During the summer of 1991 I performed what amounts to a very vicious experiment on my own body. I attempted to lower my bodyfat percentage to about 1% by limiting myself to 1000 calories a day, and working out 6 hours a day (1 hour of running 2 times per day and a daily 4 hour bicycle ride, during the heat of summer).
Yes, I lost a lot of weight (160 lbs. down to about 130), but after that, I could not run faster than 7:30/mile. My marathon PR at that time was 2:23, from November 1990.
The side effects of this insanity were remarkable:
(1) I only needed 3-4 hours of sleep per night. I was delivering newspapers at that time as a part time job in the summer, so I had to get up at 3:00am anyway.
(2) My resting heart rate was around 30 BPM. My body was clearly slowing down its system in order to conserve energy wherever possible.
(3) I could not "get it up" at all, for about 5 months. I was completely impotent, when I should have been a sexually-active 22 year old athletic male.
(4) I could have very well died during this "experiment" due to dehydration and/or depletion of sodium/potassium.
It was only when I stopped that stupidity and gained back 20 lbs. (to 154 lbs, which seems to be the best weight for me), was I able to run well.