During adulthood, the amount and quality of muscle and bone mineral decreases, whereas fat increases. Persons with low muscle mass and bone quality may not have the physical or structural strength to support the body. The dual-energy x-ray absorptiometer is used to measure the amount of muscle, fat, and the quality of bone mineral. Men have larger average amounts of muscle and bone, and greater average bone density than women. In women, the normal decrease of body mass and bone quality is accentuated by menopause. On average, blacks have a larger amount of muscle and more dense bones than whites. A person with a high peak bone mass can lose bone normally with age and still maintain good bone quality. Lifestyle, diet, and exercise are important to maintaining a healthy body and good bone quality. Physically active persons have increased levels of muscle and bone density. Age, sex, race, and lifestyle affect the level of, and changes in, body mass and bone mineral density.