Disentangling the relationship between sedentariness and obesity: Activity intensity, but not sitting posture, is associated with adiposity in women.

Background: The relationship between free-living sedentary behaviour (SB) and obesity is unclear. Studies may arrive at disparate conclusions because of inconsistencies and limitations when defining and measuring free-living SB. The aim of this cross-sectional study was to examine whether the relationship between SB and adiposity differed depending on the way SB was operationally defined and objectively measured.

Methods: Sixty-three female participants aged 37.1 years (SD = 13.6) with a body mass index (BMI) of 29.6 kg/m (SD = 4.7) had their body composition measured (BodPod, Concord, CA) then were continuously monitored for 5-7 days with the SenseWear Armband (SWA; sleep and activity intensity) and the activPAL (AP; posture). Data from both activity monitors were analysed separately and integrated resulting in a third measure of SB (activity intensity and posture; SED). SB outputs were compared according to week or weekend day averages then correlated against body composition parameters after adjusting for moderate-to-vigorous physical activity (MVPA).

Results: SED resulted in the most sedentary time 11.74 h/day (SD = 1.60), followed by SED 10.16 h/day (SD = 1.75) and SED 9.10 h/day (SD = 1.67). There was a significant positive association between SED and body mass [r(61) = 0.29, p = .02], BMI [r (61) = 0.33, p = .009] and fat mass [r(61) = 0.32, p = .01]. SED and SED were not associated with any of the indices of adiposity. Correlations between SB and adiposity were non-significant when controlling for MVPA.

Conclusions: The relationship between SB and adiposity differed depending on how SB was operationally defined and measured, and was dependent on MVPA. The definition of SB based on a sitting posture (SED) was not strongly related to body fat, whereas the accumulation of any behaviour (sitting or standing) with an intensity of <1.5 METs (SED) (offset by the presence of MVPA) was positively associated with indices of adiposity. These data suggest that the postural element of SB (sitting) is not sufficient for the accumulation of adiposity, rather activities requiring low EE (<1.5 METs) and the absence of MVPA, regardless of posture, are associated with higher fat mass.