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Application of the 2006 WHO growth standard from birth to 4 years to Pacific Island children

Abstract

Background:

Pacific adults and children have very high rates of obesity.

Objective:

Body size at birth, 2- and 4 years for the whole cohort, compliant (not smoking and breastfed) and non-compliant children was compared to the 2006 World Health Organization (WHO) growth standard using z-scores.

Design:

Longitudinal data (n=659) from the 2000 Pacific Island family birth cohort study of weight at birth and weight and height at 2- and 4 years was analysed.

Results:

At birth the average child was 3.673±0.501 kg; z-score 0.605 units higher (P<0.000001) than the WHO standard. At 2- and 4 years, average z-scores for weight and body mass index (BMI) were significantly different from 0 (mean z-scores for weight +1.062 and +1.688, for BMI +1.701 and +1.969 respectively, P<0.000001). Mean height was significantly lower (P<0.000001) z-score=−0.232 at 2 years but higher (P<0.000001) at 4 years, z-score=+0.626. Over 4 years, the daily weight gain was 11.2 g day–1 compared with 8.9 g day–1 for the WHO child. Compliant (n=287) compared to non-compliant (n=372) weighed less and were slightly shorter at 2- and 4 years measurement points. Non-compliant children categorized as non-smoking (n=229) weighed more than smoking mothers' (n=143) children at birth, 2- and 4 years.

Conclusion:

Pacific children are born heavy, over 4 years increase in weight is faster and between 2- and 4 years increase in height is faster than the reference breastfed child, independent of pre- and postnatal factors. Smoking decreases the rate of weight gain and children who are not breastfed gain weight faster. Interventions to limit weight gain need to start with the family before conception.

Introduction

New Zealand has the highest population of Polynesian people from the south Pacific in one country and most of them live in Auckland. Specifically, people originating from Samoa, Tonga, Cook Islands, Niue and Tokelau make up 6.5% of the New Zealand population.1 In 2002–2003 in a national survey of health,2 more than 80% of Pacific adults in New Zealand were classified on the basis of ethnic-specific BMIs as overweight or obese and in the 2002 national children's nutrition survey of children aged 5–14 years,3 more than 60% of Pacific children were classified as overweight or obese. The burden of chronic disease associated with obesity4 in this population group is also very high.2 What is most concerning is that the prevalence of excess weight increases with age in a population and the level of overweight (more with higher BMIs) increases at a rate faster than the prevalence.5 Growth patterns in utero and early childhood are related to genetics and the environment.6 Environmental exposures known to assert effects on growth trajectories include smoking,7, 8 behaviour of the mother during pregnancy, gestation and birth weight, and the duration of breastfeeding.9

Recently published WHO child growth standards10 provide a unique standard for comparison of growth trajectories for healthy children breastfed by their non-smoking mothers. A feature of this resource is that the modelling provided allows z-scores by sex and the actual age of children to be determined. The selection of children for the WHO standard was compliance with infant-feeding, which included exclusive or predominant breastfeeding for at least 4 months and partial breastfeeding up to 12 months, no maternal smoking before or after delivery and single term birth. This standard was derived from 1737 children from Brazil, Ghana, India, Norway, Oman and the USA. Ethnic groupings within a country and determination of involvement of children of Pacific Island ethnicity were not part of the WHO analysis (personal communication, Onyango, WHO).

The Pacific Island family study is a cohort studied from birth (2000) to increase knowledge about the health, psychosocial and behavioural characteristics of Pacific people with young children resident in New Zealand. The question is raised whether the WHO growth standards apply to this Pacific population—a primary health focus of the New Zealand government. We used longitudinal data from this study to compare the growth trajectory and body size of Pacific children with the WHO standard. We applied the WHO approach to assess the growth and development of children of Pacific Island origin.

