You already know to keep calories and fat in check, but you'll fan the flames of your metabolism by putting another nutrient on your radar: protein, the building block of lean muscle mass. “The amino acids enter your bloodstream and are then absorbed by your muscle tissues and other cells,” says UTMB’s Douglas Paddon-Jones. “Once the amino acids end up in your muscles, your body starts putting them back together — sort of like Legos — into your muscle tissue.”