Image copyright Getty Images

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.

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".

Warning signs include:

ingredients you cannot pronounce

more than five ingredients listed on the packet

anything your grandmother would not recognise as food

Image copyright Cell Metabolism Image caption Ultra-processed food consisting of quesadillas, refried beans, and diet lemonade.

Image copyright Cell Metabolism Image caption Unprocessed lunch of spinach salad with chicken breast, apple slices, bulgur, and sunflower seeds and grapes

Twenty people gave up a month of their time to live in a laboratory.

For a fortnight they were given either ultra-processed meals or unprocessed ones and then the diets were switched for the second half of the study.

The participants were allowed to eat as much as they wanted and researchers closely monitored what passed their lips.

During their ultra-processed foods fortnight, the volunteers, on average, ate an extra 508 calories a day and put on 2lb (1kg).

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

"Ultra-processed foods led to increases in calorie intake and in body weight and in fat.

"It's suggestive that this may be playing a role in the larger population."

Dr Hall said previous studies had estimated the "obesity epidemic" in the US was caused by people eating an extra 250-300 calories a day.

But why?

The explanation is, for now, elusive.

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."

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