In a study of high school girl athletes characterized by their type of sport (high impact vs. repetitive) and the regularity of their menstrual periods, researchers found that the runners who miss some periods "may not be accruing bone at the expected rate." The repetitive-sport athletes were swimmers, and cross-country and track distance runners who competed at the 800 and up. The researchers urge periodic medical evaluations of such athletes to be sure their bone health is not being compromised. This practice doesn't just apply to high school girls. All women, especially those engaged in endurance athletics, should be especially mindful of bone health. I won't go into depth here, because the research is quite complex, but basically, if you train a lot, you burn a lot of calories. And if you don't replenish those calories sufficiently, you may start missing your periods (aka amenorrhea). If that happens on a regular basis, your estrogen levels go haywire, and your bones begin to break down. Worse case scenario, you end up a 30-year-old with 60-year old bones. So eat, please.
This makes me wonder about those new birth control pills that stop your period for months on end. Would that also affect your bone density? If so, then taking those pills for years on end could be very, very dangerous. I wouldn't take those pills anyway, I had problems enough with the regular pill.