Antecedentes en los 10 municipios participantes en el proyecto Hábitat

contributing to ethnic disparities in

adiposity in primary school children in

Coventry

Chapter outline

This chapter aims to address the lack of existing evidence to understand the basis of ethnic group inequalities in childhood adiposity, as identified in the systematic review of the literature described in Chapter 2. The chapter is made up of three sections. The first is the analysis of local cross-sectional child measurement data for Coventry to ascertain the nature of any variation in BMI across ethnic groups for children aged 4- 5 years and 10-11 years. This analysis also explores the potential influence of individual, school and neighbourhood factors upon BMI using a multilevel modelling approach. The second section repeats this approach using odds of overweight and obesity as the outcome of interest, in order to help contextualise the extent of any ethnic variation in BMI. The third section provides an exploration of some of the key considerations when measuring ethnic group variation in childhood BMI and overweight and obesity and how these influence interpretation of the findings, including anthropometry (height); body composition (body fat) and diagnostic criteria for classifying child weight status.

Background

Existing cross-sectional and longitudinal analyses highlight ethnic variation in childhood obesity in the UK, with Black ethnic groups exhibiting a consistently high prevalence of overweight and obesity,

particularly Black African children, whilst Chinese children generally show a low prevalence of overweight and obesity (Health and Social Care Information Centre, 2014b; Karlsen et al., 2014a; Sproston & Mindell, 2006; Zilanawala et al., 2015). Patterns observed in other ethnic groups are less consistent across data sets, but suggest a possible increased risk in South Asian children also, especially in later childhood (Health and Social Care Information Centre, 2014b). The data also suggest variation across ethnic groups in the effect of sex, height, deprivation and time (year of measurement) upon BMI and prevalence of childhood obesity (Dinsdale et al., 2014; El-Sayed et al., 2011; Karlsen et al., 2014a).

Although researchers have widely acknowledged that the determinants of childhood obesity act and interact within a socioecological model with multiple levels of influence (Butland et al., 2007; Davison & Birch, 2001; Harrison et al., 2011b; Sallis et al., 2008), until recently, studies have tended to explore the effect of determinants across these levels upon weight status at the individual level only. Such an approach fails to account for aggregate variability in health outcomes and can result in group level correlations being inappropriately used to make inferences at the individual level (Duncan et al., 1996).

More recently, studies exploring the determinants of childhood obesity in UK populations have begun to explore and account for aggregate variability at the school, neighbourhood and regional levels using multilevel analysis (Pallan et al., 2014; Townsend et al., 2012; Williams

et al., 2015). Although these studies explored and accounted for the role

of ecological contexts within their analyses, with ethnicity incorporated as a variable for a variety of purposes, the studies did not explore potential differential effects of covariates across ethnic groups. This study therefore builds on the existing literature by aiming to explore variation in BMI across ethnic groups, and the influence of potential explanatory factors, within the context of their school and neighbourhood, using a multilevel approach.

An additional consideration when exploring ethnic variation in child adiposity is the influence of ethnic differences in anthropometry and body composition upon metrics of child adiposity. For example, BMI (weight in kg/height in m2_{) may not fully adjust for the influence of}

height upon weight in children, so may systematically overestimate the degree of adiposity in tall children (Cole, 1986; Freedman et al., 2003). Ethnic group differences in height may therefore account for some of the observed ethnic variation in child BMI and weight status. Some studies have found that weight-for-height measures underestimate adiposity in South Asian children and overestimate adiposity in Black children on the basis of body composition (Daniels et al., 1997; Haroun et al., 2010; National Obesity Observatory, 2009; Shaw et al., 2007; Wang, 2002). Hudda et al. (2017a) have recently produced a set of adjusted BMI values for children from South Asian and Black African backgrounds, based on direct measures of body fat, derived from the pooled analyses of four recent UK studies. Adjustments for the differential influence of height and body composition upon BMI may therefore overcome ethnic-specific diagnostic issues in identifying adiposity in UK child populations.

As detailed in section 1.3.4.1, child weight status is based on comparisons to reference populations. Classification of children into weight status categories can be based on centile cut-offs derived from these reference populations, or can be related to a BMI value at a given age, intended to be indicative of adult disease risk. The ethnic composition of the reference population upon which comparisons are made is an important consideration when exploring ethnic variation on adiposity. For example, the UK90 reference population is based entirely on a White British sample (see section 1.3.4.1). In addition, the use of cut-offs based on centiles versus absolute BMI values may also result in a difference in the proportion of children classified as overweight or obese.

An additional aim of this study therefore was to assess the influence of making adjustments to the metrics used for assessing child adiposity in the current analyses upon the patterns of ethnic variation in child BMI and weight status observed.