a a Department of Public Health Sciences, King's College London, Capital House, 42 Weston St, London SE1 3QD, UK. E-mail: martin.gulliford{at}kcl.ac.uk
b Nutrition and Metabolism Division, Ministry of Health, Trinidad and Tobago.
Abstract
Background To evaluate distribution of body mass index (BMI) and subcutaneous fat in children of African or Indian subcontinent descent living in Trinidad and Tobago.
Methods A cross-sectional survey was carried out in a nationally representative sample of 66 government schools. Measurements were made of children's heights, weights, triceps and subscapular skinfold thicknesses. Data were analysed by sex and ethnic group and comparison was made with international standards for overweight and obesity and with British (1990) reference curves for BMI.
Results Data were analysed for 5688/6731 (85%) eligible children including 1934 Afro-Trinidadian, 1689 Indo-Trinidadian, and 1794 of mixed ethnicity. Afro-Trinidadian and mixed children were taller, but Indo-Trinidadian children were shorter than reference. Values for BMI were lower than reference: mean standard deviation score (SDS), (95% confidence interval) Afro-Trinidadian 0.34 (0.39 to 0.28), Indo-Trinidadian 1.32 (1.39 to 1.25), mixed 0.67 (0.74 to 0.61). Overall 486 (8.5%, 7.8 to 9.3%) of children were overweight and 138 (2.4%, 2.0 to 2.9%) were obese. Triceps skinfold thickness values were lower than reference (0.45, 0.48 to 0.42 SDS) but subscapular skinfold thicknesses were higher (0.45, 0.43 to 0.47 SDS). Higher BMI were associated with higher BMI in the child's parents, higher reported birthweight, older age of the child's mother, smaller family size, and with higher maternal educational attainment.
Conclusions Overweight was prevalent and distribution of subcutaneous fat was central. Work is needed to determine whether these findings are associated with adult patterns of fat distribution and metabolic abnormalities.
Keywords Overweight, obesity, children, subcapular skinfold thickness, triceps skinfold thickness, ethnic groups, Trinidad and Tobago, West Indies
Accepted 3 May 2001
Obesity in adults contributes to morbidity, mortality and health care costs.13 Obesity in children is a cause for concern because it may predict adult obesity and increased risk of coronary heart disease in adult life.4 In high-income countries the prevalence of obesity is increasing both in adults and children.5,6 The problem is now being recognized and addressed by international organizations and national governments.7,8 In many middle-income countries, a high prevalence of diabetes9 and hypertension10 has been noted in association with obesity, and non-communicable diseases now account for the majority of deaths.11 A more long-standing concern for middle-income countries has been with under-nutrition in children, rather than over-nutrition. In the Caribbean region in the early 1980s, malnutrition in pre-school children less than 4 years old was still important.12 However, in a recent paper Martorell et al.13 concluded that although obesity in pre-school children was not a problem for most low-income countries, in some middle-income countries the prevalence of obesity was as high as in the US.
Given this background the assessment of body mass index (BMI) and body fat distribution in relation to under- and over-weight in child populations of middle-income countries is important. Evaluation of previous studies is complicated by a lack of standard definitions for obesity in children. Recently, Cole et al.14 proposed that overweight and obesity could be defined for children of different ages by identifying the centile which was equivalent to a BMI of 25 or 30 kg/m2 at age 18 years. Cole et al. included data from surveys in Brazil, Great Britain, Hong Kong, Netherlands, Singapore and the US. They acknowledged the lack of data from Africa and Asia (including the Indian subcontinent) and pointed out the need to evaluate distributions of BMI in other samples including more children of African or Asian origin. The present study reports data for children of African or Indian subcontinent descent who were living in the same geographical environment.
Trinidad and Tobago is a middle-income country with a reported per capita GNP of approximately US$4520 in 1998.15 The population is partly of African descent (about 40% of total) and partly of Indian subcontinent descent (also about 40% total) with the majority of the remainder (18%) being of mixed ethnicity.16 In the present study we report data from a cross-sectional survey of a nationally representative sample of children in Trinidad and Tobago. Our immediate aim was to evaluate the distribution of height and BMI in this population with particular reference to over- or under-nutrition. We also evaluated skinfold thicknesses as measures of body fat distribution. Finally we explored whether environmental factors were associated with overweight in this population.
