Secret Eaters.

Secret Eaters is a British documentary television series about overeating. It is shown on Channel 4 and presented by Anna Richardson. With their permission, people with eating habit problems are videoed in their home over a week. The subjects do not suspect that private investigators are also following and videoing them elsewhere. At the end of the week, the true amount of what the subjects have eaten is revealed. The subjects are given some days to change their habits, and are brought back, showing considerable weight loss. This drama is intercut with a separate short story where the presenter and food experts test some factor causing overeating.