Women in late adolescence and early adulthood should still be building bone. Studies that measure athletes at a single time point cannot tell us if osteopenic athletes simply halt this building process or if they also break down existing bone. However, three studies have been conducted in which amenorrheic/oligomenorrheic athletes were measured yearly (42-44). These studies found that amenorrheic athletes in their late teens and early twenties not only fail to gain, but actually begin to lose bone.
The magnitude of bone loss is serious. Amenorrheic/oligomenorrheic athletes have 8-31% lower bone density than normally-menstruating athletes, and 3-24% lower bone density than non-exercising, normally-menstruating controls (23-37). Alarmingly, many of these women have extremely low bone mass; some have bone densities comparable to women in their 70's and 80's (32,36).
Most studies have considered athletes with infrequent or absent periods (oligomenorrhea or amenorrhea) and have not evaluated athletes with more subtle menstrual disturbances. However, one study found that runners who menstruate monthly but who have anovulatory cycles and/or shortened luteal phase also lose bone (45). This study is of particular interest because it demonstrates that highly training females who appear to be menstruating normally may still be at risk for osteopenia; asymptomatic hormonal changes and menstrual disturbances may be sufficient to cause bone loss.
Several studies show that the lower an athlete's estrogen level falls and the longer that her menstrual irregularity persists, the greater the deficits in her bone (26,27,34,36). These observations suggest a direct relationship between a low estrogen state and bone loss. In theory, then, the establishment of normal estrogen levels should prevent and even reverse bone loss. Oral contraceptives (birth control pills) provide estrogen and regulate the menstrual cycle; thus, it has been hypothesized that oral contraceptives can be used to strengthen the bones of women athletes. However, this hypothesis has never been formally tested.
anovulatory cycle - A disturbance of the menstrual cycle in which ovulation (the release of an egg from the ovary to the fallopian tube) does not occur.
bone density - A measure of bone strength.
eumenorrhea - Having a normal number of menstrual periods per year (10 -13).
oligomenorrhea - Infrequent menstruation, 4 -9 periods per year.
osteopenia - Having abnormally low bone mineral density for one's age.
ovariectomy - Surgical removal of the ovaries.
randomized trial - A type of study in which subjects are randomly assigned to either a treatment or control group and then are monitored over time as to their progress. This type of study can be used to establish causality between a treatment and an effect.
shortened luteal phase - A type of menstrual irregularity in
which the luteal phase of a woman's menstrual cycle lasts for an abnormally
short amount of time.
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