Osteoporosis is characterized by a reduction in bone mineral density (BMD). Women more than men are at risk for osteoporosis-related fractures, especially in the wrists, lumbar spine, and hips. Numerous diet and lifestyle factors, including body weight, influence BMD, and in turn, fracture risk. BMD in the total body, hip, lumbar spine, and radius is weakly to moderately correlated to body weight, fat mass, and lean body mass in adolescent, perimenopausal, and elderly women, possibly as the result of stress on the skeleton from the mechanical loading of body weight alone. In addition, greater lean body mass may be a cause. Other explanations include increased hormonal circulation in obese women and greater conversion of adrenal androgens to estrogens linked to greater mass of adipose tissue. Currently no value is agreed on for weight-to-height versus osteoporosis and related fracture risk, but some extra fat mass yielding a body mass index > 26-28 does confer limited protection, whereas a slender figure yielding a body mass index < 22-24 increases risk.