I've been doing a lot of research for my upcoming book and have come across a lot of really fascinating information that is just not very well publicized. So I'm going to take a little time out of writing every few days and share some of the info here.
The first article is this one: Energy availability in athletes, by Anne Loucks et al.
So here are some of the tidbits from this article (it's a review, so they're other studies that are referenced. I have not read each individual study yet.)
In studies where men or women were in "room calorimeters" (basically a device where they truly can measure every scrap of energy that you're using), they found that exercise was NOT compensated for by an increase in hunger signals, which meant that it was extremely easy to operate with an "energy availability" that was too low to sustain normal physiological processes = reproductive system shut down and all the other negative effects of hypothalamic amenorrhea. This is because exercise actually produces a hunger suppressive signal.
They did note that there ARE some people for whom hunger signals increase with ongoing exercise, they called these "compensators" vs. those for whom hunger signals did not increase, "non-compensators". This difference could partially explain the difference between women with similar BMI and exercise, one who menstruates and the other who does not - nothing overt, but one IS adequately fueling her body and exercise, and the other is not.
Also, the slowdown of physiological processes happens immediately when not enough energy is being consumed; in the same experiment, they showed that the resting energy expenditure was decreased by 90 cal a day when in an energy deficit. So one day of negative energy balance isn't going to throw everything off, but throw in three or four in a row, and you're not doing your body any favors.
It explains so much about why you have to over-feed to actually recover from what you've done to your body. All those systems that have been shut down have to ramp up again, and use more and more energy as they do - and reproduction comes last!. So keep on eating, and sitting on the couch!!! (or if you do exercise, don't rely on your hunger signals to compensate for that exercise, you probably need to eat more than you think to make up for it.)