Background In many high-income countries, BMI and levels of overweight and obesity are inversely associated with socio-economic status. Recent evidence suggests that socio-economic disparities in BMI are emerging at a young age. Little is known whether patterns of these disparities vary by ethnicity, especially in the UK. This is the first UK study to our knowledge to examine the pattern of socio-economic disparities in child-to-adolescent BMI trajectories across ethnic groups.

Methods We used data from the UK Millennium Cohort Study, which oversampled children living in the disadvantaged circumstances and in England those from minority ethnic backgrounds. A total of 15,996 children with 62,051 BMI measurements between 3 and 14 years were included in this analysis. Mixed-effects fractional polynomial models were applied to estimate mean BMI trajectories for each socio-economic group (as defined by poverty and maternal education) and differences in BMI between groups at each age. Models were subsequently stratified by ethnicity and adjusted for maternal pre-pregnancy BMI, maternal smoking during pregnancy, birthweight and infant feeding.

Results Overall,the poverty group had a higher mean BMI than non-poverty group from 6 years with a small difference of 0.06 kg/m² [95% CI 0.01–0.12], which increased to 0.67 kg/m² [0.52–0.82] by 14 years. The income-BMI associations differed by ethnicity. In Whites and South Asians, the BMI difference by income was established at 3 years and widened with age. Among Black African-Caribbeans, there was a reverse socio-economic gradient in BMI, in that the poverty group had a lower BMI (-0.37 kg/m² [-0.04, -0.71] at 5 years; -0.95 kg/m² [-0.11, -1.79] at 14 years). Differences remained after adjustment for early-life factors. These distinct patterns persisted when using maternal education as the socio-economic indicator.

Conclusion These findings imply that socio-economic advantage may not necessarily be universally associated with lower BMI. The effect of socio-economic circumstances on BMI potentially differs by ethnic group. Given the increasing ethnic diversity in the UK, public health approaches to promote healthy weight need to consider the varying needs of target populations. The positive income-BMI association found in Black African-Caribbean children requires replication in other samples and further investigation into the underpinning cultural and biological mechanisms that may explain these differences.