Osteoporosis or porous bone results from reduction in the overall quantity of bone matrix. It occurs when the rate of bone reabsorption exceeds the rate of bone formation. The loss of bone mass makes bones so porous and weakened that they become deformed and prone to fracture. The occurrence of osteoporosis increases with age. In both men and women, bone mass starts to decrease at about age 40, and continually decreases thereafter. Women can eventually lose approximately one-half, and men one-quarter, of their cancellous bone. In postmenopausal women, the decreased production of the female sex hormone, estrogen, can cause osteoporosis. Conditions that result in decreased estrogen other than menopause can also cause osteoporosis. In males, reduction in testosterone levels can cause loss of bone tissue. Decreasing testosterone levels are usually less of a problem for men than decreasing estrogen levels are for women. Inadequate dietary intake or absorption of calcium can contribute to osteoporosis. Finally, osteoporosis can result from inadequate exercise or dis-use caused by fractures or paralysis. Treatments for osteoporosis are designed to reduce bone loss or increase bone formation. Increased dietary calcium and vitamin D can increase calcium uptake and promote bone formation.