Metabolism shuts off at a certain age
Not quite! It's true that as you age, your RMR decreases, but it doesn't just quit on you. Inactivity, change in body composition, and the loss of muscle mass, as well as changes in your hormone levels, all affect your RMR, explains McDaniel. "It decreases by about 1-3% per decade after the age of 30." In the grand scheme of all potential or likely age-related declines, that's hardly much to complain about... especially considering what's going to happen to your knees.
Supplements can change your metabolism
Don't believe everything you read on a label, especially one that isn't regulated by the FDA. "We have a $121 billion nutraceutical industry," says Dr. Lustig, "but none of them are required to show efficacy." That's a nice way of saying that supplement makers don't have to prove that their supplements actually work, which is probably something you should consider before splurging on a bucket of META-BOOSTER XTREME 10,000, or whatever Olympic athletes are getting busted for these days.
The only way to affect your metabolism is by losing or gaining a bunch of weight
Running, biking, and strength training are all beneficial for your overall health, but the benefits go well beyond keeping your heart in good shape and dropping a few pounds. In your body, you have white fat (bad) and brown fat (good) -- exercise seems to turn white fat into brown fat, making it more "metabolically active," which basically means it helps other cells in your body process sugar and energy more efficiently. So even though you may have the same body fat percentage, and even the same weight, working out positively affects your metabolism.