The human guinea pigs reported both meals were equally tasty, so a preference for ultra-processed was not to blame.
The nutritional content of the two diets was also carefully matched to ensure they had equal amounts of sugars, other carbohydrates, fats and fibre.
One potential explanation is the impact of industrially processed foods on the hormones that alter the desire to eat.
Dr Hall told BBC News: “When people were consuming the unprocessed diet, one of the appetite-suppression hormones (called PYY) that has been shown in other studies to be related to restraining people’s appetite actually went up despite the fact that they’re now eating less calories.”
The study also showed the levels of the hunger hormone ghrelin went down on the unprocessed diet..
Does this explain the obesity crisis?
The study is on a limited number of people only and for a short period of time, so it is unclear if the findings apply more broadly.
Some people on the diet ate an extra 1,500 calories on the ultra-processed diet, while others ate roughly the same.
Dr Gunter Kuhnle, from the University of Reading, said processing food was often important “for palatability, safety and preservation”.
He said: “This is a well designed and well conducted study with interesting, although perhaps not surprising, outcomes.
“It seems that participants found ultra-processed food more palatable, ate more quickly and consequently more – possibly because it took longer for them to feel full.
“A very interesting outcome of the study is the cost-per-energy: the ultra-processed diet was considerably cheaper than the unprocessed control diet, and this is likely to have implications from a public health point of view.”