Longevity & AgingWhy Females Have Lower VO2 Max Than Males — Hemoglobin Is the Key
A new mechanistic study using invasive cardiac and blood flow measurements in highly trained cyclists found that when adjusted for lean body mass, females and males have nearly identical cardiac output, leg blood flow, and muscle oxygen extraction. The critical difference lies in hemoglobin concentration: females averaged 10% lower levels, reducing oxygen-carrying capacity and lean-mass-normalized VO2 max by roughly 10–14%. Muscle mitochondrial and capillary densities were also equivalent between sexes. This means the cardiovascular and muscular machinery is comparably efficient — it is blood oxygen content, driven by hemoglobin, alongside body composition differences, that primarily explains the sex gap in aerobic capacity.
