When does greener mean slimmer? Longitudinal analysis of green space, trees, grass, and body mass index in a cohort of 50,672 Australians: exploring potential non-linearities and modifying influences of household relocation and gender

Studies of associations between green space and overweight/obesity are often agnostic to green space type and potential contingencies by gender and household relocation. We investigated associations between total green space, tree canopy, and open grass within 1.6 km road network buffers, on body mass index (BMI) at baseline (n = 110,234; 2005–2009) and follow-up (n=50,672; 2012–2015) using data from the Sax's Institute's 45 and Up Study (NSW, Australia). Analyses considered potential non-linearities in these associations and the possibility for effect modification by gender and household relocation. Models were adjusted for age, couple status, region of birth, household income, employment status, educational attainment and housing status. Results revealed that 20 % or more tree canopy, compared to 0–10 % tree canopy was associated with lower BMI. Comparable findings were not observed for open grass or total green space. Nonlinear associations were observed, with a rapid decrease in odds of being overweight/obese at baseline with 20 % tree canopy, that became more stable by 30 % tree canopy. This non-linearity was stronger in females than males. By follow-up, associations indicated potential protective effects with 20 % or more tree canopy for overweight/obesity in females. Logistic regression models examined the odds of becoming a healthy weight at follow-up in overweight/obese individuals at baseline. Results indicated a potentially protective influence of tree canopy on overweight/obesity for people who did not move home (odds ratio (OR) = 1.06, 95 % confidence interval (CI) = 1.01–1.12), but not those who relocated (OR = 0.97, 95 % CI = 0.83–1.11). In sum, conserving and restoring urban tree canopy to at least 20% to 30% of nearby land-use may help to reduce levels of overweight and obesity in the community These benefits may be particularly potent for females and for people who are residentially stable. More open grass, or green space in general, may not have the same benefits for adult overweight/obesity as tree canopy.