"...the average U.S. adult gradually gains about 30 pounds between the ages of 18 and 35..."

Not gaining the weight beats having to lose it

"But those excess pounds are not packed on overnight. Instead, the average U.S. adult gradually gains about 30 pounds between the ages of 18 and 35, says Deborah Tate, a professor of health behavior and nutrition at the University of North Carolina-Chapel Hill.

“There are a lot of lifestyle changes occurring in this age group, putting them at greater risk for gain,” says Rena Wing, a professor of psychiatry and human behavior at Brown University. Athletic teens may become desk-bound workers. Women gain weight with pregnancy and have trouble losing it. Busy young families may subsist on convenience food and kid-friendly snacks.

The result, Tate says, is a daily “energy gap” — the difference between calories burned and calories consumed — that slowly adds up."