The paradox of hematocrit in exercise physiology: which is the “normal” range from an hemorheologist's viewpoint?

Abstract

The paradox of hematocrit in exercise physiology is that artificially increasing it by autotransfusion or erythropoietin doping improves V_O2max and performance, while in normal conditions there is a strong negative correlation between hematocrit and fitness, due to a training‐induced “autohemodilution”. We aimed at investigating: (a) which is the physiological range of hematocrit in highly trained professional footballers; (b) what are the characteristics of athletes with high vs low hematocrit? We determined in 77 healthy male footballers the physiological range (mean±sd) of hematocrit: 42.3±2.74, (range −2σ/+2σ=36.8–47.8%) thus defining boundaries of quintiles of distribution for this parameter: 40, 41.6, 42.9, 44.6. In another sample of 42 male footballers we compared three groups: lowest quintile (n=8), highest quintile (n=5) and the three middle quintiles considered together (n=29). Athletes in the lowest quintile compared to those in the four other quintiles had a lower value of blood viscosity (−8%, p<0.01) but this difference disappeared after correction for hematocrit. These subjects with low hematocrit had also higher values of the following parameters: aerobic working capacity (p<0.01); isometric adductor strength (p=0.02); crossover point of carbohydrate oxidation (70% carbohydrates/30% lipids) (p<0.05); insulin like growth factor binding protein 1 (p<0.0001). Athletes in the highest quintile had higher red cell aggregability (Myrenne index “M1” 8.45±0.38 vs 6.82±0.62, p<0.04) and a higher disaggregability threshold γ_D (72.6±22.63 vs 44.49±1.37, p<0.01) and a lower percentage of water in fat‐free mass (p<0.02). On the whole sample hematocrit was negatively correlated with aerobic working capacity (W₁₇₀ r=−0.329, p=0.007; W_max (% of expected value) r=−0.552, p=0.008; V_O2max (% of expected value) r=−0.543, p=0.009) and with ferritin (r=−0.33, p=0.031), and positively correlated with the overtraining score (r=0.352, p=0.019) which was in turn negatively correlated with ferritin r=−0.312, p=0.02). Besides, hematocrit behaves as a major determinant of blood viscosity (correlation with blood viscosity r=0.997, p<10⁻⁷) and erythrocyte disaggregability γ_D (r=0.384, p=0.03), but the hematocrit/viscosity ratio (h/η index of O₂ delivery) remains maintained almost constant over the range of values studied. These results show that (a) physiological values of hematocrit in these athletes are comprised between 36 and 48%; (b) “low” hematocrit (<40%) was associated with a higher aerobic capacity; (c) subjects with the higher hematocrits (>44.6%) were frequently overtrained and/or iron‐deficient, and their blood viscosity (and red cell disaggregability) tended to be increased.