It’ll come as no surprise that chowing down on a microwavable pizza, caked in oily sausages and dipped in ranch sauce, isn’t going to do your health any favors. But new research has further confirmed the idea that ultra-processed foods are certainly not something to consume too often. 

Reporting in the journal Cell Metabolism, US National Institutes of Health researchers found that ultra-processed foods make you eat more, eat faster, and – unsurprisingly – put on more weight.

The study is the first randomized, controlled trial to directly compare differences in calorie consumption and weight gain between a diet rich in ultra-processed foods and an unprocessed diet. The team found that the same person will eat more food and gain more weight if they consume lots of ultra-processed foods, even if they contain similar quantities of carbohydrates, fat, sugar, salt, and other nutrients to those found in unprocessed meals. 

"I was surprised by the findings from this 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," said lead author Kevin Hall, from the National Institute of Diabetes and Digestive and Kidney Diseases.

"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."

Everybody knows that junk food is terrible for your body, but the argument against ultra-processed foods is often shrouded by other factors. One could argue that people who eat a lot of processed food are only unhealthy because, for example, they have less money and have to eat cheaper food or they have a more lax attitude towards exercise. However, this new research changes things.

Twenty volunteers – admittedly a pretty small study sample – spent 28 days under the watchful eyes of scientists in a lab while they consumed one of the two diets for 14 days, before switching to the other. The processed diet included a breakfast full of stuff like sugary cereals, margarine, and packaged muffins, while the unprocessed breakfast included plain Greek yogurt, fruit, and nuts. They were told they could eat as much as they wanted.

After two weeks of the junk diet, people gained an average of 0.9 kilograms (2 pounds). When the same people ate an unprocessed diet, they consumed over 500 calories less each day and lost body fat.

So, what could explain this pattern? It could be a question of the body attempting to fill its nutritional quotas. For example, the ultra-processed diet contained slightly less protein and dietary fiber, so people might have been trying to fill this gap. On the other hand, it could be that junk food stimulates the reward region of our brains more deeply, causing us to compulsively eat more. Alternatively, the textural or sensory properties of the food might be a factor. However, it's likely that all of these factors play a role. 

While the research has been praised by independent experts in the field, many have warned that the debate is far from over. 

"All this research actually shows is that people preferred the processed foods over the supposedly ‘unprocessed’ varieties," commented Bob Rastall, Professor of Food Biotechnology at the University of Reading. 

"Why would anyone be surprised at this? They ate more of the ‘ultra-processed’ food, resulting in a higher energy intake and gained weight as a result. To investigate the effect of food processing you would need nutritionally matched foods in each group and to control the amount eaten – then the only difference would be the degree of processing."