Channel 4 did a series called something like 'the secrets of slim people', which was a sort of reverse 'secret eaters' type programme, so instead of the people who 'lived on salad but couldn't understand why they were overweight, especially as they had a physical job' where the woman was a traffic warden on the high street who couldn't walk past Greggs without going in to buy a pasty, it featured the people who appararently lived on junk food but were slim. It's probably still on their catch up service.
Like secret eaters, they logged everything the people ate for a few days, and time after time, the big secret was ........ they didn't actually eat that much
.
The people generally didn't eat 3 big meals a day, didn't snack, stopped when they were full, if they had something like a pizza or other takeaway, saved a lot of it for later, didn't drink lots of calories, basically didn't do any of the things that make people overweight.
They calculated all their calories, and it was, with only one exception, less than 2000 calories a day and less than what they said that person burned given their sex, weight, activity level etc.
There was one woman who apparently ate more than she burned, and it couldn't be explained, but of all the other people in the series, the big secret was that they just didn't overeat.