Socioeconomic inequalities in childhood and adolescent body-mass index, weight, and height from 1953 to 2015: an analysis of four longitudinal, observational, British birth cohort studies

Background Socioeconomic inequalities in childhood body mass index (BMI) have been repeatedly documented in high income countries, yet there is uncertainty regarding how they have changed across time, how inequalities in the composite parts of BMI have changed (weight and height), and whether inequalities differ in magnitude across the outcome distributions. We investigated socioeconomic inequalities in childhood/adolescent weight, height, and BMI from 1953 to 2015 using British birth cohorts born in 1946, 1958, 1970, and 2001. Methods Associations between childhood social class and anthropometric outcomes at age 7, 10/11 and 14/16 years were examined to assess socioeconomic inequalities in each cohort using gender-adjusted linear regression models. Multilevel models were used to examine if these inequalities widened or narrowed from childhood to adolescence; quantile regression was used to examine whether the magnitude of inequalities differed across the outcome distribution. Findings Lower social class was associated with lower childhood/adolescent weight in earlier-born cohorts (1946-1970), yet with higher weight in the 2001 cohort. Lower social class was associated with shorter height in all cohorts, yet the absolute magnitude of this difference narrowed across generations. There was little inequality in childhood BMI in the 1946–1970 cohorts, yet inequalities were present in the 2001 cohort, and in all cohorts at 14/16 years (p<0.05 age x social class interactions). BMI and weight inequalities were larger in the 2001 cohort and systematically larger at higher quantiles—eg, in the 2001 cohort at 11 years there was a 0.98kg/m2 difference (0.63, 1.33) in median BMI (lowest to highest social class), yet 2.54kg/m2 (1.85, 3.22) difference at the 90th BMI percentile. Interpretation In the later 20th and early 21st centuries, socioeconomic inequalities in weight reversed, those in height narrowed, while inequalities in BMI and obesity emerged and widened. These drastic changes highlight the powerful impact of societal changes on child-adolescent growth and the insufficiency of previous policies in preventing obesity and its socioeconomic inequality. New and effective policies are required to reduce BMI inequalities in current and future children and adolescents. Without effective interventions, it is anticipated these inequalities will widen further throughout adulthood

Difference in mean zBMI by childhood physical abuse^† from fully adjusted models, males and females*.

<p>Footnotes: ^† participant report in adulthood (45y) that they had been physically abused by a parent during their childhood before 16y, i.e. punched, kicked or hit or beaten with an object, or needed medical treatment. *Difference in mean zBMI by childhood physical abuse estimated from fully adjusted models; showing equivalent differences in BMI (kg/m²) at 7y, 33y and 45y. The positive linear association of zBMI gain with age and physical abuse is given as ~0.006/y (males) and ~0.007/y (females) in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119985#pone.0119985.t004" target="_blank">Table 4</a>.</p

Supplementary information for Socio-economic disparities in child-to-adolescent growth trajectories in China: Findings from the China Health and Nutrition Survey 1991-2015

Supplementary information files for article Socio-economic disparities in child-to-adolescent growth trajectories in China: Findings from the China Health and Nutrition Survey 1991-2015   Backgrounds: Socio-economic disparities in growth trajectories of children from low-/middle-income countries are poorly understood, especially those experiencing rapid economic growth. We investigated socio-economic disparities in child growth in recent decades in China. Methods: Using longitudinal data on 5,095 children/adolescents (7-18y) from the China Health and Nutrition Survey (1991-2015), we estimated mean height and BMI trajectories by socio-economic position (SEP) and sex for cohorts born in 1981-85, 1986-90, 1991-95, 1996-2000, using random-effects models. We estimated differences between high (urbanization index ≥median, household income per capita ≥median, parental education ≥high school, or occupational classes I-IV) and low SEP groups. Findings: Mean height and BMI trajectories have shifted upwards across cohorts. In all cohorts, growth trajectories for high SEP groups were above those for low SEP groups across SEP indicators. For height, socio-economic differences persisted across cohorts (e.g. 3.8cm and 2.9cm in earliest and latest cohorts by urbanization index for boys at 10y, and 3.6cm and 3.1cm respectively by household income). For BMI, trends were greater in high than low SEP groups, thus socio-economic differences increased across cohorts (e.g. 0.5 to 0.8kg/m2 by urbanization index, 0.4 to 1.1kg/m2 by household income for boys at 10y). Similar trends were found for stunting and overweight/obesity by SEP. There was no association between SEP indicators and thinness. Interpretation: Socio-economic disparities in physical growth persist among Chinese youth. Short stature was associated with lower SEP, but high BMI with higher SEP. Public health interventions should be tailored by SEP, in order to improve children’s growth while reducing overweight/obesity.</p

Characteristics of those with no childhood maltreatment† and those abused or neglected (%).

<p>Averaged across 10 imputed datasets †No neglect at 7y or 11y and no abuse</p><p>*p<0.05</p><p>** p<0.001 for each child maltreatment group vs non-maltreated</p><p># <1/w</p><p>Characteristics of those with no childhood maltreatment† and those abused or neglected (%).</p

Characteristics of the 1958 British birth cohort study.

