Obviously, if you eat more calories, you're going to gain weight. But how your body processes those calories is also a factor; your body burns energy just staying alive during day-to-day physiological processes. There are a lot of things going on that could change how efficiently your body uses its fuel.
Getting older changes everything
Like death, taxes, and cliches, aging is one thing in life that's guaranteed. Along with gray hair and aching joints, you can also look forward to packing on the pounds.
"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.