Objectives:

To examine the impact of overweight on mean, high normal and high blood pressure in early adolescence, and how this relates to ethnicity and socio-economic status.

Design:

Cross-sectional study with anthropometric and blood pressure measurements.

Setting:

A total of 51 secondary schools in London.

Sample:

A total of 6407 subjects, 11–13 years of age, including 1204 White UK, 698 Other Whites, 911 Black Caribbeans, 1065 black Africans, 477 Indians and 611 Pakistanis/Bangladeshis.

Main outcome measures:

Mean, high normal (gender, age and height-percentile-specific 90–94th percentile) and high (95th percentile) blood pressure.

Results:

Based on the International Obesity Task Force age-specific thresholds, 19% of boys and 23% of girls were overweight, and 8% of each were obese. Overweight and obesity were associated with large increases in the prevalence of high normal and high blood pressures compared with those not overweight. The increases in the prevalence of high systolic pressure associated with overweight were as follows: boys, odds ratio 2.50 (95% confidence intervals 1.73–3.60) and girls 3.39 (2.36–4.85). Corresponding figures for obesity were: boys 4.31 (2.82–6.61) and girls 5.68 (3.61–8.95). Compared with their White British peers, obesity was associated with larger effects on blood pressure measures only among Indians, despite more overweight and obesity among black Caribbean girls and overweight among Black African girls. The effect of socio-economic status was inconsistent.

Conclusions:

The tendency to high blood pressure among adult Black African origin populations was not evident at these ages. These results suggest that the rise in obesity in adolescence portends a rise in early onset of cardiovascular disease across ethnic groups, with Indians appearing to be more vulnerable.

Introduction

The increasing prevalence of obesity in childhood signals the potential increase in the early onset of cardiovascular disease. The prevalence of obesity in young people has increased dramatically,^{1, 2, 3, 4} almost tripled in England¹ and the United States of America⁴ over the past 2 decades. In the United States, overweight children have been shown to be 2–4 times more likely to have elevated blood pressure or 'hypertension' than non-overweight children.^{5, 6, 7} Similar to US findings, blood pressure in children is closely related to weight as well as height in British children,^{8, 9} but very little is known about the effect of obesity on the risk of elevated blood pressure, nor about its interaction with ethnicity and social disadvantage. The prevalence of hypertension and related disease is higher among adult African-origin populations in Britain than white Europeans,^{10, 11, 12} but the cause of this and when in the lifespan blood pressures begin to diverge remain unclear. By adulthood, body mass index (BMI), reflecting the weight component, is a powerful driver of blood pressure as demonstrated in the CARDIA longitudinal study of African and European young adults in the United States.¹³ There is no comparable longitudinal study in Britain, but mean BMI is known to be higher in adult African-origin women than in white European women and it is a significant contributor to their higher hypertensive risks.¹⁴ This paper examines the impact of overweight and obesity on high normal and high blood pressure in early adolescence and how this relates to ethnicity and socio-economic status, using the Medical Research Council Determinants of Adolescent Social well-being and Health study (DASH).¹⁵ Data on blood pressure distributions have been published, but the impact of socio-economic disadvantage and obesity was not examined.⁸

Method

The DASH study

The sample was recruited from 51 schools in 11 London boroughs, chosen because of high proportions of the main ethnic minority groups. Pupils from Years 7 and 8 (aged 11–13 years in 1st and 2nd years of secondary school) in randomly selected mixed ability classes were invited to take part. Approval was obtained from the Multi-centre Research Ethics Committee and local education authorities. Information packs were sent to parents via head teachers. Parents were provided with a form to be signed and returned to the school if they did not wish their child to participate. Active consent was required from pupils. The pupil response rate was 81%. Fieldworkers were trained for 5 days before data collection and re-certified at regular intervals. Further details about the study can be found at http://www.sphsu.mrc.ac.uk/studies/dash.

