Abstract
SummaryBackgroundInequalities in the trajectories of body composition in childhood and adolescence have been infrequently studied. Despite the importance of environmental factors in obesity development, little research has looked at area-level socioeconomic position, independent of family socioeconomic position. We aimed to assess how inequalities in body composition develop with age.MethodsThe Millennium Cohort Study is a longitudinal study of 19 243 families who had a child born between 2000 and 2002 in the UK. Multilevel growth curve models were applied to examine change in fat mass index (FMI), fat free mass index (FFMI; using the Benn index), and fat mass to fat free mass ratio (FM:FFM), measured using Bioelectrical Impedance Analysis, from ages 7 years to 17 years by the Index of Multiple Deprivation (IMD) and household income at baseline.FindingsInequalities in FMI and FM:FFM ratio are evident at age 7 years and widen with age. At age 17 years, adolescents in the most disadvantaged IMD group had FMI 0·57 kg/mB (B=Benn parameter; 95% CI 0·43 to 0·70) higher and FM:FFM ratio 0·037 (95% CI 0·026 to 0·047) higher compared with the most advantaged group. Disadvantaged socioeconomic position is associated with higher FFMI but is reversed in adolescence after adjustment for FMI. Inequalities were greater in girls at age 7 years (mean FMI 0·22 kg/mB; 95% CI 0·13 to 0·32) compared with boys of the same age (0·05 kg/mB; –0·04 to 0·15, p=0·3), but widen fastest in boys, especially for FMI, in which there was over an 11 times increase in the inequality from age 7 years of 0·05kg/mB (95% CI –0·04 to 0·15) to 0·62 kg/mB at 17 years (0·42 to 0·82). Inequalities for the IMD were similar to income, and persisted at age 17 years independent of family socioeconomic position.InterpretationChildhood and adolescence is an important period to address inequalities in body composition, as they emerge and widen. Policies should consider FFM as well as FM, and inequalities in the environment.FundingMedical Research Council, Economic and Social Research Council.
Highlights
Systematic reviews have shown consistent associations between disadvantaged socioeconomic position (SEP) and higher body-mass index (BMI) among children in highincome countries, including the UK.[1,2] Socioeconomic position is an umbrella term that captures social circumstances and can be measured by a number of indicators such as income, education, occupation, and area-level deprivation
Added value of this study This study adds value by looking at the trajectories of three measures of body composition over 10 years across childhood and adolescence in a nationally representative sample in the UK, enabling an investigation of how inequalities in body composition develop with age
We show that the associations between socioeconomic position and fat free mass index (FFMI) differ to associations observed among studies using percentage or raw measures, indicating that inequalities in height might be an important contributing factor to inequalities in fat free mass (FFM)
Summary
Systematic reviews have shown consistent associations between disadvantaged socioeconomic position (SEP) and higher body-mass index (BMI) among children in highincome countries, including the UK.[1,2] Socioeconomic position is an umbrella term that captures social circumstances and can be measured by a number of indicators such as income, education, occupation, and area-level deprivation. Understanding the unequal burden of obesity according to SEP is important, as public health interventions aimed at reducing obesity that do not take into account inequalities could inadvertently increase them.[3] there is clear evidence in relation to inequalities in BMI, less research has investigated inequalities in body composition, which better represents health risk because central fat mass (FM),[4] and total fat free mass (FFM)[5] are associated with cardiometabolic health. Our previous systematic review found evidence that disadvantaged SEP was associated with greater FM and lower FFM in high-income countries, but found few studies indexing measures to height[11] and highlighted a paucity of studies using area-level measures of SEP
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