Junk food is precisely that

Eating At fast-food restaurants more than twice per week is associated with more weight gain and insulin resistance in otherwise healthy young adults.

Young adults who eat frequently at fast-food restaurants gain more weight and have a greater increase in insulin resistance in early middle age, according to a large multi-center study funded by the National Heart, Lung, and Blood Institute (NHLBI) and published in the January 1 issue of The Lancet.