Risk factors, confounders and outcomes

Ethnicity of white UK, Black Caribbean, Black African, Indian, Pakistani and Bangladeshi origin was determined by consistent background of parents and grandparents; self-reported ethnicity of the pupil was used in conjunction with at least one parent with the same ethnicity and at least three grandparents born in home countries. We focus here mainly on African and South Asian origin groups. The Bangladeshi group (n=219) was too small to examine separately. This group was combined with Pakistanis, as in adulthood both of these groups have a higher cardiovascular risk and are more economically disadvantaged than Indians.^{16, 17} There were many other ethnicities, but the sample sizes were too small for detailed analyses. These have been aggregated to either 'Mixed' ethnicities (mainly of white and black Caribbean ethnicity) or 'Other White' (mainly Eastern Europeans, Turks, Kurds, Greeks, Portuguese and Irish).

After careful observer training and revalidation throughout fieldwork, standardized measures were taken of height using portable stadiometers and weight using Salter electronic scales. The protocols for these measures were taken from those used for the Health Survey for England and Wales,¹⁸ and from the World Health Organization manual.¹⁹ Standing height was measured without shoes, in the Frankfort horizontal plane and at full stretch, to the nearest 0.1 cm. Body weight was measured to the nearest 0.1 kg. BMI was derived as weight (kg)/height (m)². These measures were converted to Z-scores, expressed as standard deviations (s.d.), based on the 1990 British growth reference curves,²⁰ and pupils were classified as not overweight, overweight (not obese) or obese, based on the International Obesity Task Force age-specific thresholds.²¹ The International Task Force criteria identify BMI values for each age associated with a predicted BMI of 25 or 30 at the age of 18 years. Blood pressures were taken using validated OMRON M5-I instruments and appropriately sized cuffs, the first after the pupil had sat quietly for a timed 5 min, and the second and third at least 1 min after the previous reading. The average of the second and third readings was used in analysis. Room temperature was recorded. Pupils were classified as pre-pubertal (Tanner stage 1 for breasts or genitalia and pubic hair), early pubertal (Tanner stages 2 and 3 for breasts and genitalia) or late pubertal (Tanner stages 4 and 5) using the self-complete Tanner questionnaire.²² Gender, age and height-percentile-specific 90–94th (defined as high normal blood pressure) and 95th (high blood pressure) percentile levels were defined using US normative blood pressure tables for children and adolescents.²³ Access to 17 standard of living items was used, in quartiles, as a proxy measure of socio-economic status.

Statistical analysis

The distributions of systolic and diastolic pressures approximated to normality. Treatment for high blood pressure was not a confounding influence on the distributions, as no one reported treatment. The effects of age (6 monthly), gender, height, BMI, overweight status, pubertal stage, ethnicity and socio-economic disadvantage on mean blood pressure levels were examined using simple and multiple linear regression models, and on high normal and high blood pressure levels using simple and multiple logistic regression models. There were significant interactions between gender and obesity status, and gender and pubertal status. All multivariate analyses were therefore gender-specific, with adjustment for age and temperature.

Results

From a total sample of 6644, 237 (3.5%) pupils were excluded because of extreme values (22), or missing data on blood pressure (199), anthropometry (1), sex (7) or age (8). Exclusions did not vary significantly by ethnicity. Overall, for systolic pressure, the 90–94th percentile cutoff point on the US normative blood pressure tables for children and adolescents corresponded with 6.5% prevalence for boys and 9.6% for girls in the DASH sample, and 95th percentile with 2.7% for boys and 3.8% for girls; for diastolic pressure, the 90th percentile corresponded with 9.8% for boys and 10.6% for girls and the 95th percentile with 5.1% for boys and 3.7% for girls. In the entire sample, 19% of boys and 23% of girls were overweight, and 8% of each were obese. Table 1 shows the anthropometric and blood pressure data by ethnicity. Generally, Black Caribbeans and Black Africans tended to be taller and South Asians shorter than their White UK counterparts. Compared with White UK girls, overweight was more prevalent among Black Caribbean and African girls, and obesity among Black Caribbean girls. The prevalence of high normal diastolic pressure was higher among Indian girls. There were no other significant differences in the prevalence of high normal or high blood pressures between ethnic groups.

Table 1 - Mean height, weight, BMI and blood pressure, and percentage overweight, obese, high normal and high blood pressure, by ethnicity.

