Background Obesity is associated with diseases such as cardiovascular disease and diabetes as well as mental health. Previously it was shown that the socioeconomic inequalities in adult BMI stabilised in Scotland between 2008 and 2014. The aim of this study is to describe the change in socioeconomic inequalities in BMI between 2008 and 2018.

Methods Data from the annual cross-sectional Scottish Health Survey between 2008 and 2018 for those aged 16–64 years were analysed. Survey years were grouped into 2008–2011, 2012–2015, 2016–2018 and the data were modelled using logistic modelling for outcome ‘obese/less than obese’ and linear modelling for outcome BMI, for males and females, adjusting for 10 year age group, survey year groupings and SIMD quintile. An interaction terms between year and SIMD and year and age were included. Slope Index of Inequality (SII) and Relative Index of Inequality (RII) were calculated. Data were also analysed by 10-year age group.

Results Average BMI and prevalence of obesity increased from 27.2 and 24.9% in 2008 to 27.5 and 25.9% in 2018 among males, and from 27.3 and 26.5% in 2008 to 27.6 and 29.0% in 2018 among females. When broken down by age group, the majority of this increase is observed among females aged 25–34 years and 45–54 years. When the data were modelled, socioeconomic inequalities in BMI were observed for both males (eg average BMI was 0.48 (0.23, 0.73) more in SIMD1, most deprived, compared with SIMD 5) and females (eg OR of being obese in SIMD1 was 1.45 (1.28, 1.63) of SIMD5). There was a significant rise in BMI and obesity prevalence over time. An interaction term between survey year and SIMD, however showed that this rise was not observed in the two most affluent SIMD quintiles and the increase for the three less affluent SIMD quintiles rose in an approximately stepwise fashion, suggesting a widening of inequalities, eg for outcome obese/not obese, adjusting for age, sex, year and SIMD, an interaction term between SIMD and survey year showed an OR of obesity in 2008 in SMID1 of 1.62 (1.42, 1.85) that of SIMD5, but for each additional year, OR rose by 1.03 (1.003, 1.05) for SIMD1 relative to SIMD5. Both RII and SII saw a large increase between 2012–15 and 2016–18 for males and females.

Conclusion Socioeconomic inequalities in BMI have previously thought to have plateaued, however this study shows that inequalities are now increasing.