Association between sedentary time and mortality across levels of frailty

Sedentary behaviours are associated with adverse health outcomes in middle-aged and older adults, even among those who exercise. We examined whether the degree of frailty affects the association between sedentary behaviours and higher risk of mortality.

In this prospective cohort study, we used data from 3141 community-dwelling adults 50 years of age or older from the 2003/04 and 2005/06 cohorts of the US National Health and Nutrition Examination Survey. Time engaged in sedentary behaviours was measured using uniaxial accelerometers, and frailty was based on a 46-item frailty index. Mortality data were linked up to 2011. We used Cox proportional hazard models to estimate the hazard ratio (HR) of sedentary behaviour.

We found that for people with low levels of frailty (frailty index score ≤ 0.1), sedentary time was not predictive of mortality, regardless of physical activity level (adjusted HR 0.90, 95% confidence interval [CI] 0.70-1.15). Among people who were vulnerable (0.1 < frailty index score ≤ 0.2) or frail (frailty index score > 0.2), sedentary time was associated with higher mortality only among those who were physically inactive (not meeting the criterion for moderate physical activity) (HR 1.16, 95% CI 1.02-1.33 for the group defined by 0.1 < frailty index score ≤ 0.2; HR 1.27, 95% CI 1.11-1.46 for the group defined by 0.2 < frailty index score ≤ 0.3; HR 1.34, 95% CI 1.19-1.50 for frailty index score > 0.3).

The effect of sedentary behaviours on mortality varied by level of frailty. Adults with the highest frailty level experienced the greatest adverse impact. Low frailty levels (frailty index score ≤ 0.1) seemed to eliminate the increased risk of mortality associated with prolonged sitting, even among people who did not meet recommended physical activity guidelines.