LAUSR

OBJECTIVES Lack of physical activity (PA) and prolonged sitting time (ST) are associated with increased risk of mortality and chronic illnesses, including depression. While there have been claims that the two risks are 'independent', their joint and stratified effects are unclear. The aim of this study was to explore the combined effects of physical activity and sitting time on the 12year risk of depressive symptoms (DS) in young women. DESIGN Cohort-9061 young participants in the Australian Longitudinal Study on Women's Health completed triennial surveys from 2000 (age 22-27), to 2012. METHODS Generalised Estimating Equation models were used to calculate the joint effects of PA and ST on DS, with <4h/day of ST and the highest PA quartile as the reference categories. Relationships between PA and DS, and between ST and DS, were also examined after stratification by ST and PA respectively. RESULTS In the adjusted joint effects model, compared with the reference category (low sitting, high PA), odds for DS were significantly higher in women who sat for >4, 6 and 8h/day and reported doing no PA. In every physical activity category, women who sat for ≥10h/day were at highest risk of DS (OR for lowest physical activity quartile, 1.72 [95% CI=1.38-2.14]; OR for highest physical activity quartile, 1.49 [95% CI=1.16-1.91]). After stratification by ST, odds of DS were reduced in women who reported any physical activity (compared with none), except when ST was >10h/day. After stratification by physical activity, the increased risk of sitting 8-10h/day was attenuated by any physical activity, but there was no reduction in risk of depressive symptoms with increasing PA levels in women who sat for ≥10h per day. CONCLUSIONS These data suggest that there are both joint and stratified effects of too little activity and too much sitting on the risk of depressive symptoms in young women. High levels of PA are protective against the hazards of high ST at this life stage, except in women with very high levels of sitting.