As a runner who has/had an eating disorder (severe calorie restriction), I can tell you fainting or experiencing fainting-like symptoms was pretty common. I would say most of the time for me it was just really low blood-sugar as a result of running 75+mpw and eating less than 1000cals/day. There would be times where I would stand up and immediately feel the onset of dizzyness, nausea, and tunnel-vision. It is wierd to say now, but it almost became like a high, a high indicating that I was doing the right thing in that I had restricted a sufficient amount of calories. Over the summer when I forced myself to eat less than 1000cals/day I only legitimately fainted 3-4 times, but got the feeling of coming "close to fainting" at least 3 or 4 times a week. I never told anyone and when the symptoms did happen, I would turn from anyone that was around and shut my eyes for about minute until the overwhelming sensation went away. The one time I did faint was at work (grocery store) stocking shelves and a customer freaked out. They gave me some orange juice and I was fine.