The prevalence of low bone mass and osteoporosis in athletes is difficult to address because of the differences in diagnostic criteria used among organizations and the fact that BMD data using DXA are not as easily and inexpensively collected. In general, using the World Health Organization classification for postmenopausal women , the prevalence of osteopenia in female athletes has been reported to be 22% to 50%, with a relatively low prevalence of osteoporosis . Considering the new ISCD and IOC criteria, women with low T-scores or Z-scores would now be considered as having low BMD for chronologic age or below the expected range (ISCD Position Statement, 2005). More recently, Torstveit and Sundgot-Borgen  have applied these new criteria in a sample of 186 elite athletes and found that 10.7% had a BMD below the expected range for age.
Female athletes have higher BMD than their nonathletic counterparts. Athletes exhibit a BMD at several skeletal sites that is 5% to 15% higher than the BMD of nonathletes , even when controlled for confounding variables, such as age, body mass index, and lean body mass [64,66]. Sports with loading patterns that are characterized by high impact (basketball, volleyball, gymnastics) or odd impact (squash, speed skating, and other winter sports) are strongly associated with a higher BMD [60,64,66], whereas repetitive low-impact (running)  and non-weight bearing activities (swimming) are not .
It is not surprising that athletes, when healthy, have stronger bones than nonathletes. A study showed that low BMD is two to three times more common in controls compared with athletes . Under the condition of menstrual dysfunction and the triad, however, this positive effect of exercise on bone is diminished.
When athletes have menstrual dysfunction, their BMD is significantly below that of their eumenorrheic counterparts [40,68,69], and in a sense, athletes lose the skeletal advantage of their sport involvement. Athletes with amenorrhea exhibit a BMD at the lumbar spine that is 10% to 20% below the BMD of eume-norrheic athletes [69,70]. Amenorrheic athletes also have significantly lower BMD at other skeletal sites compared with eumenorrheic athletes [71,72]. Oligomenorrhea and amenorrhea are detrimental to bone ; however, the impact of oligomenorrhea on BMD occurs likely at an intermediate stage along the spectrum of menstrual dysfunction [73,74]. Finally, the cumulative exposure of low estrogen in the form of oligomenorrhea or amenorrhea during an athlete's career also needs to be considered. The longer the duration of menstrual dysfunction, in the past and at present time, the lower the BMD .
Although most athletes with menstrual dysfunction present with lower BMD compared with their eumenorrheic counterparts, there are some exceptions. It has been shown that athletes in high-impact sports, despite menstrual dysfunction, seem to be able to maintain their higher BMD compared with athletes involved in lower impact sports who also have menstrual dysfunction  or eumenorrheic controls . The mechanical loading patterns of certain sports may override the deleterious effect of hypoestrogenism.
Athletes with menstrual dysfunction are at greater risk not only for low BMD, but also stress fractures [23,66,72,76-78]. Torstveit and Sundgot-Borgen  identified that 17% of elite athletes reported having a stress fracture. Although not significantly different from normally active controls, the athletes were more likely to have menstrual dysfunction than the controls . Besides stress fractures, athletes with one or more components of the triad also are more likely to report sprains, strains, and other soft tissue injuries , underlining the importance of the triad on health and the performance capabilities of young female athletes.
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