Think about it: most of us move less as we age. We stop playing sports, we sit more at work, we skip the gym because we’re “too tired.” Meanwhile, we’ve been told to eat less to maintain our weight, so we cut calories without considering whether we’re getting enough protein to preserve muscle. The result? We lose muscle, our BMR drops, and we blame our age.

Your metabolism isn’t broken. It’s responding exactly as it should to the inputs it’s getting—your activity level, your food intake, your muscle mass, your hormones, your sleep, your stress.