Full table

A graded increase in the prevalence of high normal and high systolic and diastolic pressure with overweight status was observed. Among boys, mean systolic pressures among those who were not overweight were 107.02 (s.e. 0.18) mm Hg, overweight but not obese 111.81 (0.38) mm Hg and obese 114.72 (0.61) mm Hg; mean diastolic pressures were 64.33 (0.14), 67.32 (0.29) and 71.29 (0.40) mm Hg, respectively. The corresponding figures for systolic pressures among girls were 106.69 (0.18), 111.20 (0.35) and 112.80 (0.68) mm Hg, and for diastolic pressures were 66.09 (0.15), 68.78 (0.26) and 72.13 (0.50) mm Hg. In terms of socio-economic circumstances, the proportions in the least advantaged fourth quartile varied significantly across the groups (White UK 20%, 95% CI 18–23; Black Caribbeans 24%, 22–27; Black Africans 31%, 28–33; Indians 17%, 14–20; Pakistani/Bangladeshi 26%, 23–30).

Comparison of the effect of overweight/obesity across ethnic groups

We examined the simultaneous effects of overweight status, height and pubertal stage on mean, high normal and high blood pressure, and then additionally of ethnicity and socio-economic status. In the partially adjusted models for systolic pressure, overweight was associated with 2- to 3-fold increase in the prevalence of high normal (boys odds ratio 2.36, 95% CI 1.81–3.09; girls 2.40, 1.84–3.13) and high (boys 2.51, 1.75–3.60; girls 3.31, 2.32–4.74) systolic pressure and obesity with a 3- to 6-fold increase in high normal (boys 3.49, 2.51–4.87; girls 3.34, 2.32–4.81) and high (boys 4.11, 2.70–6.26; girls 5.45, 3.48–8.54) systolic pressure.

Tables 2 (boys) and 3 (girls) show the findings of the fully adjusted models for blood pressure measures. The effects associated with overweight and obesity hardly changed on full adjustment, although some variations were observed by ethnic group and socio-economic status. Height remained an independent correlate for mean, high normal and high blood pressure among boys, but among girls this was evident only for mean blood pressure. Pubertal status was not a significant correlate. Ethnicity, however, was an independent correlate of the measures of blood pressure. Table 2 shows that Black African, Black Caribbean and Mixed ethnicity boys were more likely to have lower mean systolic pressures and were also less likely to have high normal systolic pressure than White UK boys. Black Caribbean and Black African boys were also less likely to have high systolic pressure. Black Caribbean boys had lower mean diastolic pressure, but no significant differences were observed for high normal or high diastolic pressure. Socio-economic status appeared to have a nonlinear relationship with high normal and high systolic pressure, and no significant association with diastolic pressure. Table 3 shows that among girls, mean systolic and diastolic pressures among Other White, Black Caribbeans, Black Africans and Pakistanis/Bangladeshis were lower than White UK girls. Only Black African girls, however, were less likely to have high normal or high systolic pressure. Indian girls were more likely than White UK girls to have a higher mean diastolic and also to have high normal diastolic pressure. As with boys, the effect of socio-economic status was inconsistent.

Table 2 - The effect of obesity and other factors on high normal (90–94th percentile) and high (95th) systolic and diastolic blood pressure in boys.

Full table

Table 3 - The effect of obesity and other factors on high normal (90–94th percentile) and high (95th) systolic and diastolic blood pressure in girls.

Full table

There were significant interactions between overweight status and ethnicity. We therefore stratified the models by overweight status to compare effects across ethnic groups (Table 4). Among boys who were not overweight, the patterns for mean systolic generally reflected those observed in Table 3, that is, lower means for Black Caribbean, Black African and Mixed groups than for White UK boys. High normal systolic pressure was less common among Black African boys who were not overweight and also among those who were overweight. Obesity had a greater effect among Indian boys. Obese Indian boys were more likely to have higher mean, high normal and high systolic pressures than White UK obese boys. Among girls who were not overweight, Black Africans had lower mean and high systolic than White UK girls, but no significant differences were observed among those overweight or obese. Overweight was associated with a greater effect on mean and high systolic pressure among Indian girls. For diastolic pressure (data not shown), among those not overweight or obese, mean diastolic was generally lower among ethnic minorities, significant (P<0.05) for Black Caribbean boys and girls, and Black African girls. For those who were overweight or obese, the only significant difference was for Indian girls. Overweight was associated with a greater effect among Indian girls than White UK girls for mean (coefficient 4.38, 95% CI 1.97–6.79), high normal (odds ratio 4.78, 95% CI 2.03–11.27) and high (odds ratio 5.55, 95% CI 1.27–24.29) diastolic pressure.

