Thanks for the question.
This is a frequently-asked question among people who exercise regularly, partially due to the warnings written in bold print telling you to refrain from intense activity immediately following donation. By donating, you are giving up both blood plasma and red blood cells. Losing blood plasma decreases hydration status and giving up red blood cells decreases the overall oxygen-carrying capability of the body, both of which are vital for physical activity.
When donating blood, 450 ml of whole blood is removed. According to clinical studies, plasma levels can fall 7 to 13 percent, but recover to normal levels within 24 hours. Hemoglobin on the other hand, requires three to four weeks to fully recover. Although recovery after donation is relatively fast, there is still a noticeable decrease in performance capabilities within the first couple of weeks.
Studies have found that maximal performance is decreased (i.e. competitive performance), but that submaximal performance is less affected (training). If you are going to donate, use that 24-hour period as a rest day, and focus on rehydrating. Due to the decreased blood volume in your body, you may feel lightheaded or your heart rate may be higher than normal during exercise, so listen to your body and stop immediately if you notice any of these symptoms.
After donation, drinking water liberally will help to replenish lost plasma. Noting the color of your urine is a good way to gauge your hydration status. The lighter and more transparent (not cloudy), the better. Also, eating a well-balanced diet, rich in iron, folic acid, and vitamin B6 and B12 can help replenish red blood cell counts more completely in the following weeks.
These nutrients are directly involved in the formation of erythrocytes (red blood cells).