Spring is when many endurance athletes start increasing training for events such as marathons and 100 mile bicycle rides. These athletes may hesitate to donate blood because they worry about compromising subsequent performance. The volume collected is 500 ml (about a pint) in most countries, with standard post-donation instructions not to exercise or do manual labor for a day or two, and to not donate again for 56 days. R research shines a light on the complicated timing of recovery from a blood donation.
A donation will have a different initial impact based on sex, size, age, and physical fitness of the donor. For example, a donation will be a much larger percent of total blood volume for a small, middle-aged, sedentary woman than it will for a large, young, athletic man. Men average 8 percent blood by weight, women 6 percent. The largest part of the difference is due to women having about 25 percent more body fat compared to men of the same athletic fitness. Age can also affect the impact of donating blood. As people age there is a trend for the amount of blood-rich muscle to decrease and for the amount of blood-poor fat to increase.
Taking an iron supplement or eating iron-rich foods will not speed recovery. With the exception of people who are iron deficient, there are adequate reserves of iron in the form of ferritin, an iron-rich protein to replace what was removed. The rate-limiting factor is how fast bone marrow can increase red blood cell production over the normal replacement rate.
A research article published in 2008 gives a picture of the donation recovery process in healthy young men. Dr. Torben Pottgiesser removed 550 ml of blood from 29 men and then measured plasma and total body hemoglobin twice a week. Plasma, the fluid part of blood, recovered within 1-3 days. This fast recovery of fluid volume is what most people think is all that affects exercise capacity. But plasma does not carry oxygen.
Hemoglobin, in red blood cells, is the protein responsible for transporting oxygen. In these men, hemoglobin did not even begin to recover in the first 5 to 10 days. It took, on average, 36 days for the subjects’ hemoglobin to return to the levels seen before the blood donation. The full range was 20 to 59 days; with 25 percent having recovered by 25 days, 50 percent by 34 days, and 75 percent by 46 days. Thus, for these young men the results support the standard practice of having donors wait at least 56 days before donating blood again. The equivalent experiments have not yet been published for women or older donors.
In theory, at anything up to moderately vigorous exercise there will be no difference in perceived exercise capacity. The heart will beat faster to compensate for less oxygen-carrying capacity per heart beat. However, the high end will be compromised. Amateur cyclists were tested before and after a blood donation. At seven days, donated hemoglobin was down 6.7 percent and maximum exercise as measured in watts output was down 7.1 percent. Unfortunately, no testing was conducted beyond seven days.
Thus, on a practical note, anyone planning to compete in a long-distance endurance event should not schedule a blood donation during the final eight weeks leading up to the event. Training can resume within a few days after the donation with the understanding that training performance will likely be compromised for weeks.