New discoveries about an enzyme that regulates cholesterol levels could help explain one of the enduring mysteries of the sexes - why men die younger than women from heart disease.
Everywhere researchers have looked in the world, heart disease strikes men at earlier ages than women. In Western countries, where heart trouble is the biggest killer of both sexes, men generally die from it about 10 years sooner.Differences in traditional risk factors - high blood pressure, smoking and obesity, among others - do not entirely explain this disparity, nor do the hormonal and physical differences.
However, scientists say their research into gender differences in a liver enzyme called hepatic lipase may well explain why women typically have better cholesterol levels than men.
Dr. John E. Hokanson of the University of Washington in Seattle presented the findings at an American Heart Association conference, which concluded Saturday.
Younger women's risk is low because of their favorable lipid profiles - the types, not just the quantity, of cholesterol in their blood.
Cholesterol and fats called triglycerides cannot dissolve in blood. So they are carried through the bloodstream by transport molecules called lipoproteins, which are produced by the liver. The liver also makes hepatic lipase, which breaks down these combinations so the fats can be used by the body.
Most of the cholesterol is transported by low-density lipoprotein, or LDL. This is generally called the bad cholesterol, since it deposits excess cholesterol on the artery walls and leads to blockages. LDL also comes in different sizes, and small, dense LDL is considered especially harmful.
High-density lipoprotein, or HDL, is helpful, because it carries cholesterol and fats out of the bloodstream before they harm the arteries. The higher the proportion of HDL, the lower the risk of heart attacks. Typically, young women have higher HDL, lower LDL and less dense LDL than do men the same age.
Hokanson and colleagues tested 25 men and 39 premenopausal women. They ranged in age from 21 to 59 and had normal cholesterol levels.
Hepatic lipase levels were 53 percent higher in the men. Hokanson found that the men's higher amounts of this enzyme could explain 42 percent of the difference between the sexes in the density of the LDL they carried. And it could explain 97 percent of the difference between men and women in HDL levels.
"We believe that hepatic lipase is an important modulator of HDL," said Hokanson. "This accounts for the difference in coronary risk lipid profiles in men and women."
While he said this does not explain all of the difference in heart disease risks between the sexes, it could play a significant part.
Experts think women's risk goes up after menopause because they lose the protective effects of estrogen, the female sex hormone. Estrogen appears to regulate the body's levels of hepatic lipase, and Hokanson found this enzyme increases after menopause.