Ultra-processed foods lead people to eat more and put on weight, the first trial to assess their impact suggests.
Volunteers had every morsel of food they ate monitored for a month.
And when given ultra-processed food, they ate 500 calories a day more than when they were given unprocessed meals.
The US National Institutes of Health said ultra-processed foods may be affecting hunger hormones in the body, leading people to keep eating.
Ultra-processed food linked to cancer
There are scientific arguments about the definition of ultra-processed food but lead researcher Dr Kevin Hall said it was like “pornography – it’s hard to define but you know it when you see it”.