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

Abstract

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 (V̇O 2 max) normalized to FFM (mL∙kg FFM −1 ∙min − 1 ). Data included body composition measured by dual-energy X-ray absorptiometry (DXA), V̇O 2 max 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, V̇O 2 max, 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.

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