Quirks & Quarks

Processed food is full of bad stuff, but the real problem is you eat too much of it

A new study found that people ate significantly more calories on an ultra-processed diet compared to a nutritionally equivalent unprocessed food diet.

A comparison of processed and unprocessed food diets showed people ate much more of the former

Example of an ultra-processed lunch meal from the study (Hall et al. / Cell Metabolism)
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Originally published on May 18, 2019.

A new study seems to suggest that there may be something about processed foods that causes people to eat more than they would of unprocessed foods containing similar levels of fat, sugar, salt and other nutrients.  In other words, the problem with processed foods is not just the kind of calories they contain, but that they seem to change our normal eating patterns.

"Something else kicks in that we still don't understand that causes people to increase calorie intake and gain body weight and gain body fat," said Kevin Hall, the lead author of the study. "This suggests that the quality of food matters for calorie intake."

Hall is the section chief in the Laboratory of Biological Modeling at the U. S. National Institute of Diabetes and Kidney Diseases in Maryland.

Two diets kept mostly the same

Twenty healthy participants of varying weights took part in the study and were housed for a month at the National Institute of Health clinic.

Researchers from the National Institutes of Health Kevin D. Hall, Ph.D., center, and Stephanie Chung, M.B.B.S., right, talk with a study participant at the NIH Clinical Center. (Jennifer Rymaruk, NIDDK)

For two weeks, they were fed a diet based on whole, unprocessed food. They then switched over to an ultra-processed diet for another two weeks. The two diets were matched in terms of the total calories available and the proportion of different nutrients such as fibre, protein, sugar, salt, and fat.  

The participants were presented with double their calorie needs for each meal, and were told to eat as much or as little as you want. They also had free access to snacks.

The experimenters measured how their weight changed over time on these two different diets. Everything they consumed was meticulously monitored, said Hall.

Example of an unprocessed lunch meal from the study (Hall et al. / Cell Metabolism)

An example of an ultra-processed breakfast served in the study was  croissants with margarine, turkey sausage and blueberry yogurt. These were typically ready-to-eat items from name brands taken from grocery store shelves.

An unprocessed meal, on the other hand, would be from whole food ingredients prepared by chefs. A breakfast might include scrambled eggs and hash brown potatoes with cream and onions.

The participants didn't prefer a particular diet, which was important to the study, Hall pointed out. They found both diets pleasant and familiar.

Ultra-processed food and weight gain

On average, Hall found that the same individuals consumed 500 more calories on the ultra-processed diet compared to the unprocessed diet, and the extra calories led to weight and body fat gains.

Dietitians at the Clinical Center of the National Institutes of Health designed recipes to test the effects of ultra-processed and unprocessed diets on study participants. (Jennifer Rymaruk, NIDDK)

It's still unclear why participants ate more of the processed food, said Hall. They did observe that appetite suppressing hormones increased and hunger hormone decreased while the participants were on the unprocessed diet.

Furthermore, participants ate processed meals much more quickly, perhaps because of the texture of the foods that made them easier to chew. This, he hypothesizes, could mean that the mechanisms that signal satiety or fullness might not have been fast enough to cause the participants to stop eating.

Hall is looking better understand why ultra-processed food leads to these effects in future studies, but his advice for now is to avoid them.

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