Subjects and methods

Initial study design and method have been reported elsewhere.11 Briefly, 1398 mothers of a cohort of live Pacific Islands infants born at Middlemore Hospital, South Auckland, between 15 March and 17 December 2000 were recruited. An infant was deemed eligible if at least one of the parents identified himself or herself as being of Pacific Island ethnicity and was a permanent resident of New Zealand. This sample represents between a quarter and one-third of all the eligible children born in this area of the highest density of the Pacific population of New Zealand (Manukau City, 2001 Census1). Body weight to the nearest grams in the new born; weight to the nearest 0.1 kg and standing height to the nearest 0.1 cm were measured at 2- and 4 years using standardized equipment and procedures and with trained operators. BMI was calculated as weight in kilograms divided by height in meter square. Breastfeeding practice was recorded at 6 weeks after the birth. At age of 4 years, body fatness of 531 children was estimated from hand-to-foot 50 kHz bioimpedance measurements using a predictor equation developed in a similar population.12 All procedures and interview protocols had ethical approval from the National Ethics Committee.

Seven hundred and thirty-four cohort members had all required measurements of birth weight, breastfeeding at 6 weeks, weight and height at 2- and 4 years. Exclusion of a total of 75 children on criteria of either low birth weight (<2500 g, n=31) and/or baby not home within 6 weeks of birth, (n=7) and/or not a twin pregnancy (n=19) and/or known diabetes (n=23) in the mother. Some had two or more reasons to be excluded. A total of 659 children; 309 girls and 350 boys; 323 Samoan, 111 Cook Island, 133 Tongan, 30 Niuean, 62 other; met the criteria.

Note, drinking alcohol during pregnancy was true for a small number of women (28/659) but these were included as the WHO compliance criteria did not specify alcohol consumption and only five in the compliant group consumed alcohol during pregnancy and no effect was determined.

The next step was to split the sample into compliant and non-compliant with the WHO criteria as at 6 weeks. Compliance, that is both not smoking during pregnancy (516) and breast or mixed feeding (332) at 6 weeks was met by 287 which left 372 in the non-compliant category.

Statistics

Results are presented as means±s.d. z-scores were calculated using the modelling formulae provided by the WHO10 for weight-for-age (wfa), BMI-for-age (bmifa), height-for-age (hfa) and weight-for- height (wfh). Body measurements, derived variables (like average weight gain) and calculated z-scores were compared using t-test for pairs of groups namely: (i) compliant and non-compliant; (ii) non-compliant smoking and non-compliant non-smoking; (iii) compliant and non-compliant non-smoking and (iv) compliant grouped by sex. Variances were compared for the same set of groups. Data were analysed using SPSS software, version 13 (SPSS Inc, Chicago, IL) and R (http://www.R-project.org). P-values <0.05 were considered significant.

Results

Characteristics of the cohort at birth, 2- and 4-year measurement points are shown in Table 1. At birth, the average child was 3.673±0.501 kg; z-score 0.605 units higher (P<0.000001) than the WHO standard. At 2- and 4 years weight and BMI were also above the standard and average z-scores were significantly (P<0.0001) non-zero. Mean z-scores for weight +1.06 and +1.69, for BMI 1.70 and 1.97 respectively. Mean height was significantly lower than the WHO standard (at 2 years, the average zhfa=−0.24 (P<0.001)) but significantly higher at 4 years (the average zhfa=0.63 (P<0.001)). Over 4 years, the daily weight gain was 11.2 g day–1 compared with 8.9 g day–1 for boys and 8.8 g day–1 for girls for the WHO children.

Table 1 Weight, height and BMI at birth, 2- and 4-year measurement points

Compliant (n=287) compared to non-compliant (n=372) weighed less and were slightly shorter at 2- and 4-years measurement points Table 2. Variance of all the variables tended to be less in the compliant group compared with the non-compliant indicating better homogeneity. Mothers who smoked (22%) were less likely to breastfeed; that is, 16% of mothers of breastfed babies smoked and 26% of those who did not breastfeed smoked (χ2 P<0.00085).

