So I've been studying the female athlete triad and in particular the causes of amenorrhea. The old hypothesis of a low body fat being the cause of amenorrhea has pretty much been refuted and replaced with the current belief that negative energy balance is the caus,. e.g. extreme cutting or an endurance athlete not replenishing her energy. How much is too big of an energy deficit? On this board we usually focuses on minimising muscle loss: No more than 2lbs a week, or the more accurate estimate of no more than a certain % of your current bodyfat.