Blood Is Less Sticky With Estrogen Replacement Therapy

Robert Rosenson, M.D., director of the Preventive Cardiology Center, Rush-Presbyterian-St. Luke's Medical Center, and collaborators at the University of Michigan at Ann Arbor, measured the viscosity of plasma -- the fluid component of blood -- in 23 women. Viscosity is the friction of a fluid that makes it resist flowing. Water has a low viscosity while molasses is highly viscous.

"Plasma viscosity is an important predictor of initial and recurrent heart attack and stroke," says Rosenson. "Studies have suggested viscosity to be as important a predictor of heart disease risk as smoking, diabetes, cholesterol or blood pressure, but this is the first study to evaluate the influence of HRT on plasma viscosity."

The study found that HRT lowered a woman's blood viscosity. This effect may be a new mechanism for the cardiovascular protection received by women taking HRT, says Rosenson.

Researchers divided the 23 women in the study into three groups. Seven women received estrogen plus progesterone therapy, eight women took estrogen alone, and the remaining eight received an inactive pill called a placebo.

"At the end of 12 weeks, viscosity in both groups receiving estrogen therapy decreased by about four percent. Based on other studies we would equate this change in viscosity to a decrease in heart disease risk by about 20 percent," says Rosenson.

Rosenson also notes that blood levels of low-density lipoprotein (LDL) or the 'bad' cholesterol of the women in both estrogen groups fell by 29 percent, and fibrinogen levels decreased by 14 percent.

Previous studies have indicated that HRT lowers blood levels of fibrinogen, a clotting factor, and LDL cholesterol says Rosenson. These factors may contribute to plasma viscosity because higher levels of fibrinogen and LDL make blood stickier, he adds.