Here’s a fact about metabolism:
It slows down as we age because we naturally lose muscle mass. So you may have burned 2‑thousand calories a day just existing at age 20. But by age 40, you’re burning 80 calories less. That may not sound like much. But if you don’t burn off 80 more calories every day – or eat 80 fewer calories – you’ll pack on 8 extra pounds in a year.