Most blood donors are a bit weakened from giving blood, despite feeling often great. The amount of blood removed from circulation reduces your possible performance level and you might tire more easily as the reduced volume of blood also means a reduction in oxygene availability. This small measurable level of performance reduction can reach up to three weeks.
Judd, TB., Cornish, S.M., Barss, T.S., Oroz, I., Chilibeck, P.D. (2011). Time course for recovery of peak aerobic power after blood donation. Journal of Strength & Conditioning Research, 25(11). .
Apart from these slightly negative impact factors the American Red Cross simply issues the recommendation:
After you give blood:
Take the following precautions:
Drink an extra four glasses (eight ounces each) of non-alcoholic liquids.
Keep your bandage on and dry for the next five hours, and do not do heavy exercising or lifting.
Because you could experience dizziness or loss of strength, use caution if you plan to do anything that could put you or others at risk of harm. For any hazardous occupation or hobby, follow applicable safety recommendations regarding your return to these activities following a blood donation.
Eat healthy meals and consider adding iron-rich foods to your regular diet, or discuss taking an iron supplement with your health care provider, to replace the iron lost with blood donation.
If you get dizzy or lightheaded: Stop what you are doing, lie down, and raise your feet until the feeling passes and you feel well enough to safely resume activities.
The main reason being the possible risk of dehydration.
Blood banks encourage the donation of whole blood by donors 50 weighing kg or who have a greater hemoglobin level of 125 g/L or greater. Following the donation of a 450-mL unit of whole blood, plasma volume, which is acutely reduced by 7% to 13%, recovers within 24 to 48 hours. This results in a decrease of the hemoglobin level of 10 to 20
g/L. Recovery of the hemoglobin level to normal requires time and an adequate iron supply. Full recovery of the hemoglobin to baseline takes 3 to 4 weeks.
It is theoretically possible that peak athletic performance may be affected during the 3 weeks it takes for hemoglobin recovery. Although there are compensatory mechanisms in anemia to improve oxygen delivery to tissues, it is not clear that they occur acutely or with such minor decreases in hemoglobin concentration.
Athletes should wait 12 to 24 hours to resume strenuous exercise after blood donation and should be sure to stay well hydrated the day after a blood donation.
Ritchard G. Cable: "Execise and Blood Donation", JAMA. 1993 Jun 23-30;269(24):3167.
More information and a nice selection of links at
Jena Walther: "Donating Blood and Exercise: What Athletes Should Know" (National Academy of Sports Medicine) February 6, 2016.