Mighty Things From Small Beginnings

  • Tiny fragments of cells called platelets are responsible for clotting
  • Clotting after injury stops bleeding; clotting within a blood vessel can obstruct blood flow to the heart muscle, leading to a heart attack
  • Alcohol may ‘thin’ the blood, thereby reducing the chances of vessel obstruction
  • The anticoagulant properties of alcohol are dose specific, occurring at blood-alcohol concentrations associated with moderate drinking

Like many parts of the human body, a platelet is a tiny but essential team member. One of its primary functions is to form blood clots - this is called platelet aggregation - both internally and externally. A study in the April issue of Alcoholism: Clinical & Experimental Research has found that certain levels of alcohol consumption may have positive consequences for coronary artery disease by inhibiting internal platelet aggregation.

"Platelet aggregation is an extremely important event in a heart attack," said Adam K. Myers, professor of physiology at Georgetown University Medical Center and lead author of the study. "The threatening event in coronary artery disease is the formation of a clot within the blood vessel by platelets."

Platelets are made in bone marrow; a drop of blood contains some 15 million platelets. Although they are often called cells themselves, platelets are really fragments of other cells that are activated whenever blood clotting or repair to a vessel is necessary. Under normal circumstances, when an injury or cut occurs, platelets rapidly accumulate at the site of blood-vessel damage, swell into odd, irregular shapes, grow sticky and clog at the cut, thereby creating a plug. Under abnormal circumstances, such as coronary artery disease, a blood clot occurs within a blood vessel and obstructs the normal flow of blood, leading to a heart attack. . The inability to clot following injury is called "hemophilia." Blood clotting that obstructs flow is called "thrombosis."

Anticoagulants, medications that prevent blood from clotting by thinning it, help prevent the formation of blood clots. Aspirin can serve as an anticoagulant. Myers’ study found that, within specific parameters, alcohol might also act as a kind of anticoagulant. "Our study found that a dose-dependent amount of alcohol can reduce the potential for these platelets to aggregate," said Myers. "The mechanism by which this occurs is similar to aspirin’s protective effects against coronary heart disease and heart attacks, by blocking platelets from forming clots." However, just like too much alcohol can lead to a number of health complications, excess aspirin can be "too much of a good thing."

"We are able to measure aspirin’s effects on platelet function through bleeding times," explained Norberta Schoene, research chemist at the U.S. Department of Agriculture. "If you slice your skin before taking aspirin, it may take around three minutes for blood to stop oozing out. After taking aspirin, your bleeding time might go up to nine or 10 minutes." The important factor, she noted, was that aspirin, alcohol, and other drugs all have a dose-dependent effect, a dose response. Any positive consequences of alcohol ingestion seem closely tied to specific levels of consumption.

Myers and Schoene are both concerned about people incorrectly interpreting the study’s results to justify personal drinking habits. Accordingly, Myers pointed out two important components to his study. One, researchers looked at the effects of alcohol consumption in one sitting. They were interested in the mechanisms behind thrombotic activity, and how alcohol may effect that. They did not investigate long-term or cumulative use of alcohol.

"We only looked at a single episode," said Myers. "I can’t predict what one drink a day, or two drinks a day, or three drinks a week are going to do on a long-term basis."

The second component relates to past media attention to the "French paradox" involving red wine. Sometimes - for example, the touted health benefits of red wine, soy products, and specially formulated margarines - publicity can precede research. Indeed, studies investigating the French paradox have had mixed results, which is why Myers chose to look at ‘straight’ alcohol rather than any specific beverage.

"There is something about alcohol itself," he said, "as opposed to a particular beverage, that could have beneficial effects. More and more research shows that it doesn’t seem to matter what people drink, as long as they are moderate drinkers."

Another finding of interest in Myers’ study was the greater effect that alcohol seemed to have on platelet aggregation in women. In other words, alcohol seemed to ‘thin’ their blood more than it did for men. Myers speculated this finding may be due to how women absorb and metabolize alcohol, or that estrogen may be a factor. "We know that women, prior to menopause, have much lower rates of coronary heart disease," he said.

Schoene agreed: "Premenopausal women produce adequate levels of estrogen, which is supposedly protective for cardiovascular disease," she said. "It may be that what we’re seeing here is a synergistic interaction between alcohol and estrogen, and this interaction is causing the platelets from the female subjects to be less active in clot formation." She added that such unexpected findings are one of the exciting benefits of research. "This is the kind of finding that gives scientists good evidence to do a further study of women. Pursuing these findings in greater detail is what helps us understand the mechanisms of disease."

<P><img src="../images/null.gif" width="25" height="1">Zhang, Q.H., Das, S., &amp; Siddiqui, S. (2000, April). Effects of acute, moderate ethanol consumption on human platelet aggregation in platelet rich plasma and whole blood. <I>Alcoholism: Clinical and Experimental Research, 24</I>(4), 528-534.</P>