A source of energy used by the body to make substances it needs. Fat helps your body absorb certain vitamins from food. Some fats are better for your health than others. To help prevent heart disease and stroke, most of the fats you eat should be monounsaturated (mon-oh-uhn-SACH-uh-ray-tid) and polyunsaturated (pol-ee-uhn-SACH-uh-ray-tid) fats.