A large study of more than 2 million siblings suggests that taller people are at an increased risk of developing blood clots, while shorter people are less susceptible.

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A new cohort study suggests that susceptibility to blood clots may have something to do with your height.

Venous thromboembolism (VTE) is a condition characterized by the formation of blood clots in the veins. VTE can include deep vein thrombosis, in which blood clots form in the "deep veins," usually in the legs, and pulmonary embolism, wherein a blood clot travels through the system and ends up blocking the pulmonary artery, which carries blood from the heart to the lungs.

VTE can lead to reduced mobility or even immobility, and, in the most severe cases, death. In the United States, approximately 300,000 to 600,000 individuals experience VTE each year.

Some of the known risk factors for VTE include major surgeries, multiple trauma, heart attack, obesity and lack of physical mobility, pregnancy, and hereditary factors. But now, a new study from Lund University in Sweden suggests that height should also be taken into account as a risk factor.

Lead researcher Dr. Bengt Zöller, in collaboration with his colleagues, worked with a large cohort of 2,704,212 Swedish siblings - both men and women - to see whether there was any link between VTE and physical height.

The researchers' findings were published yesterday in the journal Circulation: Cardiovascular Genetics.

Taller people more predisposed to VTE

In their study, the authors explain that an association between VTE and height had already been observed by researchers, but that the links remained inconclusive due to various confounding variables.

That is why in the current study, the team decided to work with siblings, as they would offer a stronger basis for comparison - especially since height is often an inherited characteristic.

Two groups were studied to begin with: one comprised men born between 1951 and 1992 recruited for military service and without any history of VTE, and another comprised women experiencing their first pregnancy and who also did not have a history of VTE.

The men were followed from the time of their enlistment (1969 to 2010) until 2012, while the women were followed from their first pregnancy (1982 to 2012) until 2012. From these groups, the scientists were able to identify pairs of natural siblings who exhibited a significant difference in physical height.

The researchers, as expected, found that taller people have a higher risk of developing blood clots, whereas shorter individuals are exposed to a lower risk.

Some differences based on biological sex were also noted. Men whose height was under 5 foot 3 inches had a 65 percent lower risk of VTE compared with men who were 6 foot 2 inches or taller.

In the case of women, those shorter than 5 foot 1 inch had a 69 percent lower risk of developing blood clots than women who were 6 feet or taller. All the women had only been through one pregnancy.

What is the mechanism?

Dr. Zöller hypothesizes that the length of the "deep veins," as well as gravity, may well play a role in the association between height and VTE.

"It could just be that because taller individuals have longer leg veins there is more surface area where problems can occur. There is also more gravitational pressure in [the] leg veins of taller persons that can increase the risk of blood flow slowing or temporarily stopping."

Dr. Bengt Zöller

The researchers do acknowledge some limitations to their study, however. For instance, the lack of access to data regarding family lifestyle factors including smoking, dietary practices, and degree of physical activities.

Another point of caution is linked to the "translatability" of this study to other populations. The research was conducted on a Swedish cohort, so the same findings may not hold true for a U.S.-based cohort. Nevertheless, the researchers argue that Swedish and North American populations both boast a similar degree of ethnic diversity.

The team maintains that future assessments for risk of blood clots should start taking physical height into account but warns that further studies are still needed to ascertain the mechanism at play.

"I think we should start to include height in risk assessment just as overweight, although formal studies are needed to determine exactly how height interacts with inherited blood disorders and other conditions," concludes Dr. Zöller.