Microwave foodCopyright of the image
Getty Images

Ultra-processed foods cause people to eat more and gain weight, suggests the first attempt to evaluate their impact.

Volunteers watched every piece of food that they ate for a month.

And when they receive ultra-processed foods, they consume 500 more calories a day than when they were receiving unprocessed meals.

The US National Institutes of Health have stated that ultra-processed foods can affect hunger hormones in the body, causing people to continue eating.

  • BBC Food: What's a processed food?
  • Ultra-processed foods linked to cancer

There are scientific arguments on the definition of ultra-processed foods, but the lead researcher, Dr. Kevin Hall, said that it was like "pornography – it's hard to define, but you know when you see it. "

Warning signs include:

  • ingredients that you can not pronounce
  • more than five ingredients listed on the package
  • everything your grandmother would not recognize as food

Copyright of the image
Cell metabolism

Legend

Ultra-processed foods including quesadillas, stir-fried beans and diet lemonade.

Copyright of the image
Cell metabolism

Legend

Unprocessed lunch of spinach salad with chicken breast, apple slices, bulgur, sunflower seeds and grapes

Twenty people spent a month of their time living in a laboratory.

For fifteen days, they received either ultra-processed meals or unprocessed meals, and then the diets were replaced for the second half of the study.

The participants were allowed to eat as much as they wanted and the researchers kept a close watch on what was happening to them.

During their fifteen or so ultra-processed meals, the volunteers consumed an average of 508 extra calories a day and consumed 1 kg (2 lbs).

Dr. Hall, of the National Institute of Diabetes and Digestive and Kidney Diseases, told BBC News: "This is the first study to show that there is a cause and effect relationship.

"Ultra-processed foods have resulted in increased caloric intake, body weight and fat.

"This suggests that this could play a role in the general population."

Dr. Hall said previous studies had estimated that the "obesity epidemic" in the United States was caused by people consuming 250 to 300 extra calories a day.

But why?

The explanation is, for the moment, elusive.

Human guinea pigs reported that both meals were also tasty; so there was no preference for ultra-processed ones.

The nutritional content of both diets has also been carefully matched to ensure an equal content of sugars, carbohydrates, fat and fiber.

One potential explanation is the impact of industrially processed foods on hormones that alter the desire to eat.

Dr. Hall told BBC News: "When some people were eating an untransformed diet, one of the appetite suppressing hormones (called PYY) that, according to other studies, was linked to people's appetite control was increasing as they were now eating fewer calories. "

The study also showed that levels of the hormone hunger, ghrelin, decreased in the untransformed diet.

Does this explain the crisis of obesity?

The study covers a limited number of people and over a short period. It is therefore difficult to know if the conclusions apply more widely.

Some people with a particular diet ate an extra 1,500 calories in the ultra-processed diet, while others ate just about the same.

Dr. Gunter Kuhnle, of the University of Reading, said that food processing was often important "for flavor, safety and conservation".

"It's a well-designed and well-conducted study with interesting results, although perhaps not surprising.

"It seems that participants found the ultra-processed foods tastier, faster and therefore more, perhaps because they took longer to feel satiated.

"A very interesting result of the study is the cost per energy: the ultra-processed diet was much cheaper than the unprocessed control diet, and that would probably have implications from a public health point of view."

Follow James on Twitter.