foods sold in the U.S. are ultra-processed, with about 60% of calories consumed by average Americans coming from ultra-processed foods, according to a study published in BMJ Open. But companies who manufacture these products could refute the results.  

The researchers acknowledged a limitation of the study was that eating ultra-processed foods is easier and cheaper than preparing unprocessed whole foods. Because the meals were prepared and given to participants for free, they didn’t choose them nor how they were presented. Therefore, the question was not addressed about how consumers choose between ultra-processed and minimally processed meals given the variables of cost and convenience. The small number of participants was also noted, although Kevin Hall, the study’s lead author, indicated the results were still important.

“Though we examined a small group, results from this tightly controlled experiment showed a clear and consistent difference between the two diets. This is the first study to demonstrate causality — that ultra-processed foods cause people to eat too many calories and gain weight,” he said in a release from NIH.

Many food manufacturers are aware of the already proposed connections between ultra-processed food and health concerns, and some have reformulated products to achieve a healthier profile and respond to consumer demand for less processed foods. If people become aware of study results like this NIH one, they may double down on better-for-you products and continue to turn away from ultra-processed foods loaded with salt, sugar and fat.

One thing the study found that might influence dietary preferences has to do with PYY, an appetite-suppressing hormone, and ghrelin, a hunger hormone, NPR noted. Participants who ate the minimally processed diet had higher levels of PYY and lower levels of gherelin, while it was the opposite for those eating the ultra-processed foods. This raises interesting questions about what and how these gut hormones are triggered, which could be the subject of followup studies on this topic.

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