PLEASE NOTE: I am not a doctor and everything in this post is based on me, an experiment of one.
If I look in the index of any of my running books, and I have A LOT, for "blood donation", I find no entry in any of them. So when I got a call about two weeks after running the Toronto marathon, I said sure, no problem. I used to give blood regularly when I lived in Quebec City because the Red Cross (back then) came to the building I worked in about 3 times a year. I didn't really run back then. Since moving to Toronto I hadn't given blood, I felt a bit guilty and felt the need to atone.
So I went, gave my bag and came home. I didn't run until 3 days later because I was taking it easy after the marathon and all. When I did run, I felt incredibly sluggish. I was running fairly slow but I felt like I had no energy. This was the shortest run I had done in 6 months and I was TIRED. When I downloaded my Garmin 305 data and saw my heart rate, I was worried. It was WAY high. I had run at my recovery pace and my heart rate was basically the same as when I had run my first 10k race! WTF? Did I damage my heart or my leg muscles? That whole week I felt like I'd lost half my fitness. I could run slow just fine, but hills and striders were really hard. The second week was a bit better and now in the 3rd week I finally am starting to feel more energetic. I have a 10k race on Sunday, so we shall see.
I looked in all my books and found little of interest, except a little blurb in "Lore of Running" about inducing anemia with blood donations, but nothing specific. I found a few comments from other runners in running forums that complained of the same symptoms after donating blood. When I talked to my doctor she laughted and said it takes up to 3 months to eliminate all the effecst of a blood donation but that I should be back on track within 3 to 4 weeks of donating.
I am so happy I didn't do it a few weeks before my marathon. This would have been devastating. Training for a marathon takes the better part of 6 months and wrecking a race because of a lack of information would have been very frustrating, to say the least. I had always been told the effects of a donation would only last a couple of days. But then again, I should have known better since they won't let you give more for another 53 days! Duhhh!
So yes, I will give blood again. But I will do it knowing what the effects are and how long they will last (at least for me).