Table 2 Size and growth characteristics of compliant and non-compliant children

The compliant groups were further analysed for differences between male (n=143) and female (n=144). Boys were slightly heavier, taller and had a higher BMI than girls at all measurement points. Weight gain in boys over the 4-year period was 11.3±2.3 g day–1 compared to girls which was 10.62.0 g day–1 (P<0.016). The z-scores by sex were also significantly different at all points for example wfa z-score at 4 years for girls was 1.40±1.16 and that for boys was 1.75±1.17 (P<0.017 for sex difference). The sex differences were relatively small given the large z-score difference to the WHO standard so for reasons of presenting an overall picture boys and girls were combined for the comparison of effects on growth trajectories of the influences of breastfeeding and smoking. Body fatness showed a tendency to be higher in girls 21.7±5.1% than boys 19.7±4.8%, P=0.09. The Fat percentage was slightly higher (0.5%) in non-compliant children and smokers but the difference was not statistically significant.

Figure 1 demonstrates the differences in z-score of this study population from the WHO standard at each age point. All the subgroups were significantly different from the standard but at 4 years, the breastfed children whose mothers did not smoke had the lowest weight (P=0.02 compliant vs all non-compliant). The pattern of weight gain in children is the same as that in the WHO standard with a higher rate of growth in the first 2 years. (0–2 years is greater than 2–4 years) (Table 2.)

Figure 1
figure1

z-score deviation from WHO standard of weight for age for compliant (breastfed and not smoking during pregnancy), and non-compliant (not smoking or smoking) non-compliant. Mean and s.e. are shown. At each age the group marked with * was significantly different compared to the other two groups P<0.05.

Discussion

Body size and growth trajectory of Pacific children, compliant and non-compliant are markedly different from the 2006 WHO growth standard.10 The WHO standard differs from the centres for disease control (CDC) 2000 and the national centre for health statistics (NCHS) growth charts in both the populations used and the method of construction with the biggest differences in infancy. From 0–8 months, the NCHS and CDC z-scores at lower birth weights (−3 z-score) are less than the new WHO standard10 and at 5 years the previous z-scores were higher for the very heavy (+3 z-score) children—otherwise the growth charts are similar. Therefore the data from Pacific children would be interpreted as being slightly more overweight at birth than with the present analysis and less overweight at 5 years. Comparison with CDC and NCHS (US children, mixed ethnicity) would still lead to the finding that the mean weight of the population is more than 1 z-score higher at 2- and 4 years. Weight is markedly greater at all measurement points but height at 2 years is less than the standard and greater at 4 years. The same difference between height and weight is seen in the New Zealand health survey data2 that show within an age group, Pacific adults are not taller than Europeans but they are on average heavier at all ages by at least 12 kg. If the children in this study show the same growth pattern and are not as tall as adults then the early rapid growth demonstrated in this analysis means that these children are at particularly high risk for adult obesity13, 14 and for associated chronic disease.15

It is recognized that for the same height and weight, Pacific adults16, 17 and girls12 have more lean mass and less fat mass than Europeans. The mean fatness of this Pacific cohort at 4 years is above the 90th percentile of fat reference curves for 5 years British European children18 and for the study population in general (but not for individuals) conclude that the level of fatness is high at 4 years. We also note that we do not have the information to know about other important growth mediators such as exposure to smoke in the home after birth.

As previously published for other ethnic groups, we are able to show that for Pacific children that a lower birth weight19 and rapid growth20 is related to smoking and that smoking is prevalent in this population.8 We are also able to show in this cohort that the ‘catch-up’ growth rate of babies whose mothers smoke exceeds breastfed babies but does not exceed non-breastfed babies.

