Tall, obese individuals have a significantly higher risk of developing blood clots in veins deep in the body, this is especially the case for men, Norwegian researchers revealed in the journal Arteriosclerosis, Thrombosis and Vascular Biology.

The authors explain that obese individuals are known to have a higher risk of developing clots in deep veins, especially in the lower limbs, as well as clots in blood vessels of the lungs (pulmonary embolism) - which can strain the heart and cause sudden death. Venous thromboembolism (VTE) refers to both these conditions.

Sigrid K. Braekkan, Ph.D., from the Hematological Research Group at the University of Tromsø, Norway, and team compared tall obese men to short-normal weight men (BMI 25, 5ft 7.7 inches tall) and tall, obese women to short normal weight women (up to 5ft 2.6 inches tall). The risk of VTE (age-adjusted) was:
  • In obese, tall men - 5.28 times higher
  • In obese, tall women - 2.77 times higher
  • In normal weight, tall men - 2.57 times higher
  • In normal weight tall women (over 5ft 6 inches) - no increased risk
  • In obese, short men - 2.11 times higher
  • In obese, short women - 1.83 times higher
They found that the risk of developing VTE among tall, obese males was similar to the risk that exists in pregnancy, using oral contraceptives, or having a gene for an inherited predisposition to clotting (Factor V Leiden).

Braekkan said:

"We believe that we observed the increased risk in tall and normal-weight men, but not women, because most women do not get sufficiently tall. The risk may be present in very tall women, but there were too few to investigate this properly."

The authors believe further studies are required to find out what mechanisms are at play in tall, obese individuals regarding VTE risk.

Braekkan said:

"In tall people the blood must be pumped a longer distance by the calf-muscle pump, which may cause reduced flow in the legs and thereby raise the risk of clotting.

Understanding and preventing VTE is important because even the first occurrence may be fatal. Obesity, in combination with other VTE risk factors, has been shown to substantially increase the risk, so we wanted to assess the combined effects of tall stature and obesity."

The investigators gathered data from the Tromsø study. Tromsø is a Norwegian town in which periodic health surveys of adults aged 25 to 97 years are carried out. The height and weight of 26,714 adult males and females were collected, and were followed up for a median of 12.5 years, up to 2007. A total of 461 VTEs occurred.

Obese people have high pressure in the abdomen, this can undermine the calf-muscle pump's ability to return the blood from the lower limbs.

Braekkan said:

"Obesity is also linked to a state of constant low-grade inflammation, and inflammation may render blood more susceptible to clotting."

When assessing their patients' overall risk for dangerous clots, doctors should take into account their height and weight, the authors wrote.

Braekkan said:

"Since body height is not easy to modify, the most important thing is to stay slim, especially if you are tall."

According to the American Heart Association, over 275,000 individuals are hospitalized in the USA annually with deep vein clots or pulmonary embolism.

"Joint Effects of Obesity and Body Height on the Risk of Venous Thromboembolism - The Tromsø Study"
Knut H. Borch; Cecilie Nyegaard; John-Bjarne Hansen; Ellisiv B. Mathiesen; Inger Njølstad; Tom Wilsgaard; Sigrid K. Brækkan
Arteriosclerosis, Thrombosis, and Vascular Biology. 2011. doi: 10.1161/ATVBAHA.110.218925

Written by Christian Nordqvist