The key findings of the present study are that (1) maximal power output, VO2peak, and hemoglobin mass decreased up to 4 weeks after a single whole blood donation in moderately trained people; (2) training adaptations seemed somewhat lowered by repeated whole blood donations as endurance capacity did not increase following multiple maximal exercise tests in the donation while it did in the placebo group; (3) key hematological parameters for oxygen transport were lowered by a single whole donation and cumulatively further affected by the repetition of the donations; and (4) submaximal lactate levels, reflecting submaximal endurance capacity, were not modified by whole blood donation.
For the first time, a comprehensive study was performed including repeated blood donations and assessment of endurance capacity, hematological parameters, and hemoglobin mass by the optimized CO rebreathing method, with a follow-up of 4 weeks after each donation. In addition, a placebo group was included, which strongly increased the power of the study and thereby, the interpretation of the results. Except one study , all others failed to include such group and the comparisons were made before and after blood donation. When looking at short-term adaptations of a few hours or days, the lack of a placebo group is less critical and a pre-post analysis can be considered as sufficient. When changes are expected on the long term, the inclusion of a placebo group is mandatory, as the time between two measurements becomes a variable by itself. In the present case, the placebo group allowed the distinction between the effects caused by blood donation and the effects caused by the repetition of the exercise tests, which was expected to induce a global training effect. A typical example is the improvement of the maximal power output during test series 2 and 3 in the placebo group only. Without a placebo group, we could have concluded that a single donation alters maximal power output while repeated donations did not. The results of the placebo group indicate that the normal response to 15 maximal incremental tests is a global improvement in maximal power output. The conclusion is therefore that repeated blood donations reduce adaptations to training, at least at the level of maximal power output. On the other hand, the decrease in VO2peak in the donation group during the first test series is not totally attributable to blood donation as a decrease was observed after 1 and 4 weeks in the placebo group as well. It is therefore likely that the decrease in VO2peak we observed during the first test series was not totally attributable to donation itself but also to fatigue induced by repeated exercise tests. Interestingly, not only endurance parameters were affected by the repetition of the exercise tests, MCH, MCHC, and RDW were also training-sensitive, independently of the group. All together those results highlight the importance of including a placebo group when studies are performed on the long term.
We found a reduced maximal power output and VO2peak of a few percent up to 4 weeks after the first blood donation. The largest reduction in maximal power output was 4% and in VO2peak 11%, which corresponds to values previously reported [15, 20, 22, 24]. Only one study extended the recovery period until 4 weeks . The main results of that study were that running performance on a 3-km time test series and VO2peak were reduced up to 1 week and hemoglobin concentration up to 2 weeks. In another study, VO2peak was shown to be reduced up to 2 weeks after donation while time to fatigue was not affected . Here, we used a similar exercise protocol, i.e., maximal incremental test on a cycle ergometer, to determine endurance capacity and VO2peak. It is therefore surprising that endurance capacity was differently affected after blood donation. It is possible that the training status influences the effect of donation on endurance capacity as our subjects had an averaged VO2peak of 57 mlO2/kg/min and in Judd et al. a VO2peak of 47 mlO2/kg/min. One could postulate that somewhat better endurance-trained athletes will have a different reaction to blood donation than less well trained and could develop iron deficiency more rapidly as they usually have a low iron status. To confirm this hypothesis, subjects with different endurance training levels should be tested in the same study design but elite athletes are not willing to undergo blood donation, knowing the negative impact on their performance, even if temporary. The reduction in performance and VO2peak after blood donation is probably due to the limitation of blood oxygen transport capacity. Hematocrit, hemoglobin concentration, hemoglobin mass, ferritin, iron, and RBC were all reduced after blood donation and their values were still lower than basal values after 4 weeks, except for iron and hemoglobin mass. We hereby confirm previous reports showing that recovery of hematological parameters may take a few weeks or even more [15–17]. Here, hematocrit, hemoglobin concentration, ferritin, and RBC were still lower 3 months after the first blood donation, at the time of beginning the second test series, indicating that the recovery of those parameters was incomplete before the next blood donation. It is therefore not surprising to observe an additive effect of repeating blood donation on those parameters.
