The most important function of estrogen with respect to bone health is related to estrogen's suppressing effect on osteoclast activity . As mentioned previously, osteoclasts are bone cells that tear down bone in the process of bone resorption. In the hypoestrogenic state, the female athlete likely exhibits accelerated bone resorption through the impact of irregular or absent menstrual cycles. In addition, a direct effect, through low energy availability, may be possible . Some studies have shown that athletes, at risk for disordered eating, present with low BMD in the absence of menstrual dysfunction [74,78]. Studies aimed at correcting the hypoestrogenic state, using estra-diol replacement (without an increase in energy intake), generally have not succeeded in the normalization of BMD after years of treatment [81-84], indicating that factors other than estrogen also are important for bone. The most convincing evidence that low energy availability may have a direct effect on bone was published in an article by Ihle and Loucks , who showed that markers of bone formation and resorption changed unfavorably within 5 days in sedentary women who were exposed to low energy availability through dietary restriction or increased exercise energy expenditure . Whether this is also the case in athletic women has yet to be determined. Nevertheless, it seems highly plausible that an energy and nutrient deficit affects metabolic substrates and hormones, including insulin, growth hormone, insulin-like growth factor-1, cortisol, and thyroid hormone, which all are considered important hormones for bone metabolism, independent of the hypoestrogenic state .
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