Sex-based differences in hematological values after normalization to body mass or fat-free mass in adults matched for aerobic fitness

Blood properties influence aerobic exercise performance. While vascular volumes and hemoglobin mass (Hb(mass)) are elevated in trained individuals, evidence of sex differences in vascular volumes is equivocal due to inadequate matching of aerobic fitness between males and females. This cross-sectional study aimed to compare hematological values normalized to body mass (BM) and fat-free mass (FFM) between males (n = 45) and females (n = 34) matched for aerobic fitness ((V)overdotO(2max)) normalized to FFM (mL center dot kg FFM-1 center dot min(-1)). Data included body composition measured by dual-energy X-ray absorptiometry (DXA), (V)overdotO(2max) from an incremental test, and hematological values derived from a CO rebreathe test. Fat mass was unrelated to blood volume (BV; R-2 = 0.02, P = 0.26) and Hb(mass) (R-2 = 0.03, P = 0.16), while FFM was the strongest predictor of both (R-2 = 0.75 and R-2 = 0.83, respectively, P < 0.001). Females exhibited higher FFM-normalized BV (+4%, P < 0.05) and plasma volume (PV) (+14%, P < 0.001) and lower red blood cell volume (RBCV) (-8%, P < 0.001) and Hb(mass) (-8%, P < 0.001) compared to males. Positive correlations between aerobic fitness and relative Hb(mass) and BV were observed in both sexes when normalized to BM and FFM (0.48 < r < 0.71; P < 0.003). Stepwise multiple regression models, including FFM, (V)overdotO(2max), height, and [Hb], provided accurate predictions of Hb(mass) (R-2 = 0.91) and BV (R-2 = 0.85). Overall, sex differences persist in relative Hb(mass), BV, PV, and RBCV after matching of aerobic fitness, though relative BV and PV were greater in females. These findings suggest sex-specific strategies in oxygen delivery and/or extraction, and they underscore the importance of carefully selecting normalization practices when assessing sex-based differences in hematological variables .