At present child growth velocity standards are not available from the WHO, that is the rate of growth at each age rather than cross-sectional data at each point; our data are only able to provide a coarse snapshot of growth velocity at 2-years measurement points. Therefore we do not know when for example the maximum velocity of growth for each child was within each 2-year period or when the periods of ‘catch-up’ growth, if any, mainly occurred. The relatively short hfa at 2 years is worthy of note but we cannot provide an explanation. We do know that the rate of weight gain in g day–1 is faster in the first 2 years, 13 g day–1 compared with 8 g day–1 in the next 2 years. On average, Pacific children are heavier at birth, heavier at 2 and even heavier at 4 years. The ‘thrifty genotype’ hypothesis21 that a diabetogenic gene or genes persist at a high level in a population will favour a survival advantage in times of nutritional deprivation but at times of a positive imbalance in nutrition is harmful22 and has been thought to be a possible driver of obesity in this population. With globalization and migration Pacific people now have a plentiful supply of a diet with an excess of energy, simple carbohydrates and saturated fats and a reduction in physical activity. Both factors may therefore cause the previously favourable metabolic profile seen in ‘survivors’ to become a handicap, which results in obesity and type 2 diabetes23 which are very high in this population;2 the profile is increasing and occurring at younger ages. In common with the rest of the world, the risks for this population are likely to accumulate with each adverse factor24 and generation.25

One of the major limitations of this analysis is that we do not know the glycaemic status of the mothers during pregnancy. This group often present late in pregnancy and are not necessarily assessed for gestational diabetes mellitus (GDM). It has been shown by Simmons and Brier26 that the birth weight of Polynesian babies whose mothers (n=55) were known not to have GDM was 3590±500 g. More recently Simmons15 presents evidence that in 916 glucose tolerant Pacific women increased maternal weight and glycaemia are associated with increased size of babies (3630±530 g). The birth weight of the compliant children is comparable to their group at 3.685±494 g.

We have attempted to overcome limitations of the long periods between measurements by identifying those children who met the WHO criteria at 6 weeks, that is breastfed and mother not smoking; compliant and non-compliant and tracking their growth. The non-compliant group has been further divided into smoking and non smoking and clear differences are shown. This study therefore presents strong evidence for the growth trajectories of Pacific Island children and gives insights into the rate of body weight gain and early accumulation of risk associated with rapid weight gain for chronic disease in this population, that is before birth and tracking into an early childhood. Given the increasing obesity in the Pacific and international community and the known intergenerational effects27 of the fetal environment, the data presented here show the other side of the relationship between birth weight and risk for childhood and therefore adult obesity. Further studies are needed to better define the risk factors but the management of weight and screening for hyperglycaemia during pregnancy are immediate concerns.

The major conclusion/recommendation is that to be effective, interventions to limit the weight gain should start with the family before conception. In this population, interventions targeted at the environment of pre-school children should be initiated. The question is also raised as to how the WHO growth standards should be applied in practice. Pacific population groups lead the world in the prevalence of obesity and the deviation of this group from the WHO standard is markedly different even in compliant children. The steep growth trajectory of this young, vulnerable population demands urgent, informed and targeted action to improve the prognosis and reduce the accumulation of risk across the life course.

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Acknowledgements

We gratefully acknowledge the families who participated in the study as well as other members of the research team. In addition, we wish to express our thanks to the PIF advisory board for their guidance and support. ER and JP were involved in the study design, data collection and data analysis. VO led the data analysis and KN was involved in the study protocols and data interpretation. All authors were involved in writing the manuscript. None of the authors had any conflict of interest.

The Pacific Island Family Study is funded by grants from the Foundation for Research, Science & Technology, the Health Research Council of New Zealand, the Child Health Foundation and the Maurice & Phyllis Paykel Trust.

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Rush, E., Paterson, J., Obolonkin, V. et al. Application of the 2006 WHO growth standard from birth to 4 years to Pacific Island children. Int J Obes 32, 567–572 (2008). https://doi.org/10.1038/sj.ijo.0803751

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Keywords

  • early environment
  • Pacific children
  • growth standard
  • weight gain

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