Methods
Subjects
We carried out a cross-sectional survey of school children in Trinidad and Tobago. There are 468 primary schools in Trinidad and Tobago, 433 in Trinidad and 35 in Tobago. The sample of 66 schools was drawn by stratifying the nation into health administrative areas and randomly selecting schools with probability proportional to size. The sample of schools was drawn by the Central Statistical Office for an earlier survey carried out in 1989. The same sample was used for this survey because the geographical distribution of children in that survey corresponded closely to the distribution observed in the 1990 census. Fieldwork was carried out in the first 6 months of 1999, using the methods of the British National Study of Health and Growth. Within each school we measured all children in the first year classes and in the classes for children aged 89 years.
Measurements
Measurements were made of children's heights, weights, and triceps and subscapular skinfold thicknesses. Height was measured on a Holtain stadiometer using the method described by Tanner et al.17 Height was measured to the last 0.1 cm and 0.05 cm was added to correct the bias. Children were weighed in underpants with weight recorded to the last complete 100g using electronic digital scales. Triceps skinfold thickness was measured as recommended by Tanner and Whitehouse18 except the measuring point was marked with the arm hanging straight and not bent. Fieldwork was carried out by the nutritionists and food demonstrators from the Nutrition Division of the Ministry of Health. Staff were trained thoroughly in measurement techniques before the start of the study, but measurement error was not specifically quantified.
Questionnaires
The parents of each child were asked to complete a questionnaire. Where necessary the questionnaire was interview-administered by a class teacher or a fieldworker. For the present analyses the following items were used. Parents were asked How would you describe this child's ethnic group? Responses were classified into the categories, African, Indian, white, Chinese, mixed, other or not known. This represented a shortened form of the categorization used in the Trinidad and Tobago national census.16 For analysis the categories were further reduced to Afro-Trinidadian, Indo-Trinidadian, Mixed, and Other and not known. The self-reported heights and weights of the child's natural mother and father were used to estimate their BMI. The child's reported birthweight was divided into quartiles, after conversion to kilogrammes where necessary. The number of children in the family was included in analyses as a categorical variable. The mother's education was divided into the categories primary, secondary, technical, university, other and not known. The mother's employment was divided into the categories housewife, paid employment, not in paid employment, other and not known. The mother's age at the child's birth was calculated from the mother's year of birth and the child's age. The degree of overcrowding in the home was calculated by dividing the number of people by the number of rooms (excluding the bathroom and kitchen, if the latter was only used for cooking and washing). Both of these variables were divided into quartiles for analysis. Other variables including the birth order, paternal education and employment, whether grandparents were living in the home, whether the parents were living in the home, and the type of drinking water supply in the home were analysed using the categories shown in Table 2.
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In order to analyse data for children from more than one age group it was necessary to standardize measurements for age and gender. As measurements increase in their variation with age, as well as mean value, a standard deviation score (SDS) was calculated for each measurement. The SDS for height is given by the difference between the child's measurement and the mean for a child of the same age and gender from a reference population divided by the standard deviation for that age and sex in the reference population. For weight and BMI, which have positively skewed distributions, a Normalizing transformation was used. Data from British children were used for reference. The height and weight SDS19 and BMI SDS20 were calculated from the British 1990 growth references as recommended. Data for white children from the 1990 survey of the National Study of Health and Growth (NSHG) constituted the majority of the data for ages 511 years.20 No concurrent UK reference curves for skinfold thickness were produced because of the lack of data from studies other than the NSHG. The SDS were therefore calculated directly from NSHG 1990 data for English white children. Reference curves for triceps and subscapular skinfold thickness were obtained using the method of Cole.21 By definition each SDS had a Normal distribution mean 0.0 and standard deviation 1.0 for the English white 1990 population. In boys, one weight SDS is equivalent to about 1.9 kg at 5 years and 4.5 kg at 11 years.
