Why do men generally have higher hematocrit and hemoglobin levels than women?

Men generally have higher hematocrit and hemoglobin levels than women primarily due to the influence of testosterone production. Testosterone plays a critical role in erythropoiesis, the process of producing red blood cells. This hormone stimulates the kidneys to produce more erythropoietin, which in turn promotes the formation of red blood cells in the bone marrow, leading to increased hemoglobin concentration and hematocrit levels.

Although higher muscle mass may be related to increased oxygen demands and, consequently, higher blood parameters, it does not directly account for the sex differences observed in hemoglobin and hematocrit. Nutrition also affects blood parameters, but the differences between men and women in these metrics are more substantially attributed to hormonal differences rather than nutritional status alone. While increased oxygen consumption may correlate with higher levels of hemoglobin and hematocrit in athletic populations, it is a result, rather than a cause, of these differential levels between genders.

Therefore, the influence of testosterone is the most significant factor in explaining why men typically exhibit higher levels of hematocrit and hemoglobin compared to women.