I am going to be donating blood later tomorrow and was wondering on your opinions regarding being fasted or fed when I do it? I will be very well hydrated (as always) but I will be giving it around 3-4 hours after I have woken so I normally wouldn't have eaten by then as I typically eat 2 meals a day, a late lunch and dinner...
Obviously paleo man would have often 'given blood' fasted during or due to the hunt, but this is not purely about paleo reenactment (its just a really good thing to do!) and like dr harris says medical science first, paleo second...
It probably does not make much practical difference when a healthy person gives blood, since your internal blood pressure regulation can compensate for the fluid loss. As has been pointed out, adequate hydration is important when you donate.
Theoretically, you would not want to give blood after a large meal, because blood flow is shunted to your digestive system, which would leave less blood flow to your heart and brain. A small snack or breakfast would probably not cause a significant shunting effect. Also, there could be a theoretical advantage to donating blood early in the morning, when your blood pressure is at its highest.
I found no problem when donating blood while fasting recently.
I am overweight by about 25 pounds and as such eat 1 meal a day and sometimes a snack of cream and some sardines within a couple hours after dinner. My 1 meal a day is dinner at approximately 6PM.
I donated blood at around 2PM. At that point, I had been fasting for approximately 20 hours. I too was well-hydrated and recommend the same for anyone else before giving blood since you lose a significant volume of blood in the process.
After my donation, I sat around briefly and downed a bottle of water, then went on my way.