The effect is pretty trivial on the donor, even if it’s vital for the recipient and a significant public good.
A pint of blood weighs something more than a pound. If you don’t weigh enough to be able to temporarily spare a pound — say 1% of your weight, or a pint of the ~11 pints of blood in your body, the blood bank may turn you away. The blood bank also will encourage you to take aboard replacement fluids and some sugar before you leave.
Filed in: Aerobic Exercise » Does donating blood help you lose weight?