Are taller men at greater risk of blood clots?

Men who are six feet tall or taller may have a higher risk of blood clots in their veins than do their shorter counterparts, while height does not seem to have an impact on women's risk, a new study suggests.

The blood clots, known as venous thromboembolisms, include clots in the deep veins -- usually in the legs -- and pulmonary embolism, a potentially fatal condition that occurs when a deep-vein blood clot travels to the lungs.

Certain situations can lead to clots in some people, such as being immobilized after surgery or taking a long-haul flight. There are also lifestyle-related factors, such as obesity and smoking, which can leave a person more vulnerable to the blood clots.

Some studies have found a correlation between height and clot risk, with greater height equaling greater risk. But those studies have only included men.

Overall, the tallest men -- those who were taller than 181 centimeters, or roughly 6 feet -- had double the clot risk of men who were shorter than 173 centimeters, or 5 feet, 8 inches. That was with factors such as weight, smoking habits and diabetes taken into account.

The findings, reported in the American Journal of Epidemiology, do not prove that greater height itself is a risk factor for clots.

The researchers accounted for a number of factors that could have explained the link, but there may be other variables at work that were unmeasured in the study, according to lead researcher Dr. Sigrid K. Brakken, of the University of Tromso in Norway.

However, Brakken told Reuters Health by email, it is plausible that taller height could affect the odds of blood clots in the veins -- due to what's known as "venous stasis," or slowed blood flow in the veins, particularly those of the legs. That slower flow increases the potential for blood clotting.

The reason the association is seen only in men, according to Brakken, is likely because fewer women are tall enough for their height to affect clot risk.

The findings are based on data from 26,727 Norwegians between the ages of 25 and 96 who were followed for 12 years. During that time, 462 developed a first-time clot; they were often related to specific causes, like surgery, trauma or medical conditions such as cancer -- but in 42 percent of cases, there were no such "provoking" factors.

Among men who stood at six feet or taller, there were 1.68 clots per 1,000 men per year. That compared with a rate of 0.84 per 1,000 men per year among those who were shorter than 5 foot 8.

When the researchers accounted for weight and several other factors that affect clot risk, the tallest men still had double the risk of the shortest men.

If further research confirms that greater height is a risk factor for clots, Brakken said, it could help identify those people who need to take steps to prevent the blood clots.

Not smoking, losing excess weight and getting regular exercise are among the ways to lower clot risk.

SOURCE: here American Journal of Epidemiology, online April 23, 2010.

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