In 2019, he put together the first randomized controlled trial to compare the ultra-processed diet to one based on less processed foods.

Hall used the NOVA classification system — developed by the Brazilian researchers — which categorizes foods based on how much processing they undergo. It distinguishes between unprocessed or minimally processed foods, like an ear of corn or frozen peas; processed foods – like tuna canned in oil or smoked meats, which generally have two or three ingredients; and ultra-processed foods, which are created with formulations of ingredients made using industrial techniques.

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The results took Hall by surprise.

"I had sort of expected that ... there wouldn't be any difference," says Hall. "But in fact, what we saw was that when they were on the ultra-processed diet, they were eating about 500 calories per day more than when they were on the unprocessed diet and they were gaining weight and gaining body fat" – they put on about 2 pounds on average.

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