Beyond reductions in circulating estrogen in postmenopausal women, other factors can increase the loss of bone mineral. These factors include poor calcium and/or vitamin D intake as well as abnormalities in metabolism. Additionally, physical activity increases the mechanical stress placed on bone and stimulates a reinforcement of bone strength. Perhaps this effect is most obvious in the absence of any weight-bearing demands upon bone. For instance, astronauts subjected to extended periods of time in space at zero gravity (weightlessness) experience decreases in bone density. On the other hand, regular weight-bearing exercise seems to help strengthen bone and also to slow the gradual loss of bone material as the body ages.
Smoking seems to exert a negative influence upon bone mineral content and the rate of bone mineral loss, especially in postmenopausal years. Smokers tend to have lower bone densities than nonsmokers. One reason for this occurrence is that smoking reduces blood estrogen levels. Smokers also seem to reach menopause at a younger age.
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