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Metabolic slowdown is a real thing. It's due to your muscle mass declining—by up to eight percent each decade after age 30 and up to 10 percent after the big 5-0. "You lose about a half pound of lean muscle every year in your twenties, thirties, and forties, and once you hit your fifties, you lose roughly a pound a year," says Wayne Westcott, Ph.D., an exercise physiologist at Quincy College in Boston. Although researchers aren't exactly sure how this happens, the leading theory is that your body starts breaking down muscle at a faster rate than it can build it back up again. Muscle is metabolically active, so your metabolism slows as it declines.