When I started teaching fitness classes almost five years ago, I was 46 years old—and about 15 pounds lighter. Every once in a while, when I step on the scale, I catch myself thinking, What happened? I work out five days a week. I teach classes. I move my body for a living. So how could I possibly have gained weight? Over time, I’ve come to recognize a few honest truths. I’ve grown older. My metabolism has slowed. And while I’m still active, I’m not moving in quite the same w