I am a frequent blood donor; needles don't bother me and I like the idea of literally helping to save lives (yes, I try to do that every day at my job as well...) Shortly befre I ran the Jacksonville Marathon last December, I was "due" for a donation and the vampires are usually set up somewhere in the lobby of my hospital, so I have to walk past their curtained-off area every morning and evening. I figured though, that donating right before a race was probably and bad idea, especially if I was trying to score a BQ.
If you take the contrasting side of this, think about what athletes try to do to improve their performance. Accusations of "blood doping" are all over competitions like the Olympics and the Tour de France. Oxygen is carried by red blood cells which are full of hemoglobin, so more hemoglobin and more red blood cells equal more oxygen. The general idea is that if you can carry more oxygen, then your endurance is improved. People accomplish this legally by training at high altitude or sleeping in oxygen deprivation tents (low oxygen levels force your body to make more hemoglobin to try and carry more). They accomplish it illegally by using drugs like erythropoietin (which is a hormone your body makes, but there is a synthetic form as well).
So, it stands to reason that if you have less hemoglobin and fewer red blood cells, you will have less endurance. In this study by Birnbaum et al., ten healthy young men, donated a unit of blood and significantly decreased VO2 max by about 3 liters/min. So, if you're Steve Prefontaine (VO2 max 84.4) then a drop of 3.5% in you VO2 max might not make much difference, but for us mortals, it might mean more of a 7.5% drop. By the way, this page gives you an idea what your VO2 max might be, and depressingly demonstrates why we slow as we age.
Tha full text of the study's abstract is here:
1. J Sports Med Phys Fitness. 2006 Dec;46(4):535-9.
Effect of blood donation on maximal oxygen consumption.
Birnbaum L, Dahl T, Boone T.
Exercise Physiology Department, The College of St. Scholastica, Duluth, MN, USA.
AIM: This study determined the effect of donating one unit of blood on various
physiological parameters associated with a VO2(max) test. METHODS: Ten healthy,
male subjects (23+/-4 years, 178+/-7.6 cm, 74.4+/-12.3 kg) completed a VO2(max)
test 24 h before donating one unit of blood (~500 mL) and 24 h after donating
blood. The Bruce protocol was used to determine the subjects' VO2(max).
Physiological responses were measured at the end of the VO2(max) test. A repeated
measures ANOVA was used to determine if there were significant (P<0.05)
differences in the subjects' physiological responses between the VO2(max) before
and after blood donation. RESULTS: Significant differences were found in VO2(max)
(mean+/-SD, 3.18+/-0.74 vs 2.87+/-0.53 L.min(-1)), cardiac output (Q, 25+/-5 vs
22.5+/-3.3 L.min(-1)), stroke volume (SV, 134+/-37 vs 121+/-22 mL.beat(-1)),
delivery of oxygen (DO(2), 5+/-.87 vs 3.97+/-.68 L.min(-1)), and hemoglobin
concentration (Hb, 153+/-12 vs 135+/-16 gm.L(-1)). No significant changes were
observed for heart rate (HR); arteriovenous oxygen difference (a-vO(2) diff),
systolic blood pressure (SBP), and diastolic blood pressure (DBP). CONCLUSIONS:
These findings indicate that donating one unit of blood decreased VO2(max) due to
the decrease in Q, which resulted from the decrease in SV since HR was unchanged.
The lower VO2(max) along with the decrease in DO(2) would be expected to have a
negative effect on athletic performance.
PMID: 17119517 [PubMed - indexed for MEDLINE]