When you give blood, you lose the oxygen-carrying hemoglobin in those red blood cells, and as a result you run slower. It takes about 5 weeks to fully restore the hemoglobin. But will your running really be affected for that long? A new study from the University of North Texas, published in the journal Applied Physiology, Nutrition, and Metabolism, offers a few interesting new wrinkles on this longstanding question.

A total of 19 healthy volunteers were divided into two groups; nine of them gave a unit of whole blood, while the other 10 gave about 700 mL of plasma only (the liquid component of blood was taken, while the red blood cells were returned). After the blood donation, they did an exhaustive exercise test two hours later, again two days later, and finally seven days later. (They'd also done the same test twice before donating to establish a baseline.) The test involved pedaling a stationary bike at a set power output until exhaustion, with the power set so they'd finish after about four minutes. This allowed the researchers to estimate the effects of the blood donation on both aerobic (i.e. VO2max) and anaerobic energy delivery.

Donating whole blood did pretty much what you'd expect: total hemoglobin levels dropped immediately, and were still lower seven days later. VO2max dropped from 40 mL/kg/min before the test to 34 two hours later. After two days, it was up to 36, and after seven days it was up to 37. Time to exhaustion followed a similar pattern: baseline 268 seconds, then 218, 250, and 258. After seven days, there was no longer a statistically significant difference in time to exhaustion compared to baseline. That definitely doesn't mean the effects had totally disappeared, but it suggests that you can be close to normal after a week or so.

Donating plasma was a different story. There was no difference in VO2max at any point. Instead, it was lactate and "maximum accumulated oxygen deficit" -- measures of anaerobic capacity-- that changed. The researchers suggest that this is because the lost plasma includes a bunch of bicarbonate, which is used to buffer lactate. However, this recovers quite quickly: time to exhaustion in the test dropped dramatically (from 256 s to 229 s) two hours after the donation, but was back to normal (259 s) after two days. This suggests that endurance athletes can give plasma with minimal lasting consequences on their race performance -- perhaps a good way around a persistent dilemma.

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