By Sigrid Brækkan, Associate Professor TREC
Obesity is associated with increased risk of blood clots (thrombosis). The mechanism behind this association remains unclear, but studies have shown that overweight and obese subjects display higher levels of several components of the coagulation system. 
If the risk of blood clots in obese is caused by increased coagulation activity, one would expect that obese people are protected against bleeding. However, is this the case? Do obese people bleed less frequently than those that are normal weight?
In collaboration with researchers from the Netherlands, we investigated the association between obesity measures and risk of bleeding. We found that the skinniest subjects had the highest risk of bleeding, while there was no difference in the risk of bleeding between normal weight and overweight or obese subjects.
We included 9736 patients, aged 18-79 years, who participated in the SMART study (Second Manifestation of ARTerial disease) conducted in the Netherlands. The patients were examined at the time of study entry, and obesity measures including body mass index, waist circumference, hip circumference and waist-to-hip ratio were assessed. The patients were then followed for an average of 6 years, and all events of clinically relevant bleeding were recorded.
There was no difference in the risk of bleeding among those normal weight and those overweight or obese, but those who were underweight (BMI<18.5) appeared to have an increased risk. This could potentially be explained by other concomitant diseases among the underweight that we were unable to take into account in our analyses. Several factors can explain the absence of an association between obesity and lower risk of bleeding.
Obesity is associated with hypertension, diabetes and renal failure, which is also associated with risk of bleeding. Thus, the presence of such bleeding-related factors may counteract the increased coagulation activity and thereby explain the absence of protection against bleeding in the obese. Moreover, it is possible that increased coagulation activity alone is insufficient to cause thrombosis, and that concomitant presence of other thrombosis-related risk factors is necessary to yield the increased thrombosis risk observed in obese subjects.
Obesity was not associated with a lower risk of bleeding. Our findings suggest that the presumed protection against bleeding due to a procoagulant profile may be counterbalanced by other bleeding-related factors in obese subjects.
Reference:
Braekkan SK, var der Graaf Y, Visseren F, Algra A. Obesity and risk of bleeding. The SMART study. J Thromb Haemost. 2015 Oct 30.
