Study links sitting for longer stretches to risk of blood clots in older women

Taking more breaks from sitting could potentially lower the risk 

New research suggests that older women who sit for longer periods during the day have an increased risk of developing venous thromboembolism (VTE), a potentially serious condition in which a blood clot forms in a vein. The study, published in the Journal of Thrombosis and Haemostasis, provides some of the best evidence to date linking sedentary behavior (sitting or lying down for long periods) to VTE risk. 

“To our knowledge, this study is the first to use a wearable monitor called an accelerometer to objectively measure sitting time in relation to VTE risk,” said lead author Laura Harrington, PhD, MPH, an associate investigator at Kaiser Permanente Washington Health Research Institute (KPWHRI). “By doing so, it provides new insight on how daily sitting patterns impact the risk of VTE for older women — and suggests possible targets for how people might lower this risk.” 

What do we know already? 

VTE is the third most common cardiovascular disease in the United States and can have serious health consequences. Notably, if a blood clot travels to the lungs, it can partially or fully block blood flow — a condition called pulmonary embolism, which can be fatal.   

As people get older, their risk of VTE increases. The reasons why are complex and incompletely understood, but scientists think sedentary behavior can play a role. Many older adults sit or lie down for long stretches during the day. This can lead to slower blood flow (a condition called venous statis), which can encourage the development of blood clots.  

Several previous studies have linked sedentary behavior to VTE risk. However, they have relied on participants to “self-report” their sedentary time (for example, through questionnaires and logs), which doesn’t always provide accurate data. “People tend to underestimate how much they are sitting and lying down,” Harrington said. 

To better understand the relationship between sitting and VTE risk, researchers looked at data on 5,591 older women (ages 63 to 99) who took part in the Women’s Health Initiative Objective Physical Activity and Cardiovascular Health (OPACH) study. The participants wore an accelerometer at their hip for 7 days, which allowed researchers to calculate their total sitting time, the average duration of their sitting bouts, and their total time spent in prolonged sitting bouts of 30 minutes or longer.  

What did the new study find? 

During an average follow-up of 8.2 years, 229 participants developed VTE. The researchers did not find evidence that total sitting time and total time spent in prolonged sitting bouts were associated with the risk of VTE. However, women whose average sitting bout was greater than 15 minutes were 59% more likely to have developed VTE than those whose average bout was 9 minutes or less.  

When the researchers looked separately at participants who had developed a pulmonary embolism, the results were even more striking: Women whose average sitting bout was above 15 minutes were twice as likely to have developed an embolism as those whose average bout was 9 minutes or less. 

To arrive at these findings, the researchers took into account many variables that can affect the risk of VTE, including participants’ age, activity level, family history of VTE, and health. They found that one factor — body mass index (BMI), which estimates whether someone is overweight or obese — may influence the relationship between sedentary time and VTE risk. Future studies will need to explore this relationship further.  

“Our findings show it’s not necessarily how long over the course of the day that older women sit that affects their VTE risk, but how long they sit without getting up,” said Harrington. “These are compelling results, as they suggest that taking more frequent breaks from sitting may lower older women’s risk of VTE — a promising area for further research.” 

Harrington’s coauthors on the study include KPWHRI affiliate investigator Andrea LaCroix, PhD.  

By Sophie Ramsey