Rolling up your sleeve to share those valuable cells is an admirable thing to do. But you shouldn't race for at least 24 hours—and maybe even longer—after giving a pint of blood.
The Red Cross explicitly tells blood donors, "Do not do any heavy lifting or vigorous exercise for the rest of the day." It also recommendeds that donors drink extra fluids and avoid alcohol for 24 hours, and athletes are advised not to compete on the day of a blood donation. And hard-core athletes should avoid donating blood during the race season.
That's because pint or so of blood you'll be giving up makes up about 10 to 15 percent of your blood volume, says Dr. Anne Eder, executive medical officer of the American Red Cross Biomedical Services. Your plasma volume will recover within about 24 hours, but the hemoglobin in your red blood cells—the protein that transfers oxygen from your lungs to your muscles—won’t return to normal levels for two to four weeks.
The jury is still out regarding exactly how much that affects performance and recovery time. Researchers have studied exercise tolerance and performance after blood donation, with conflicting results. In one study, volunteers reported no differences in symptoms (like fatigue) and physiological parameters (like heart rate and pulse) after giving blood and exercising. Another found that blood donation made no difference in the physical performance of active-duty soldiers. However, says Eder, other studies have detected decreases in oxygen delivery to muscle tissue.
Because of this, she says, high-performance competitive athletes may experience marginally decreased exercise tolerance for about one week, and perhaps as much as five weeks, before red blood cells are adequately replaced. If you're not concerned about your performance level, but want to play it safe, 12 to 24 hours of rest (and plenty of fluids) should be sufficient, Eder says. But if you're training seriously for a race, then hold off on donating blood until the season is over. There's a reason athletes get in trouble for blood-doping, not blood-letting.
That said, there are proven ways to speed that recovery, such as eating foods rich in iron. If you're a frequent donor, taking an iron supplement is a good idea, too.
And there are other perks to giving blood besides the ethical ones. Doctors give donors quick physicals, check their blood pressure, and test for diseases and infections. It might even lower your risk of heart disease, according to a new study on zebrafish, although the Red Cross maintains that there are no proven cardiovascular benefits to donating blood.
Bottom line: While individual responses vary, if you're looking to PR avoid donating blood during your race season.