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I have given up on the idea of giving Blood. On Two occasions I have turned up at a so called drop in sessions, only to be told that they were full. I was then advised that the next time I should telephone in advance, and make an appointment. Guess what I tried doing this and they didn`t have any appointments available at the times that I could attend. What made me laugh was the fact that they were advertising on national TV quoting "Blood stocks are low".
I always book, I wouldn't turn up without an appointment after a few times when I would just turn up & have to wait ages.
I used to donate in one of their mobile units, they were having to turn people away as they didn't have the space to accommodate. Even those with appointments had an additional 15 minute delay.
Just to say that I gave blood in July (for the first time in 5 donations - I either don't have enough iron, or, last time, the flow was too slow and I didn't give a full bag). The day after I ran my local parkrun and although I did an ok time, I was absolutely exhausted and faded half way through. About 2 weeks later I did a local 10k and faded half way through again and the same happened on another 10k a few days later.
About 3 weeks later I did my parkrun again and although I was only 4 seconds faster than the previous run it felt a lot more comfortable. I did the Overton 5 on Saturday and feel like I am back to normal - 6 weeks on.
So my conclusion is that giving blood and running (well) do not mix and I will try to make sure that I give at least 3-4 weeks before any race I want to do in future. I wouldn't let it put people off though - if you can give, please give. There are so many people who can't or won't.
Having browsed around a bit, I think this gives a reasonable summary. If you time your donation well before an endurance event (maybe even 8 weeks) you will have recovered. But anyway if I were a donor I think I would still carry on donating even knowing I had a possible loss of performance of 7% at one week after, for example.
The fluid loss is recovered in a couple of days, but the red blood cells need a month or more, apparently. Since they carry oxygen, but VO2 max is not the be all and end all of performance, one might expect some diminishing effect as time goes on.