<p>Table based on observed data, N varies due to missing data.</p><p>*Neglect at 7y and/or 11y (if one missing, other age used).</p><p>^† IQR = inter-quartile range</p><p>^‡ defined as BMI≥20.63 at 7y, 25.10 at 11y, 28.88kg/m² at 16y for males; ≥20.51, 25.42 and 29.4kg/m² respectively in females; in adulthood BMI≥30kg/m².</p><p>Characteristics of the 1958 British birth cohort study.</p

Mean differences in zBMI (95% CIs) at 7y and rate of change in zBMI (7–50y) by childhood maltreatment, estimated using multilevel models.

<p>†Mean difference in rate of change (i.e. additional rate of change associated with maltreatment) is represented by the coefficient for a linear age interaction term (and for 7y/11y neglect only it is a linear function of age: i.e. coefficient for interaction with age +2*(coefficient for interaction with age²)* age (where age is centred at 7y)</p><p>*A: adjusted for: social class at birth (or 7y if missing), identified from maternal reports, based on Registrar General’s classification of the father’s occupation: I&II (professional /managerial), IIINM (skilled non-manual), IIIM (skilled manual) and IV&V (semi-unskilled manual, including single-mother households), maternal smoking during pregnancy: smoking ≥1 cigarette/day after the 4th month of pregnancy recorded shortly after birth, mean parental zBMI: 1969 reported maternal and paternal BMI, standardised using internally derived standard deviation scores, mean parental z-BMI calculated as the average z-BMI of both parents (where missing, either mother or father zBMI was used), 7y amenities: having no access or sharing amenities (bathroom, indoor lavatory, and hot water supply), 7y household overcrowding: defined as ≥1.5 persons/room, 7y housing tenure: owner-occupied, council rented, private rental or other, birthweight: measured in ounces and converted into grams, gestational age (in weeks) estimated from the date of the mothers’ last menstrual period, breastfeeding reported in 1965 by the mother, categorized as ‘never’ or ‘ever’ breastfed, 7y ill health identified from medical examiner’s report of major handicap or disfiguring condition.</p><p>** A+B: adjusted as for A above + pubertal timing from parental report at 16y for age of voice change for males (three groups < = 12, 13–14, > = 15y) and menarche for females (five groups < = 11 to > = 15y), time-varying concurrent employment (in paid employed, others) 23–50y; educational qualifications by 50y (five groups: none, some, O-levels, A-levels or degree level); time-varying concurrent smoking 23–50y (non-smoker/ex-smoker/smoker); time-varying concurrent leisure-time physical activity frequency 23–50y (<1 vs ≥1 /week) which identifies those at elevated risk of all-cause mortality [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119985#pone.0119985.ref044" target="_blank">44</a>,<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119985#pone.0119985.ref045" target="_blank">45</a>]; time-varying concurrent drinking 23–50y (males: non/infrequent drinker, 1–21, ≥22 units/week; females: non/infrequent drinker, 1–14, ≥15 units/week)</p><p>*** A+B+C: adjusted as above + time-varying depressive symptoms 23–50y (indicated by the 15 psychological items of the Malaise Inventory (8-items available at 50y were pro-rated to the 15 item scale used at other ages))</p><p>Mean differences in zBMI (95% CIs) at 7y and rate of change in zBMI (7–50y) by childhood maltreatment, estimated using multilevel models.</p

Difference in mean BMI (kg/m²) and OR for obesity (95% CIs) from 7 to 50y by physical abuse^† in males and females.

<p>Footnotes: ^† participant report in adulthood (45y) that they had been physically abused by a parent during their childhood before 16y, i.e. punched, kicked or hit or beaten with an object, or needed medical treatment.</p

Trajectories of the probability of overweight or obesity (versus normal weight) from sex- and study-stratified multilevel logistic regression models.

<p>NSHD: Medical Research Council National Survey of Health and Development, NCDS National Child Development Study, BCS: British Cohort Study, ALSPAC: Avon Longitudinal Study of Parents and Children, MCS: Millennium Cohort Study.</p

The 98^th, 91^st, and 50^th adulthood BMI centiles from sex- and study-stratified LMS models plotted against the normal cut-offs.

<p>BMI: Body Mass Index, LMS: Lambda-Mu-Sigma, NSHD: Medical Research Council National Survey of Health and Development, NCDS National Child Development Study, BCS: British Cohort Study.</p

The 98th, 91st, and 50th childhood BMI centiles from sex- and study-stratified LMS models plotted against the IOTF cut-offs.

<p>BMI: Body Mass Index, IOTF: International Obesity Task Force, LMS: Lambda-Mu-Sigma, NSHD: Medical Research Council National Survey of Health and Development, NCDS National Child Development Study, BCS: British Cohort Study, ALSPAC: Avon Longitudinal Study of Parents and Children, MCS: Millennium Cohort Study.</p