Differences in Cardiac Output and Aerobic Capacity Between Sexes Are Explained by Blood Volume and Oxygen Carrying Capacity

Abstract

Whether average sex differences in cardiorespiratory fitness can be mainly explained by blood inequalities in the healthy circulatory system remains unresolved. This study evaluated the contribution of blood volume (BV) and oxygen (O2) carrying capacity to the sex gap in cardiac and aerobic capacities in healthy young individuals. Healthy young women and men (n = 28, age range = 20–43 years) were matched by age and physical activity. Echocardiography, blood pressures, and O2 uptake were measured during incremental exercise. Left ventricular end-diastolic volume (LVEDV), stroke volume (SV), cardiac output (Q), peak O2 uptake (VO2peak), and BV were assessed with precise methods. The test was repeated in men after blood withdrawal and reduction of O2 carrying capacity, reaching women’s levels. Before blood normalization, exercise cardiac volumes and output (LVEDV, SV, Q) adjusted by body size and VO2peak (42 ± 9 vs. 50 ± 11 ml⋅min–1⋅kg–1, P < 0.05) were lower in women relative to men. Blood normalization abolished sex differences in cardiac volumes and output during exercise (P ≥ 0.100). Likewise, VO2peak was similar between women and men after blood normalization (42 ± 9 vs. 40 ± 8 ml⋅min–1⋅kg–1, P = 0.416). In conclusion, sex differences in cardiac output and aerobic capacity are not present in experimental conditions matching BV and O2 carrying capacity between healthy young women and men.

No sex differences in oxygen uptake or extraction kinetics in the moderate or heavy exercise intensity domains

“…Females did demonstrate lower HbO 2 +MbO 2 levels during heavy intensity cycling, perhaps indicating a lesser O 2 availability. This would fit with the notion that O 2 carrying capacity is inferior in females during high-intensity exercise (Diaz-Canestro et al ., 2022); however, the lack of difference in the speed and amplitude of HHb+Mb onset kinetics (and pulmonary V□O 2 ) implies that oxygen extraction is not negatively affected by this in the moderate and heavy intensity domains.…”

“…Despite more aerobically-suited skeletal muscle, females have lower levels of haemoglobin (Murphy, 2014), which is thought to impair O 2 carrying capacity during exercise (Harms et al ., 1998; Diaz-Canestro et al ., 2022). During exercise where O 2 delivery and utilisation are both limiting factors (e.g., cycling, Goulding & Marwood, 2023), these factors are thought to counteract each other to enable comparable relative metabolic thresholds (i.e., critical power) between the sexes (Ansdell et al ., 2020b).…”

Heart rate and gas exchange dynamic responses to multiple brief exercise bouts (MBEB) in early‐ and late‐pubertal boys and girls

“…We did observe larger V̇O 2 , V̇CO 2 , and V̇E costs of MBEB in females compared to males (n.b., there was no sex × pubertal stage interaction). In the case of V̇O 2 , it has been challenging to directly compare oxidative capacity in response to exercise between women and men due to factors such as differences in body composition, (Diaz‐Canestro et al, 2021 ) blood volume, (Diaz‐Canestro et al, 2022 ) distribution of muscle fiber types, (Fournier et al, 2022 ) and gauging overall levels of conditioning. Men may demonstrate greater V̇O 2 responsiveness to exercise training than do women, (Diaz‐Canestro & Montero, 2019 ; Montero et al, 2018 ) suggest that even though muscle mass is smaller in women, mitochondrial volume is greater, perhaps explaining our finding of great V̇O 2 costs when normalized to the work performed.…”

The Chinese cardiorespiratory and circulatory system at work in women and men: a case–control study

“… 27 The average time of day at which the measurements were performed was matched between EA and HC by scheduling prospective participants in EA or HC groups according to the average time of previous participants in the opposite group along with covariate adaptive randomisation, including age (<45/≥45 yr), sex (female/male) and physical activity (<5/≥5 h of MVPA-endurance), to ensure that these variables were matched between groups According to previous studies, the specific day of the menstrual phase was noted (if applicable) but not fixed for the day of testing as it does not influence the study outcomes. 28 , 29 , 30 Prior to testing, the participants completed health and clinical questionnaires in English and/or Chinese and rested in supine position for 20 min in order to stabilise cardiovascular, hemodynamic and haematological variables. These questionnaires, in addition to the aforementioned self-identified data, covered disease history (including family history of disease), physical activity history, diet/supplement history (if the participant reported any type of food restriction, e.g., with vegetarian diets), female-specific (e.g., menstrual-related) health information, blood donation history (if any), as well as detailed information regarding physical activity, food, supplement, medication (if any) and fluid intake over the last 24 h. Resting measurements were performed on the horizontal (supine) platform that contained the exercise testing equipment, with the legs straight and relaxed, and the feet not attached to the cycle ergometer (KICKR Core, Wahoo, USA).…”

“…O 2 uptake (VO 2 ) and CO 2 output were continuously measured throughout an established incremental exercise protocol using a mixing chamber system (CardioCoach VO 2 , KORR Medical, USA), as previously described. 7 , 15 , 29 , 31 , 32 Following a warm-up period at 10–30 W, the workload was progressively increased by 10–30 W increments every 50 s until exhaustion was reached in the recommended total duration of 7–10 min. 45 Calibration of the gas analysers and the flowmeter was performed prior to each test.…”