Blood donation and endurance athletes: how does it affect performance?

There’s always a shortage of blood supplies so one of the best gifts you can give to help others is to donate blood. The human body contains around 8 pints of blood; donation of a pint of blood results therefore in a drop of around just over 10% in total blood volume, which is made back over time by the synthesis of new blood cells and plasma. For those who don’t engage in exercise, this temporary drop in blood volume is barely noticeable. But because the role of the blood is to transport oxygen around the body, this is not the case for endurance athletes engaged in heavy training. And some research on the effects of blood volume and endurance performance suggests that athletes wishing to donate blood would do well to time their donations away from periods of competition.

In a study carried out last year, scientists investigated the effects of blood removal on the pacing and performance of cyclists during a 4-minute cycling time trial [Int J Sports Physiol Perform. 2017 Jan 17:1-25. doi: 10.1123/ ijspp.2015-0778]. Seven trained male cyclists completed an incremental cycling test followed twenty minutes later by a 4-minute self-paced cycling time trial. In total, the cyclists completed six trials on six separate occasions over 42 days. The first two sessions acted as a familiarisation and baseline testing. After these, all the cyclists had 470mls of blood removed (slightly less than is typically donated when blood is given). They then performed the four remaining trials at 24 hours, 7 days, 21 days and 42 days following blood removal. During all of the trials, the cyclists were monitored for their power outputs and oxygen uptake capacities. The cyclists’ aerobic and anaerobic contributions to power were also measured.

The findings

The first key finding was that the cyclists’ average power outputs during the time trial fell significantly following blood removal. At 24 hours, 7 days and 21 days after blood removal, power output dropped by 7%, 6% and 4% respectively. Secondly, the results showed that the proportion of power coming from aerobic energy also fell; at 24 hours after donation, it fell by 5%. After 7 days it was still down by 4% - a decline that was maintained even after 21 days, which accounted for a large part of the performance drop noted in the cyclists.

Sports Performance Bulletin verdict

A drop in endurance performance following blood removal is not exactly surprising. Blood is the means by which oxygen (and thus energy) is transported to working muscles. This explains why any shortfall in iron nutrition can adversely impact endurance, because iron is essential for the formation of red blood cells that actually carry the oxygen in the blood. It also explains why the illegal practice of blood doping (injecting extra blood before competition) has proved popular among some of the cheating elite!

What is perhaps more surprising is the persistence of the drop in performance. After nearly a month, performance was still down by around 4% - a huge margin for any athletes who is engaged in serious competition. This is almost certainly because the body takes time to replenish red blood cells – even when plenty or iron-rich foods are eaten and supplementation is given. It would have been informative if the researchers had carried out further power tests at two and three months to see whether full recovery had been achieved. However, after such a long time when so many other variables can come into play (changes in training volume/intensity, illness, injury etc) this may not have produced a definitive answer. But given that power was down by 4% a month after blood removal, a sensible approach for serious athletes who want to donate blood would to ensure that at least three months elapse between any blood donation and subsequent competition.

Practical implications

• If you donate blood, a good time of year would be at the start of the off-season (October/November). This will give you plenty of time to regenerate your red blood cell count before serious training begins again.
• If you donate blood at other times, you should allow three months or more between blood donation and competition.
• Having donated blood, it’s important to consume plenty of iron-rich foods for a few weeks afterwards such as liver and red meats, seafood, kidney beans, lentils and soy beans, dark green leafy veg, and nuts and seeds.
• A temporary iron supplement (around 15mgs per day) can also be a useful insurance policy.