I like to give blood because it feels good, and because I can lose a pound of body weight in about fifteen minutes.
I have been worried that giving blood makes me tired sooner so I can't exercise as hard.
There have been a number of studies on this question. One by Birnbaum et al at College of St Scholastica showed that oxygen absorbed decreased by 10% the day after blood donation.
Another article shows 9%. In this article, an exercise that would tire college students in 11 minutes, now tires them in 10.1 minutes. So this means at the end your workout, you are going to feel tired for those last few minutes.
How long does this decrease in performance last? Strangely, it seems to last a long time1. In women, half of subjects did not recover their original blood iron content in four weeks, however most recovered faster when given megadoses of iron. In men, recover took between 20 and 59 days, with the average being 36 days. That is, the effect of blood donation endures for sometime, and be sure to take iron pills. The effect of gender seems hard to measure. Presumably menstruating women who are prone to low iron, have an especially difficult time recovering from blood donation. 3
A related topic is blood doping, which is transfusing oneself with extra red blood cells so that the blood carries extra oxygen, and presumably this reduces fatigue. There is not good evidence that this actually works because the body removes the extra blood cells quickly. More specifically, capillary pressure removes liquid from the blood increasing its viscosity. Total blood volume equalizes within an hour or so, and blood viscosity rises. There is also a problem with maintaining the stored blood in prime condition.
The same article talks about natural adaptations of endurance athletes, and these include relatively higher blood volume especially plasma volume, so that iron concentration is actually relatively lower. On the other hand blood viscosity is lower, and this improves heart stroke volume, and oxygen transfer. Faster blood flow also aids cooling of the body.
All this is going to slow me down in donating blood. And if I do, I am going to double up on the iron pills.