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Oral Contraceptive Use and Bone Mineral Density
Osteoporosis is a condition in which the mineral content of bone is decreased and bones may break more easily and heal less effectively. Osteoporosis has no symptoms in the early stages but a bone mineral density test (BMD) can accurately tell whether the bones are showing evidence of mineral loss. As bone mineral density decreases, risk of fracture increases.
This test is recommended for all women over age 65, as well as both men and women who are at high risk of having an abnormal bone density because of a history of fracture during adulthood, advancing age, cigarette smoking, use of corticosteroids for more than three months, early onset estrogen deficiency, alcohol intake of more than two glasses per day, and low calcium diet, among other factors. The results of a bone mineral density test can be used to predict a person's risk of a fracture.
Low bone mineral density can be addressed in a variety of ways including changes in vitamin D and calcium intake, increasing weight bearing and strength training exercise, and taking prescription medications. The goal of treatment is to prevent debilitating fractures. Although many contributors to osteoporosis and low bone mineral density have been determined, researchers continue to look for effective ways for patients to decrease their risk.
A recent study published in the January issue of the journal Contraception found an association between the use of oral contraceptives and decreased bone mineral density. Almost 12 million women in the United States use oral contraceptives. The highest use is seen among women younger than 30, the age period during which much of our bone mass is gained. The researchers wanted to see whether oral contraceptives would have different effects on bone density when used by women who hadn't yet achieved their total adult bone mass as compared with those who had.
They studied 606 women, ages 14−30 years old and divided them into two groups, 14−18 years (adolescents) and 19−30 years (young adults). They compared their bone mineral densities with control groups who were not taking oral contraceptives. They obtained health, reproductive, menstrual, smoking, alcohol, physical activity, caffeine and dietary histories and recorded height, weight. They discovered that the 14− to 18− year−old participants, who were current oral contraceptive users, did not have bone mineral density values that were significantly different from young women of the same age who were non−users.
But in the 19−30 year old group, the bone mineral densities of oral contraceptive (OC) users were significantly lower than those of non−users. In the young adult group, the longer the duration of OC use, the lower the mean BMD. For young adults who had used oral contraceptives for 24 months or longer, the average BMD at the spine was 5.9% lower than non−users. (The average period of use for adolescents was 12 months, and this may have contributed to the reduced effect noted above.) Additionally, in the young adult group, use of oral contraceptives with lower doses of estrogen (less than 30 micrograms vs. 30−35 micrograms) was more strongly associated with lower bone mineral content.
The researchers described a number of potential ways that the estrogen in oral contraceptives could affect bone mineral density. One is that they may decrease the turnover of bone and thus act to increase bone mineral density. They may cause the growth plates in long bones to close, leading to limitations in bone length. They may also act on the tissue that surrounds the bone (periosteum) and impacts bone expansion.
Estrogen also effects the action of other hormones that affect bone growth and density including androgens and insulin−like growth factor. The ultimate impact of estrogen on bone growth and density is likely the sum of multiple biochemical processes. The age of the women and the state of skeletal maturation are also likely to contribute to the effect of estrogen on bone mass.
Oral contraceptive use may have had less impact on bone mineral density in the adolescent group because their bones were still in a process of active growth and mineralization, and/or the length of exposure to oral contraceptives was shorter than the young adult group, according to researchers. They did not draw conclusions about the clinical importance of the difference in bone mineral density observed in young adult OC users or how their BMD and fracture risk would be impacted later in life.
The current population of menopausal and postmenopausal women is the first group to have used oral contraceptives for extended periods of time during their lives, so detailed information about effects of long term exposure is only now becoming available. Although a decrease in bone density similar to that seen in the study population (5.9% for young adults who used OC for >24 months) is associated with almost 50% more osteoporotic fractures in post menopausal women, there has not yet appeared to be a similarly increased risk of fractures in young OC users.
More study will be needed to determine how a low bone mineral density in young adults affects their immediate as well as their later risk of bone fractures. Similarly, more study is needed to determine what happens to bone mineral densities when a woman discontinues oral contraceptive use. The investigators note that there are health risks from other types of contraceptives as well as from unintended pregnancies, and they recommend that this new information be weighed with other risks/benefits of oral contraception. However, osteoporosis rates are increasing and more study is needed to determine the impact of oral contraceptives on bone mineral density and fracture risk throughout a women's life cycle. Clarification of the safest dose, duration of treatment, and appropriate bone density surveillance for women who take the pill will add to the ability of doctors and patients to improve health and quality of life in women.
March 3, 2010
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