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A study conducted at the University of Pittsburgh and published in Clinical Hemorheology and Microcirculation demonstrated on average 62% higher systolic blood viscosity and 25% higher diastolic blood viscosity in men over premenopausal women.
Premenopausal women have lower hematocrit and improved red blood cell properties, decreasing their blood viscosity levels and protecting the arteries close to the heart from injury, plaque and rupture. Women typically lose about 50-100 mL of blood every menstrual cycle, and lost blood cells are replaced with younger, healthier cells. The blood of premenopausal women has about 80% more young RBCs and 85% fewer old RBCs than male blood. Younger RBCs have been shown to be softer (more deformability) and less sticky (lower aggregability) – two properties that cause a significant reduction in blood viscosity. Numerous scientific studies have established that cardiac morbidity and mortality rates are much higher in men than in premenopausal women. Various hypotheses have been offered for the mechanism behind this gender difference including the estrogen hypothesis and the iron hypothesis. The authors of the Pittsburgh study however refuted the estrogen and iron hypotheses citing other published findings and, instead, suggested that reduced viscosity is the best candidate for explaining the superior cardiovascular profile of premenopausal women.
The Pittsburgh group wrote “the increase in the concentration of young RBCs and decrease in the concentration of old RBCs in the human blood resulting from blood loss is of a great importance for the prevention of cardiovascular diseases.”
The authors’ conclusions offer insight to a remarkable study performed in 1978 which followed a cohort of 2,873 women from Framingham, Massachusetts for 24 years (their ages at the beginning of the study ranging from 29 through 62 years). Over the entire time period, no premenopausal women developed myocardial infarction or died of coronary heart disease despite approximately 8,500 premenopausal person years’ experience. The Framingham authors concluded the following: “A careful investigation of this subject may lead to a better understanding of the factors that account for the remarkable protection against coronary heart disease enjoyed by premenopausal women. Somewhere in this tantalizing mystery may lie a lesson of profound importance in understanding the genesis and course of this disease, perhaps in men as well as women.” Based on the University of Pittsburgh study reviewed above, viscosity stands as a strong candidate for bringing these factors to light.