"Your metabolic rate goes down as you age," Dr. Grunvald says. "Your peak metabolic rate is kind of around your late 20s, maybe 30, around there. After that, it's downhill."
So all those times you could get away with late-night pizza and French fry binges will definitely catch up with you as you get older, as will the number of beers you end up physically regretting the next morning. OK, your metabolism slows down. That's something you can deal with...
You lose muscle with age
"As we age, we naturally lose lean muscle mass at a rate of 1% per year starting around age 30, and that accelerates starting around age 40," says Dr. Caroline Apovian, director of Nutrition and Weight Management at Boston Medical Center.
So your metabolism slows down, and you lose lean muscle mass?! The two go hand-in-hand, she explains, as having more lean muscle mass means you'll have a faster metabolism. So if you're eating the exact same calories now as you did in your early 20s, you could still be putting on weight.