Spaghetti hoops on toast, sausages and bacon are served in a canteen

White toast topped with spaghetti hoops is the epitome of ultra-processed food. Such foods have been linked with weight gain. Credit: Paula Solloway/Alamy

Nutrition

First strict test shows why a junk-food diet packs on weight

A steady repast of pancakes, packaged snacks and processed meats prompted people to consume more calories.

Harried humans around the world are embracing cheap, ultra-processed foods such as white bread, bacon and hash browns. But the first randomized controlled trial on the health effects of these foods shows that people offered such a diet ingest more calories — and pack on more weight — than they do when presented with more wholesome meals.

To determine how processed foods affect health, Kevin Hall at the National Institute of Diabetes and Digestive and Kidney Diseases in Bethesda, Maryland, and his colleagues fed study participants ultra-processed foods for two weeks. The same participants also spent two weeks eating unprocessed foods, such as fish and fresh vegetables. Both types of meal had the same number of calories, and the same levels of nutrients such as sugar and fat. Participants chose how much to eat.

When offered ultra-processed foods, people ate more quickly and took in an average of 500 more calories per day than when they were offered unprocessed foods. Participants gained roughly 1 kilogram during the trial’s junk-food phase and lost roughly the same amount during the whole-foods phase.