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Toby Talbot/The Associated Press

Ask a dedicated runner to choose between civic duty and a personal best, and you may not like the answer you get.

But that's the dilemma confronting tens of thousands of runners as they consider whether to donate blood in the weeks leading up to some of the country's biggest road races this month. New research confirms the endurance-sapping effects of removing whole blood, but it also suggests a compromise: By giving just part of your blood, you can help your fellow citizens without torpedoing your training.

The link between blood loss and endurance is straightforward. Red blood cells contain hemoglobin, which carries oxygen molecules from your lungs to your muscles. Extract a standard unit of blood – about 450 millilitres – and you lose roughly 9 per cent of the hemoglobin in your body. Less oxygen gets to your muscles, and as a result you run more slowly.

In fact, blood donation is the mirror image of the banned blood-doping technique used by Lance Armstrong and other disgraced endurance athletes, says Dr. Andrew Jones, a professor of applied physiology at the University of Exeter in England who has studied the performance implications of blood donation and blood doping. By reinfusing fresh blood, these cheating athletes boost their circulating hemoglobin so they can get more oxygen to their muscles and consequently, say, win the Tour de France.

You have about five litres of blood in your body; after you take out a unit, it takes 24 hours or so to restore that volume, assuming you drink enough fluids. For that reason, Jones suggests being cautious about training in the first 24 hours after donating blood.

Even after your blood volume returns to normal, the oxygen-carrying hemoglobin takes much longer to regenerate. A German study in 2008 found that volunteers took an average of 36 days (with a range of 20 to 59 days) before their hemoglobin levels returned to predonation levels – which is why the Canadian Blood Services stipulates a minimum interval of 56 days between donations.

That doesn't mean your performance will be noticeably subpar for a full 36 days. Researchers at the University of North Texas recently put a group of volunteers through a series of high-intensity cycling tests, both before and after donating blood. The results, which will appear in a forthcoming issue of Applied Physiology, Nutrition, and Metabolism, showed the volunteers reached exhaustion 19 per cent earlier when tested two hours after donating whole blood, and 7 per cent earlier two days after the donation.

But after seven days, their time to exhaustion was just 3.7 per cent lower than before donating, no longer a statistically significant difference. This suggests that the biggest deficits occur during the first week, and the differences should be too small to notice within a couple of weeks.

The North Texas study also looked at another option: donating just plasma instead of whole blood. In this case, your blood is extracted, processed by an apheresis machine that extracts the plasma, and the rest of the blood – including the endurance-linked red blood cells – is returned to the donor. (A similar procedure also allows you to donate just platelets, another useful blood component.)

As expected, plasma donation didn't affect the donors' hemoglobin levels. Their aerobic capacity – the maximum rate at which they could transport oxygen to their working muscles, known as VO2max – was also unchanged. Still, their time to exhaustion dropped by 10.5 per cent in the test two hours after plasma donation. The reason, according to the researchers: Plasma contains crucial substances that buffer the rise in lactic acid associated with intense anaerobic exercise.

So both whole blood and plasma donation make you slower, for different reasons. But there's a key distinction: Unlike the slow regeneration of hemoglobin, plasma levels return to normal within about a day. Sure enough, in the tests performed two and seven days after plasma donation, the volunteers had fully returned to their predonation performance levels, which suggests you can donate plasma without interrupting your training for more than a day. (While researchers have yet to study platelet donation, there's no reason to think it would affect endurance performance either.)

If you do choose to donate whole blood, take note of how you feel while running or biking in the week after the donation and pretend it's your baseline. Then compare that feeling to the same effort a few weeks later – and you'll have first-hand experience of what it feels like to cheat in the Tour de France.