Sitting Pretty: Only Prolonged Sedentary Time Linked With Incident CVD Risk

Sitting too long is associated with an increased risk of cardiovascular disease, but that risk exists only for those who are sedentary for large portions of the day, according to the results of a new study. 

Published July 13, 2016, in JAMA: Cardiology, a meta-analysis of nine studies examining the potential link between sedentary time and incident cardiovascular disease showed there was a nonlinear association between too much sitting and heart disease, with an increased risk of cardiovascular events evident only for those who sat for more than 10 hours per day. 

“We see that the independent relationship between sedentary time and cardiovascular disease is more or less flat until you reach a certain threshold,” said lead researcher Ambarish Pandey, MD (University of Texas Southwestern Medical Center, Dallas, TX). “After that [threshold], higher sedentary time becomes associated with more risk.”  

Speaking with TCTMD, Pandey said the American Heart Association recommends 150 minutes  of moderate-intensity physical activity per week—or 75 minutes of vigorous physical activity—and that studies have suggested prolonged sedentary time is also associated with an increased risk for cardiovascular disease. To date, though, there is little information on the risks of cardiovascular disease associated with different amounts of sedentary time, he said. As a result there are no recommendations from professional societies as to what constitutes “too much sitting” or recommendations on limiting sedentary time to optimize cardiovascular disease prevention. 

“There has been a lot of interest in the field and there have been a number of small studies, even some large studies, looking at sedentary time duration and risk of cardiovascular disease,” said Pandey. “These studies have had mixed findings, with no consensus on what should be the threshold for sedentary time, beyond which the risk of cardiovascular disease increases. That’s been the void in the literature that our study aims to address.”  

In the meta-analysis, which included 720,425 individuals followed for 11 years, those with the highest amount of sedentary time—mean duration 12.5 hours per day—had a statistically significant 14% relative increase in the risk of MI, stroke, and cardiovascular mortality compared with those with the least amount of sedentary time (mean duration 2.5 hours per day). However, individuals with moderate amounts of sedentary time—defined as 7.5 hours per day—did not have a greater risk of these events when compared with the least sedentary group.

The researchers also performed a “continuous dose-response” analysis examining the relationship between sedentary time and risk of cardiovascular disease. Overall, the relationship was nonlinear, with a nonsignificant increase in risk first observed among individuals who sat for 6.8 hours per day. The relationship between sedentary time and incident cardiovascular disease became statistically significant at 10 hours per day. 

The group assessed the role of body mass index (BMI) as a potential confounder on the association but found similar results in pooled analyses with and without adjustment for BMI. Risk models were also adjusted for physical activity levels and cardiovascular disease risk factors. To TCTMD, Pandey said this is the reason why sedentary time does not show an association with cardiovascular disease until very high levels of inactivity are reached. 

Carl “Chip” Lavie, MD (John Ochsner Heart and Vascular Institute, New Orleans, LA), who was not involved in the research, said the strength of the meta-analysis is that the individual studies adjusted for physical activity levels when assessing the impact of sedentary time on cardiovascular disease risk. There are some questions the analysis is unable to answer, however. 

What would be interesting to see is whether sedentary time still carries added risk for people who have a very high level of activity, Lavie suggested. “Let’s say somebody is doing 45 minutes or an hour of exercise a day, does sedentary time still add to their risk? The meta-analysis is adjusted for physical activity, but it’s adjusted for physical activity in everyone. I’d like to see the subgroup with high physical activity—does it matter in them? My prediction is that higher physical activity partially abolishes the negative effect of sedentary time, but it might not totally abolish it.”

Lavie said previous studies have shown a significant association between sedentary time and cardiovascular risk at around 8 hours and that he suspects sitting for 4 to 6 hours per day is unlikely to cause harm. “If you think about it, there are an extremely high number of people who would be sitting for 6 hours per day,” said Lavie. “If you have an office job, right off the bat you’re sitting for a good chunk of time, and then people sit for a couple of hours in the evening when they’re home. So until you start getting to 8 to 10 hours, I really don’t think you have too much excess risk.” 

As for next steps, Pandey said their study needs to be replicated, and that it’s premature to make any specific recommendations on sedentary time for cardiovascular disease prevention. “We also need some intervention studies, where we can actually see whether or not reducing sedentary time improves your risk-factor burden and clinical outcomes,” said Pandey. “That’s where the knowledge gap still exists. We know that sedentary time is bad, but what we don’t know is if reducing sedentary time improves cardiovascular risk.” 

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