Extreme values for SDS (>10 SDS or <10 SDS) were examined and SDS and associated measurements set to missing where these appeared impossible. Eight items of data were excluded in this way. A conservative approach was taken to allow the possibility that some extreme departures from the UK reference data would occur. Children were divided into two age groups, <7 years and 7 years, in order to examine the hypotheses in relation to age. The proportion of children who were overweight or obese was estimated using the method suggested by Cole et al.14 This method uses age and sex specific cut-points which are equivalent to a BMI of 25 or 30, respectively, at age 18 years. Random effects models (with school as the random effect) were used to estimate associations between BMI or subscapular skinfold thickness and explanatory variables. Models were fitted using the xtreg command in Stata with the maximum likelihood option.22
Results
There were 6731 children eligible for inclusion in the study and measurements were obtained for 6405 (95%). Children were excluded if they included missing values for height, weight, skinfold thicknesses or reported ethnicity. There were then 5688 (85%) children with data available for further analysis. These included 1934 Afro-Trinidadian children, 1689 Indo-Trinidadian children, 1794 of mixed ethnicity and 271 with other or not known ethnic group. In the younger children the mean age was 5.8 years (range 4.386.99 years) in the older children the mean age was 8.6 years (range 7.0010.44 years).
Tables 1 and 2 show the characteristics of the children in the sample according to ethnicity. The distribution of reported birthweight was lower in the Indo-Trinidadian children than in other groups. The distribution of maternal BMI was also lower for the Indo-Trinidadian children. Educational attainment tended to be slightly higher for the Afro-Trinidadian parents than for Indo-Trinidadians. Afro-Trinidadian children, and to a lesser extent those of mixed ethnicity, were less likely to have their father living with them at home. However, Indo-Trinidadian mothers were less likely to be in paid employment.
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Table 6 shows the results of regression analyses with BMI or subscapular skinfold thickness as dependent variable. The analyses show that ethnic differences in BMI in children were not explained by measured environmental factors. BMI was significantly higher in the children aged 89 years compared with children aged 56 years. Children were significantly heavier if their mother or father had higher BMI, or if they were heavier at birth. BMI decreased as the number of children in the family increased. In additional analyses there was no independent effect of birth order after adjusting for sibship size. BMI was higher in children born to older mothers. There was a weak positive association of BMI with increasing maternal education. Subscapular skinfold thickness measurements were similar in the ethnic groups and in both age groups but were higher in boys than girls. Subscapular skinfold thicknesses were associated with the BMI of the parents, and were inversely associated with the sibship size. There was no consistent association with birthweight except for those children categorized as not known birthweight. Subscapular skinfold thickness was associated with older age of the mother at the child's birth, but there was no association between subscapular skinfold thickness and maternal education.
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Table 7 shows coefficients for other explanatory variables and BMI or subscapular skinfold thickness SDS after adjusting for the model described above. It can be seen that neither BMI nor subscapular skinfold thickness were associated with paternal education or employment, maternal employment, the number of parents or grandparents in the home, nor the type of water supply available to the home. Both BMI SDS and subscapular skinfold SDS were negatively associated with the degree of overcrowding (persons per room). There were no significant interactions between any of the explanatory variables and ethnicity. This suggested that associations were broadly similar across the ethnic groups included in the study.
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Our main aim in this study was to evaluate indicators of under- and over-nutrition in children in Trinidad and Tobago. Our results document wide variations height, weight, BMI and skinfold thicknesses in Trinidadian children but these were strongly associated with ethnicity. The validity of comparisons made between ethnic groups, and with external reference standards, needs to be considered before evaluating the data.
Validity of BMI comparisons
BMI is more strongly associated with height in children than adults.23 Shorter children have lower BMI values, and tall children have higher BMI results.24 Some caution is there fore required when comparing groups with different height distributions at the same age, because this form of bias might exaggerate differences between ethnic groups.25 Children of African descent are taller than British children so their low BMI values suggest that they are truly leaner than their British counterparts. For Indian children BMI values may be biased to lower levels by their lower mean height. The prevalence of obesity could be overestimated in Afro-Trinidadians and underestimated in Indo-Trinidadians. The association between height and BMI is weaker in children under 5 years, and this bias will then be less important. Furthermore, BMI was strongly correlated with skinfold thicknesses in each group.
Maturation and obesity
We noted that obese children were considerably taller than their non-obese counterparts. This observation, which was first noted by Garn and Haskell26 and confirmed by Himes and Roche,27 may be explained by the earlier maturation of heavier children.26 This finding would explain why height and obesity are not independent in childhood and suggests that comparisons made within groups with the same height distribution at a given age, would be valid.25 This observation would also explain why the height dependency of body mass diminishes during adolescence.25
Body composition
In addition to differences in height, there are ethnic differences in body composition at given height and weight. People of African descent have greater bone and muscle mass at a given BMI,28 and use of BMI as a measure of adiposity may result in overestimation of the degree of body fatness. Conversely, in men of Indian subcontinent descent, body fat content is higher (and fat free mass is lower) than for African Americans or white Americans at any given level of BMI.29,30 If the same is true in children, then use of BMI would lead to underestimation of the degree of body fatness of Indo-Trinidadian children.
