by Alexandria Bachert MPH, Staff Writer, MedPage Today
September 11, 2017

Sitting more than half the day, regardless of how much exercise people got otherwise, was associated with increased risk of all-cause mortality in a large prospective study of middle-aged and older adults in the U.S.

Those spending more than 12.5 hours of sedentary time per day with bout duration of more than 10 minutes had the highest risk for all-cause mortality (HR 2.00; 95% CI 1.45-2.75; P<0.001), said researchers led by Keith M. Diaz, PhD, of Columbia University Medical Center in New York City.

Writing in the Annals of Internal Medicine, the group also that total sedentary time, more than the average duration of individual bouts of sedentary time. Over about 4 years of follow-up, individuals with low total sedentary time showed about the same mortality rates whether bout duration was classified as low (<10 minutes on average) or high (>10 minutes).

“The simple, straightforward implication is that excessive sedentary time may be a toxic, hazardous behavior and that regardless of whether a person exercises on a given day, they still need to be mindful of how much time they spend sedentary outside their exercise time,” Diaz explained to MedPage Today. “If you have a job or lifestyle where you have to sit for prolonged periods, our findings suggest that taking a movement break every half hour could reduce your risk of death.”

In an accompanying editorial, David A. Alter, MD, PhD, of the University of Toronto, commented, “Daily sedentary time, uninterrupted sedentary bout length, and moderate to vigorous physical activity may each play an important and distinct role in long-term health behaviors and survival.”

“Interventions with the greatest effect on population outcomes may be those that take each into account,” he stated.

Barry A. Franklin, PhD, director of Preventive Cardiology and Cardiac Rehabilitation at Beaumont Hospital in Royal Oak, Michigan, who was not involved with the study, told MedPage Today that the findings support recent studies which suggest that intermittent low-intensity standing throughout the day may be as important for disease prevention and optimization of health as is structured exercise.

“Contemporary physical activity guidelines should target reducing and interrupting sedentary time to reduce the risk of all-cause mortality,” Franklin said in an email.

“Outcomes of this study indicate what other studies have found and that is the more sedentary we are, the greater the health risk,” agreed Connie Diekman, MEd, RD, of Washington University in St Louis. “The good news is that regular activity is important to overall health. Whether we can say with great confidence that it will reduce risk of death is not clear but we can say that reducing sedentary lifestyles is an important part of a pattern to promote health.”

Diaz and colleagues analyzed data on 7,985 black and white adults 45 and older who were participating in the REGARDS (Reasons for Geographic and Racial Differences in Stroke) study.

Sedentary time was measured using a hip-mounted accelerometer and prolonged, uninterrupted sedentariness was expressed as mean sedentary bout length.

In multivariable-adjusted analysis by quartile of total sedentary time, longer time spent sitting was associated with higher risk for all-cause mortality: HR 1.22 for Q2 versus Q1 (95% CI 0.74-2.02), HR 1.61 for Q3 versus Q1 (95% CI 0.99-2.63), and HR 2.63 for Q4 versus Q1 (95% CI 1.60-4.30; P for trend <0.001). Longer sedentary bout duration by quartile was also associated with higher risk for all-cause mortality: HR 1.03 for Q2 versus Q1 (95% CI 0.67-1.60), HR 1.22 for Q3 versus Q1 (95% CI 0.80-1.85), and HR 1.96 for Q4 versus Q1 (95% CI 1.31-2.93; P for trend <0.001).

People with greater total sedentary time were more likely to be black, to smoke, and to have diabetes, hypertension, dyslipidemia, estimated glomerular filtration rate less than 60 mL/min/1.73 m², and atrial fibrillation. They were also less likely to be moderate or heavy drinkers and had greater BMI and lower levels of light-intensity physical activity.

While it’s not entirely clear why excessive sedentary time might contribute to all-cause mortality, the researchers listed poor blood glucose control and blood vessel damage as two possible reasons.

“Some of the best available evidence suggests that excessive sedentary time can cause abnormally high levels of sugar and, over time, could lead to diabetes. One of the reasons for this is related to our skeletal muscles, which require fuel such as glucose to operate and take in glucose from our blood,” Diaz said in an email.

He also explained that sitting for long periods can cause blood to pool in the legs and that the combination of blood pooling, decreased blood flow, and turbulent blood flow is believed to be conducive to the development of plaque formation in the blood vessels.

Peter Katzmarzyk, PhD, of Pennington Biomedical Research Center in Baton Rouge, Louisiana, commented that using accelerometry to measure sedentary behavior was a major study strength.

“Many previous studies have relied on self-reported assessments of sitting time, whereas this study was better able to objectively quantify the level of sedentary behavior,” he told MedPage Today. He also referenced adding sedentary bouts into the analysis, noting that “this study was able to look at how bout duration also is related to mortality, which is a novel analysis.”

Franklin agreed that objectively measuring sedentary time with an accelerometer was a huge improvement in study methodology, as self-reported activity and inactivity levels are often inaccurate.

Diaz and colleagues noted several study limitations, such as that the accelerometer did not distinguish between postures so they were forced to use an intensity-only definition of sedentary behavior and that only 7 days of accelerometer data were collected.

Importantly, they also pointed out that some of the risk factors were collected at baseline and may have changed during follow-up, leaving room for residual confounding. Additionally, the findings may not be fully generalizable to the entire REGARDS cohort, or to the general population.

• Reviewed by F. Perry Wilson, MD, MSCEAssistant Professor, Section of Nephrology, Yale School of Medicine and Dorothy Caputo, MA, BSN, RN, Nurse Planner