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Time spent being sedentary and weight gain in healthy adults: reverse or bidirectional causality?^1,2,3

¹ From the Medical Research Council Epidemiology Unit, Institute of Metabolic Science, Cambridge, United Kingdom

Background:Whether obesity is a cause or a consequence of a sedentary lifestyle has not yet been fully elucidated, which leaves uncertainty about the direction of causality.

Objective:We aimed to assess the longitudinal associations between objectively measured time spent being sedentary (sedentary time) and obesity indicators.

Design:The study was a prospective, population-based cohort study in 393 middle-aged healthy whites (n = 176 M, 217 F). Sedentary time (% of daytime hours) was measured by individually calibrated monitoring of the heart rate. Body weight (BW), body mass index (BMI), and waist circumference (WC) were assessed by standard clinical procedures. Fat mass (FM) was assessed with bioimpedance. All measurements were collected at baseline and at 5.6-y follow-up.

Results:At baseline, sedentary time was significantly correlated with FM (partial r = 0.10, P = 0.043) and WC (partial r = 0.11, P = 0.027) after adjustment for sex and age. At follow-up, sedentary time was significantly correlated with BW (partial r = 0.19, P < 0.0001), BMI (partial r = 0.20, P < 0.0001), WC (partial r = 0.15, P = 0.003), and FM (partial r = 0.19, P < 0.0001). Sedentary time did not predict any of the obesity indicators at follow-up. In contrast, BW (β = 0.33; 95% CI: 0.15, 0.50), BMI (1.10; 0.58, 1.63), FM (0.59; 0.11, 0.40), and WC (0.44; 0.23, 0.66) predicted sedentary time at follow-up after adjustment for sex, baseline age, baseline sedentary time, baseline physical activity energy expenditure, and follow-up time.

Conclusion:BMI, FM, and WC may predict sedentary time, but our results do not suggest that sedentary time predicts future obesity.