John, I will let Eliana answer your question to her...
But I definitely agree on the detrimental consequences of extreme restriction of calories with intense exercise...adrenal issues (which I suppose is directly related to cortisol), the impact on the thyroid gland, the reproductive system, the heart and so on. Ok, well I will give MY answer to where the body is getting a "reserve" from..... It is taking away important function from all of the above-mentioned! I am a definite case study of it, ruining my body for over a year in the quest for a perfect body.
Like I said in my very 1st post though, in HINDSIGHT I was not too big to begin with so I malfunctioned very early on...and kept doing it anyway.
And I have to really watch those obsessive tendencies because now that my body is accustomed to being starved, if you will, it doesn't take much to put me back on that path of "robbing peter to pay paul". Our bodies will only bounce back for so many times in these extreme conditions. There are several studies out now which is searching to tie a link of thyroid disease to the great famine and how it seems to become a hereditary issue. Of course there are other causes.