Sitting Too Much May Have Serious Consequences

A study of more than 4,500 Scottish people over age 35 found those who spent at least four hours of screen time each day were more than twice as likely as those with less than two hours of screen time to suffer a cardiovascular event (HR 2.30, 95% CI 1.33 to 1.96), according to Emmanuel Stamatakis, PhD, of University College London, and colleagues.

People with the most screen time also had an increased risk of death during an average of four years of follow-up (HR 1.52, 95% CI 1.06 to 2.16), Stamatakis and co-authors reported in the Journal of the American College of Cardiology.

Adjusting for physical activity levels only slightly attenuated the magnitude of the associations between sitting and cardiovascular risk and death.

Although the study could not establish causal relationships between screen time — a proxy for being sedentary — and the risk of adverse outcomes, the authors concluded, “Our results support the inclusion of a sedentary behavior guideline in public health recommendations for cardiovascular disease prevention.”

Previous studies have observed a link between screen time and both death and cardiovascular risk factors. A study published in July found that sitting was associated with mortality^[1], regardless of physical activity levels.

For the current study, Stamatakis and his colleagues analyzed data from 4,512 respondents to the 2003 Scottish Health Survey. All were at least 35 years old and had follow-up information available from national registries through 2007.

The participants reported the amount of leisure time they spent using screen-based entertainment each day — less than two hours, two to less than four hours, or four or more hours.

During an average follow-up of 4.3 years, there were 215 cardiovascular events and 325 deaths from all causes.

Cardiovascular risk was increased in respondents reporting at least two hours of screen time a day, and mortality risk was elevated for those with at least four hours of screen time a day.

After adjustment for age, sex, ethnicity, obesity, smoking, social class, long-standing illness, marital status, diabetes, and hypertension, the hazard ratio for all-cause mortality was 1.52 (95% CI 1.06 to 2.16) and for cardiovascular events was 2.30 (95% CI 1.33 to 3.96) for those with four or more hours of screen time a day compared with those with less than two.

Additional adjustment for physical activity only slightly attenuated the risks, with a hazard ratio 1.48 for all-cause death and 2.25 for cardiovascular events.

Although the mechanisms tying screen time to cardiovascular risk are unclear, an analysis of a subset of the survey respondents who provided blood samples suggested metabolic and inflammatory pathways might be involved.

C-reactive protein levels accounted for about 18% of the relationship, and a combination of C-reactive protein, body mass index, and HDL cholesterol explained 25% of the relationship.

Another possible mechanism proposed by the researchers was related to the decreased expression of endothelial nitric oxide synthase, a result of reduced local shear stress from lower blood flow resulting from excessive sitting.

“Further experimental studies will be required to determine the exact mechanisms accounting for increased cardiovascular disease risk during prolonged inactivity in humans,” Stamatakis and his colleagues wrote.

The noted some limitations of the study, including the use of self-reported screen time, the inability to account for diet, and the small sample sizes used for the various explanatory analyses.

References

1. ^{^} sitting was associated with mortality (www.medpagetoday.com)