Smoking reduces the amount of cal-cium your bones absorb. Vitamin D helps bones to absorb calcium, but smoking in-terferes with how your body uses vitamin D. Less calcium is then available to build strong bones. As a result, your bones start to get brittle.Smoking lowers estrogen levels in both men and women. Estrogen is important because it helps the bones to hold calcium and other minerals that make them strong.At menopause, a woman’s body makes muchless estrogen, and this puts her naturally at risk for osteoporosis. Smoking increases her risk even more.To hold calcium, the bones also need help from weight-bearing exercise, such as walking. Smokers, however, tend to get less exercise than non-smokers do. Smok-ing is also toxic to osteoblasts (bone-forming cells).