Sex differences in cardiorespiratory fitness are explained by blood volume and oxygen carrying capacity

Intrinsic sex differences in fundamental blood attributes have long been hypothesized to contribute to the gap in cardiorespiratory fitness between men and women. This study experimentally assessed the role of blood volume and oxygen (O₂) carrying capacity on sex differences in cardiac function and aerobic power.

Healthy women and men (n = 60) throughout the mature adult lifespan (42–88 yr) were matched by age and physical activity levels. Transthoracic echocardiography, central blood pressure, and O₂ uptake were assessed throughout incremental exercise (cycle ergometry). Main outcomes such as left ventricular end-diastolic volume (LVEDV), stroke volume (SV), cardiac output (Q), and peak O₂ uptake (VO_2peak), as well as blood volume (BV) were determined with established methods. Measurements were repeated in men following blood withdrawal and O₂ carrying capacity reduction matching women’s levels. Prior to blood normalization, BV and O₂ carrying capacity were markedly reduced in women compared with men (P < 0.001). Blood normalization resulted in a precise match of BV (82.36 ± 9.83 vs. 82.34 ± 7.70 ml·kg⁻¹, P = 0.993) and O₂ carrying capacity (12.0 ± 0.6 vs. 12.0 ± 0.7 g·dl⁻¹, P = 0.562) between women and men. Body size-adjusted cardiac filling and output (LVEDV, SV, Q) during exercise as well as VO_2peak (30.8 ± 7.5 vs. 35.6 ± 8.7 ml·min⁻¹·kg⁻¹, P < 0.001) were lower in women compared with men prior to blood normalization. VO_2peak did not differ between women and men after blood normalization (30.8 ± 7.5 vs. 29.7 ± 7.4 ml·min⁻¹·kg⁻¹, P = 0.551).

Sex differences in cardiorespiratory fitness are abolished when blood attributes determining O₂ delivery are experimentally matched between adult women and men.