Excellent thread, but I just wanna point out that 17% or whatever number is not the magic number of body fat that will interfere with periods. The number changes from woman to woman. There are women that at 19% will stop having regular periods, and women who will have regular periods at 14% or even 13%.

The healthiest body fat for a woman can be as low as she wants, provided it does not interfere with regular periods. The other side effects are not from low body fat per se, but from the amenorrhea.

My mother was a competitive swimmer for years and her body fat was around 13-14% and she always had her period. There was a time when she dropped to 12% and stopped getting her periods though. So it is very individual.