Type 1 (postmenopausal osteoporosis) occurs in 5% to 20% of women, affecting those within 15 to 20 years of menopause. The incidence is peaking in the 60s and early 70s, and it is eight times higher in women than men. The frequency of postmenopausal osteoporosis accounts for the overall female-male ratio of 2:1 to 3:1.
The decrease in estrogen is thought to cause this form of osteoporosis. It makes the skeletal bones more sensitive to parathyroid hormone (PTH), resulting in increased calcium resorption from bone. This then turns on a negative feedback mechanism that decreases PTH secretion, 1,25-dihydroxy vitamin D production, and calcium absorption and ultimately causes bone loss, leading to fractures and osteoporosis.
Women can lose around 2% to 3% of their bone per year for the first 5 years after menopause. Because of the drop in estrogen production, women lose nearly 50% of their trabecular (spongy) bone and 35% of their cortical (outer covering of the bone) bone throughout their lifetime. In contrast, men lose only 25% of both types of bone. At least 75% of the bone loss that occurs in women during the first two decades after menopause can be attributed to a lack of estrogen rather than to aging. Bone loss associated with menopause does not begin with the onset of amenorrhea but may occur 1 to 3 years before the actual cessation of menstrual periods.
