An article in Omega Cycling by Dr P A Lambeti (MBBcH), said, in part:
"A study has been done looking at the effects of blood donation on exercise performance in competitive cyclists. This study evaluated 10 male cyclists before and after phlebotomy (donating blood), to determine the effect of donation of one unit of blood on exercise performance. Each subject underwent maximal exercise testing with oxygen consumption measurement at baseline, 2 hours after phlebotomy, 2 days after phlebotomy, and 7 days after phlebotomy. The results found that maximal performance was decreased for at least one week and that submaximal performance was unaffected by blood donation.
Thus, if you are a competitive cyclist, do not donate blood within 7 – 10 days of a competitive race, as your performance will be compromised. If you are a casual cyclist performing submaximally, you may not experience any deleterious effects apart from a higher heart rate than normal from the day after donating."
Monster facts. Also 150,000 people faint or feel faint after giving blood each year.