Table 4 - Mean, high normal (90–94th percentile) and high (95th) systolic blood pressure by ethnicity and overweight status.

Full table

Effect of overweight/obesity within ethnic groups

In the tables above, relative differences between minorities and the UK groups have been presented. Table 5 shows the effect sizes associated with overweight within each of the minority groups for mean, high normal and high systolic pressure. In fully adjusted models for each group, substantial effects were observed within each group. The effects were not significant for White Other boys and girls and Pakistani/Bangladeshi boys for high normal systolic; and for White Other boys and girls, Black Caribbean and Pakistani/Bangladeshi boys for high systolic pressure.

Table 5 - Effect of overweight/obesity (combined) on mean, high normal (90–94th percentile) and high (95th) systolic blood pressure within each ethnic group.

Full table

Effect of continuous measure of BMI

'Overweight' and 'obesity' neglect the continuous relationship between adiposity and blood pressure. All models were re-run with BMI (s.d.) as a continuous percentile variable. In fully adjusted models, BMI (s.d.) remained a significant correlate for increases in mean systolic (boys: coefficient 0.06, 95% CI 0.04–0.07; girls 0.06, 0.05–0.07), and for the prevalence of high normal (odds ratio 1.01, 95% CI 1.01–1.02; girls: 1.02, 1.01–1.02) and high systolic pressure (boys: 1.02, 95% CI 1.01–1.02; girls: 1.02, 95% CI 1.01–1.03). The patterns for ethnicity, gender and socio-economic status remained similar to those reported above.

Discussion

This is the first known study in the United Kingdom to examine the effect of obesity on high blood pressure in adolescence in a large multi-ethnic sample. Children require age- and height-specific ranges of normal blood pressure because of the rise of blood pressure with growth. There are no published standard tables for Britain, and we used the recently updated US population-based reference values for detecting hypertension in children.²³ Arguably, these values may not accurately represent normative British population-based profiles, but the general correspondence between the 90th and 95th percentile cutoffs and 10% and 5% prevalence in our sample suggest fairly similar age- and height-specific blood pressure distributions for Britain. The discrepancy was greatest for systolic pressures among boys with slightly lower prevalence than expected. Ideally, to detect 'hypertension', blood pressure should be measured on three separate occasions. Sorof et al.⁷ found that the prevalence of hypertension declined on subsequent measurements but, as with studies based on single sets of measurements, the effect size associated with obesity was generally the same. A recent study suggested increased variability among overweight or obese youths than among those of normal overweight.²⁴ The clinical importance of this finding in terms of future cardiovascular risk is unclear, but the implications are that the ambulatory blood pressure measures would be useful, as some individuals with potential 'hypertension' could be missed.

The prevalence of overweight or obese children aged 10–15 years in the Health Survey for England is 23%, lower than the 30% reported here for children aged 11–13 years.^{25, 26} There are few studies with which to compare age- and ethnic-group- specific prevalence of overweight in adolescence in the United Kingdom, but generally Black African origin girls appear to be at greater risk,^{25, 26, 27} although in one study there were no ethnic differences in obesity.²⁸ Defining obesity during childhood and adolescence is complicated because of the variability in growth rates and in body composition that occur at different maturation stages. An alternative method is the use of 85–95th BMI percentiles to detect overweight, and above the 95th percentile for obesity.²⁹ Using this definition, more pupils were classified as obese (23.1% for boys and 21.6% for girls) but the pattern of ethnic differences in obesity remained the same; the effect of obesity on high systolic pressure was broadly similar (associated with 3- to 6-fold increase in high systolic pressure) and the protective effect of Black Caribbean and Black African ethnicity among boys was also consistent.

