This question was originally posted on Quora.com and was answered by Viktoria Keller, Co-creator of Zerxza.com and possibly a coffeeholic Here’s a picture that sums it up pretty well: Some people might seem to eat loads of fast food but in reality, their diet might be pretty clean in general. If they are also physically active, then the mystery’s solved: plenty of exercise and mostly healthy diet means they won’t really gain weight, even if they eat fast food quite often. Source