To the best of our knowledge, no study has prospectively analyzed the impact of repeated blood donations on endurance capacity. We found that maximal power output was mainly affected after the first donation with a minor impact after the second and the third one. Nevertheless, the expected improvement in maximal power output after repeated exercise tests, as observed in the placebo group, was not present in the donation group. It seems therefore that training adaptations are impaired due to repeated donations, probably due to a decrease in key hematological parameters. Hematocrit, hemoglobin concentration, ferritin, and RBC all decreased globally over time, not only within one test series. We hereby extend the results of a previous study looking at the effect of repeated blood donations on serum ferritin concentrations . Ferritin levels were directly inversely proportional to the number of donations per year, with the lowest concentrations measured in people who gave blood thrice a year . Of note, our subjects did not receive any iron supplementation as they had a normal iron status at baseline and were not considered at risk for anemia. Our results show that even in a population a priori without risk for iron deficiency, it would be interesting to consider an orientation of the diet towards an iron-rich diet and/or iron supplementation to try to limit the negative effect of repeated blood donations on iron status. At the same time, this could limit the reduction in endurance performance. Further investigation should therefore focus on possible countermeasures to limit the side effects of repeated blood donation on iron status and endurance performance. Of note, our subjects were not sensu stricto iron deficient as their ferritin levels were above 15 μg/l (World Health Organization) but it should be acknowledged that endurance performance can be altered before iron deficiency really takes place.
Measurement of hemoglobin and ferritin concentration as well as hematocrit to assess hematological recovery after blood donation may not reflect the true amount of blood as both are affected by changes in plasma volume. Instead, total hemoglobin mass has been proposed to be the most sensitive variable to assess hematological recovery . After one blood donation, total hemoglobin mass has been shown to be reduced by about 8–9% and to recover pre-donation values after 35 days in average . Of note, this recovery period was highly variable, ranging from 20 to 59 days. Here, we found similar amplitude in the decrease in total hemoglobin mass, which was recovered after 4 weeks in test series 1 and 2 but took more than 4 weeks in test series 3, indicating that the recovery period was longer when the donations were repeated. Concomitantly, serum erythropoietin levels were higher during the third donation, slightly increasing over time from the first measurement to the end of the study in the donation group. Erythropoietin is a critical hormone in the formation of red blood cells in the bone marrow, thereby increasing total hemoglobin mass . Taken together, the evolution of hemoglobin mass and erythropoietin suggests that the regenerating capacity of total hemoglobin mass was slower with the repetition of blood donations. Interestingly, the decrease in hemoglobin mass was preceded by a decrease in hematocrit, hemoglobin concentration, RBC, and ferritin. As the quantification of the latter parameters is performed in routine when donating blood, a continuous decrease in their level could be interpreted as a premature sign of a later decrease in hemoglobin mass, which is more complex to assess. As hematological parameters are key factors in the adaptation to endurance exercise [14, 30], it is not surprising to see no improvement of maximal power output in the donation group while this was the case in the placebo group with the repetition of the maximal efforts. Notably, blood lactate concentrations at 190 W, which reflect submaximal training adaptations, decreased similarly over time in both groups. We hereby confirm a previous report showing that submaximal endurance capacity is not altered by blood donation .
The interpretation of the present results is limited to the population we studied, i.e., moderately trained people. As mentioned above, one can speculate that athletes with higher hemoglobin mass and VO2peak  would suffer greater losses in performance from blood donation. In addition, at the beginning of the study, half of the subjects were regular donors (three times a year) while the other half were not (one time or less a year). As already shown retrospectively in male Saudi blood donors, the higher the frequency of blood donations, the lower the ferritin concentrations , and probably the higher risk of developing iron deficiency. Here as well, regular donors had lower ferritin concentrations at the beginning of the study than non-regular donors (data not shown). However, no difference was observed between those two groups in the drop of Pmax or VO2peak after the first donation, indicating that the antecedent of the donors had a limited impact on our main outcome, endurance capacity. Another limitation of the present study is that we did not determine the anaerobic threshold and as a consequence no distinction could be made between the aerobic and anaerobic muscular work. Whether blood donation induces a shift in the anaerobic threshold due to reduced capacity for carrying oxygen remains to be tested.
Practically, the changes we observed in maximal power output and VO2peak were rather limited for a moderately trained population. Endurance capacity was reduced by a few percent for 4 weeks, which should not prevent moderately trained athletes to give blood as other uncontrolled factors such as the form of the day, a poor hydration status, motivation, work-induced fatigue, or familial emergency would have an impact of the same magnitude on their performance. We can recommend to those athletes not to plan an important race the month following a blood donation or not to donate blood the month before an important race. For elite athletes, blood donation is not to be recommended for the direct effect on endurance performance but also for the impairment repeated donations have on training adaptations. Reducing performance by a few percent can have dramatic consequences on the ranking. But this concerns only a small proportion of the total number of people practicing physical activity and participating in sport events.