Most people get fatter as they get older…but they don’t have to. It is a matter of reduced physical activity levels and lower metabolic rate caused by a loss of lean body mass [muscle].

The lifelong loss of lean body mass reduces our basal metabolic rate as we age. It’s a very subtle change that begins between ages 20 and 30. The percentage of body fat gradually increases, and it produces an ever-decreasing calorie requirement. That’s because fat cells burn fewer calories than muscle cells. And a lower metabolic rate means that unless you eat less, you’ll gain weight over the decades.

But exercise can mount a two-pronged attack on middle-age spread and muscle loss. Any activity makes you burn more calories (so you’re less likely to wind up with an excess). And strength-training can offset the loss of muscle mass.