The key to maxing the mitochondria you have all over is cardio. In fact, endurance exercise can double the amount in your muscle, says Dr. Mark Tarnopolsky, a professor at McMaster University in Ontario. “By swabbing athletes’ cheeks, we’ve found that they even have more mitochondria there,” he says. Translation: If lowly cheek cells benefit, think of what your heart and skin reap. Indeed, Dr. Tarnopolsky’s research found that twice-a-week cycling for 30 to 45 minutes (plus one weekly walk) helped skin increase its collagen-filled underlayer and refresh its outer layer more quickly—in effect, “youthening” within three months.