BMI may be a less reliable measure of fat mass for some ethnic groups. For the same unit of BMI, African Americans appear to carry less fat³⁰ and South Asians appear to carry more fat³¹ than White Caucasians. This may have led to an overestimation of obesity among Africans and an underestimation among South Asians. Stage of maturation is known to influence body composition and, more rapid growth and earlier pubertal development have been observed for obese subjects.³² Black Caribbean and Black African girls were more likely to be in late puberty compared with White UK girls.⁸ In contrast to the position for African-American girls,^{33, 34} differing rates of stage of maturation did not appear to play an important role here. In stratified analyses by pubertal stage among girls, Black Caribbean and Black African ethnicity was nonsignificant in both early and late puberty for high normal and high systolic pressure. Mean blood pressures of Black African girls have been shown to be actually lower than White UK girls in late puberty.⁸ Boys appeared to find the Tanner questionnaire a less comfortable experience than girls, even though the questionnaire was completed in an enclosed private area after a nurse had explained what was required. With over 20% missing data for the boys, it was difficult to assess the role of stage of maturation. Generally, accuracy among those who did report could be expected as after completion of the forms, random checks were done with pupils from different age and ethnic groups to ensure that they had understood the questions.

Mean blood pressure levels vary by socio-economic status. In contrast to consistently negative correlations (higher socio-economic status associated with lower blood pressure) among adults in industrialized countries, findings vary in developing countries.³⁵ Findings for children are generally inconsistent across studies,³⁵ but there is evidence to suggest an indirect relationship, as childhood socio-economic conditions may affect growth velocity, which in turn influences later blood pressure.^{36, 37} In the DASH sample, the relationship with socio-economic status was inconsistent. We cannot discount that this may be related to the measure of socio-economic circumstances. Measuring self-reported socio-economic status among children is problematic. Other work has shown that multidimensional indices of socio-economic status may be more relevant as a measure of inequality among ethnic minorities, given the disruption of occupational careers and general life chances on migration. The distribution of the standard of living measures by ethnicity was as expected from national surveys, with all groups except Indians significantly less likely to be in the most-advantaged top quartile than White UK. Reported paternal economic activity and family type provided some verification of our measure of socio-economic status, as children with a working parent (17%) or in a two-parent household (15%) were more likely to be in the most-advantaged top quartile of the standard of living index than children with an unemployed parent (7%) or in a lone-parent household (8%). Neither parental economic activity nor family structure was consistently related to the blood pressure measurements.

The relationship between obesity and blood pressure is well established in adults.^{38, 39} Since obesity in childhood is likely to continue into adulthood,⁴⁰ obese children are particularly at risk of becoming hypertensive as adults. The possibility that adult hypertension may begin during childhood has been suggested by a number of studies showing that there is a degree of consistency of peer rank order or tracking of blood pressure over the life course,⁴¹ but there are few longitudinal data on blood pressure in ethnic groups. The US Bogalusa study is an exceptional source and recent findings suggested that the key determinant of African-White differences in adolescent blood pressure was a combination of birthweight and growth trajectories.⁴² The follow-up of the DASH cohort, recently completed, will provide novel longitudinal information about the relationship between relative weight change, growth velocities and blood pressure among British ethnic minority adolescents.

Conclusion

Obesity was significantly associated with a higher prevalence of high blood pressure at ages 11–13 years. Indians were less likely to be overweight or obese than whites, but the effect of excess weight appeared to be greater. Despite more excess weight among Black Caribbean and Black African girls, and more socio-economic disadvantage, they were no more prone to high blood pressure than Whites UK. On a more general note, as with the US studies,^{7, 43} our results suggest that the trend of decreasing cardiovascular disease in adults over the past 50 years may reverse if these children become obese adults. Blood pressure reductions in adolescence can be achieved by weight loss through reducing excessive energy intake⁴⁴ and increasing physical activity strategies,⁴⁵ although the long-term pathogenic effects of obesity on blood pressure, such as altered vascular structure, remain uncertain.

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