Inequalities in body mass index, diet and physical activity in the UK: Longitudinal evidence across childhood and adolescence

Abstract

We use longitudinal data across a key developmental period, spanning much of childhood and adolescence (age 5 to 17, years 2006-2018) from the UK Millennium Cohort Study, a nationally representative study with an initial sample of just over 19,000. We first examine the extent to which inequalities in overweight, obesity, BMI and body fat over this period are consistent with the evolution of inequalities in health behaviours, including exercise and healthy diet markers (i.e., skipping breakfast) (n = 7,220). We next study the links between SES, health behaviours and adiposity (BMI, body fat), using rich models that account for the influence of a range of unobserved factors that are fixed over time. In this way, we improve on existing estimates measuring the relationship between SES and health behaviours on the one hand and adiposity on the other. The advantage of the individual fixed effects models is that they exploit within-individual changes over time to help mitigate biases due to unobserved fixed characteristics (n = 6,883). We observe stark income inequalities in BMI and body fat in childhood (age 5), which have further widened by age 17. Inequalities in obesity, physical activity, and skipping breakfast are observed to widen from age 7 onwards. Ordinary Least Square estimates reveal the previously documented SES gradient in adiposity, which is reduced slightly once health behaviours including breakfast consumption and physical activity are accounted for. The main substantive change in estimates comes from the fixed effects specification. Here we observe mixed findings on the SES associations, with a positive association between income and adiposity and a negative association with wealth. The role of health behaviours is attenuated but they remain important, particularly for body fat.

Keywords: Adolescence; BMI, Body Mass Index; Body mass index; Childhood; Diet; FE, Fixed Effects; MCS, Millennium Cohort Study; OLS, Ordinary Least Squares; Obesity; Physical activity; SES, Socioeconomic status; Socioeconomic inequalities; United Kingdom.

Fig. 1

z-BMI across childhood to adolescence by permanent income quintile. Notes: Lines shows the estimated z-BMI and widths of the shaded area are 95% CIs at each age, estimated with multilevel general linear regression models (Table S11 in Supplementary data). z-BMI = body mass index z-scores. Income quintiles characterised by permanent income during childhood based on the average equivalised weekly net family income recorded in the first three sweeps, from 9 months to 5 years.

Fig. 2

Body fat (%) across childhood to adolescence by permanent income quintile. Notes: Lines shows the estimated body fat percentage and widths of the shaded area are 95% CIs at each age, estimated with multilevel general linear regression models (Table S11 in Supplementary data). Income quintiles characterised by permanent income during childhood based on the average equivalised weekly net family income recorded in the first three sweeps, from 9 months to 5 years.

Fig. 3

Eats breakfast every day of week across childhood to adolescence by permanent income quintile. Notes: Lines shows the estimated proportion of participants who eats breakfast every day of week and widths of the shaded area are 95% CIs at each age, estimated with multilevel general linear regression models (Table S12 in Supplementary data). Income quintiles characterised by permanent income during childhood based on the average equivalised weekly net family income recorded in the first three sweeps, from 9 months to 5 years.

Fig. 4

Weekly physical activity (5 + days) across childhood to adolescence by permanent income quintile. Notes: Lines shows the estimated proportion of participants who report 5 or more days of weekly physical activity and widths of the shaded area are 95% CIs at each age, estimated with multilevel general linear regression models (Table S12 in Supplementary data). Income quintiles characterised by permanent income during childhood based on the average equivalised weekly net family income recorded in the first three sweeps, from 9 months to 5 years.