http://www.nytimes.com/2010/02/04/fashion/04best.html?_r=0"Dathan Ritzenhein, an American who is one of the world’s top runners, used a similar system. Mr. Ritzenhein, who broke the national record last year in a 5,000-meter-race and who, at ninth place, was the top American finisher in the marathon at the Beijing Olympics in 2008, said that it took him about 12 years of trial and error to learn his best racing weight. He discovered it last year, his best racing year ever: 121 or 122 pounds (he is 5 feet 8).
That weight is not a natural one for him, he said. If he were to stop training, he would weight about 127 or 128 pounds, and when he is training but not trying to control his weight, he is about 124 or 125. His goal is to try to be at his perfect racing weight a couple of weeks before a big event, losing about a pound a week in the preceding weeks to get there.
He has learned, he said, that if he tries to lose weight too fast or if he continues to lose weight up until his race day, he does not have the energy he needs for his best performance. And if he tries to lose weight too fast, his training suffers."
This quote ended up making the opposite point I wanted it to. Point is, don't do anything sudden, and you can find your best weight through trial and error. But if you get really negative feedback from your body, like feeling tired or sick a lot, stop it. Running isn't worth endangering your health.