The experiment was simple enough. Volunteers were fed either an ultra-processed diet or a diet that was identical in terms of fat, salt, sugar and fibre but without any UPF. After two weeks, both groups then switched to receive the other diet. During both phases, participants could eat as much as they liked. On the ultra-processed diet, participants ate more and gained weight, whereas on the unprocessed diet they actually lost weight, despite having access to as much food as they wanted.