Main findings
Initially, we wanted to evaluate whether under-nutrition was a problem for children in Trinidad and Tobago. Since the proportion with height 2.0 SDS was low, it would not appear that growth restriction was an appreciable problem. When judged according to the proportion with BMI
2.0 SDS, Trinidadian children were lighter than reference and this was especially so in Indo-Trinidadians. By contrast, the proportion of children who were classified as overweight or obese according to the international standard was similar to that observed in England and Scotland14,31 but slightly lower in Indo-Trinidadians than Afro-Trinidadians. In English boys in 1994, 5.4% were overweight at 46 years and 9.0% at 78 years. In English girls, the equivalent figures were 10.9% and 12.5%.31 Overall, 1.7% of English boys, and 2.6% of English girls were obese at ages 411 years in 1994.31 The proportion classified as obese was broadly similar in this sample, although the proportion classified as overweight was slightly lower. However, the age distributions of the two samples differed.
Our analyses confirmed that higher BMI was associated with the child having a higher birthweight, parents with higher BMI, an older mother, or a smaller sibship size. These associations have been noted in previous studies.32,33 Although we have not conducted a validation study, these parental self-reports may be inaccurate and the associations shown were likely to be underestimated. There was a weak association of BMI with higher levels of education in the mother, as has been noted in some but not all populations.13,32,33 There was also evidence for an association of household overcrowding with lower BMI and subscapular skinfold thickness measurements. Taken together these associations did not explain ethnic differences in BMI. It is unlikely that a difference in material conditions accounted for the observed anthropometric differences between ethnic groups. In Trinidad and Tobago, there are cultural differences between the ethnic groups but measures of affluence act as stratifying factors within each group.34 Ethnic differences of similar magnitude have also been reported among ethnic minority children living in Britain.23,35
Pomerlau et al.36 analysed BMI in European, Afro-Caribbean and South Asian women. They found that South Asians had lower BMI than Afro-Caribbeans but both groups had higher BMI than Europeans. This suggests that a marked transition in body fatness occurs between child and adult life in people of Indian subcontinent descent. Most studies have associated adult obesity with higher BMI in childhood and with higher birthweight.32,33 However, it has been proposed that under-nutrition and thinness in early life may predispose to adult obesity, in particular a higher waist circumference.37
Skinfold thicknesses
In addition to a high prevalence of obesity in relation to the low mean value for BMI, Trinidadian children also showed high values for subscapular skinfold thickness. Given the low values for triceps skinfold thickness, the results suggest a different profile for body fat patterning when compared with the reference data. This more central distribution of body fat could have metabolic implications.38 In data from the Bogalusa Heart Study, Freedman et al.39 reported that higher subscapular skinfold thickness was associated with higher low density lipoprotein cholesterol, triacyl glycerol and insulin concentrations. These differences were independent of height and weight. However, other studies have given more equivocal results, with an effect of body fat distribution not entirely independent of body fatness.40
Conclusions
Anthropometric measurements of children in Trinidad and Tobago showed large differences from reference standards derived from European populations. There were also large differences between the two main ethnic groups in Trinidad and Tobago with children of mixed ethnicity giving intermediate values. These differences made it difficult to assess children's nutritional status. Low mean values for BMI gave a high proportion of children who were underweight as judged by British reference data, but the prevalence of overweight and obesity according to the international standard was broadly similar to that seen in Britain. Skinfold thickness measurements suggested a central pattern of body fat distribution which was seen in each ethnic group. It will therefore be important to elucidate whether body fat distribution in childhood is related to fat distribution in adulthood, and with the development of features of the metabolic syndrome.
KEY MESSAGES
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Acknowledgments
The authors thank the Chief Medical Officer of Trinidad and Tobago for permission to report this work. They also thank the Principal Medical Officer (Community Services), the Ministry of Education, and the staff, pupils and parents of the participating schools for their support of the survey. Special thanks are due to the staff of the Nutrition Division for their skill and dedication in working on the survey.
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