Effect of exercise on bone: permissive influence of estrogen and calcium.

Estrogen deficiency in postmenopausal women is associated with low lumbar bone mineral density and an increased incidence of fractures of the vertebrae and proximal femur. Estrogen deficiency in premenopausal women with secondary amenorrhea related to athletic training or anorexia nervosa is also associated with decreased lumbar bone mineral density. The purpose of this review is to present four concepts related to the adaptations of bone to physical exercise, as a basis to explain the loss of bone mass in women with athletic amenorrhea. These concepts are based on Lanyon's theory of a Minimum Effective Strain-Related Stimulus. The bone remodeling response to estrogen deficiency is an increase in the rate of bone remodeling activity and in the rate of bone resorption relative to formation, resulting in a net loss of bone mass. In the presence of estrogen deficiency, the stimulus of physical activity is thought first to decrease the rate of turnover and secondly to increase bone formation. Endurance exercise training appears to be an insufficient stimulus to accomplish both tasks, which may explain why these athletes often have low lumbar bone mineral density.