High affinity RBCs would actually be a disadvantage for athletics. You actually don't need very high affinity to pick up oxygen from the lungs -- your lungs are comparatively extremely high in oxygen. What matters more is being able to drop the oxygen off in peripheral tissues. Higher affinity means that it's harder to actually deliver the oxygen, which is why we evolutionarily developed the switch away from fetal hemoglobin.
I thought the evolutionary impetus for fetal hemoglobin was because it greatly increases the efficiency of fetal oxygen uptake across the placental interface?
From shadowgovt:
> I have seen no literature on whether having fetal RBCs in adulthood has any benefits or drawbacks (besides changing the affinity ratio for their fetus if the patient gets pregnant
This was exactly the question that popped into my mind when I read about switching from normal adult RBCs to fetal RBCs: does this therapy reduce the likelihood of carrying a baby to term?
Yes, that is true. I phrased that badly -- it's more that we didn't take the evolutionary branch where we retain the fetal hemoglobin because it is maladaptive in adults.