ABSTRACT:

Objective:

To describe the methodological features of a study on the association between restricted intrauterine growth and prevalence of overweight, obesity and hypertension in school aged children.

Methods:

The study was conducted in two stages in two public schools in Niterói (RJ), from June through December 2010. All students aged 6 to 14 years were eligible to participate. The first stage consisted of an interview to collect information on demographic variables, diet and other variables. A sample was selected for the second stage, in order to conduct an equivalent of a case-cohort study. There was an interval of about 15 days between the two stages. Cases were overweight students, defined as a Z-score for BMI/age/sex > +1.00 in the first stage. Controls were selected by using a random schedule in which the sample frame was the whole cohort. Bioelectrical impedance analysis, carotid ultrasound to measure intimal-medial thickness, blood measurements and interviews were obtained. Gestational age and weight at birth were used to define proxy variables of restricted intrauterine growth. Early health information was obtained from medical registers.

Results:

The study participation was 76.4% (n = 795) out of 1,040 eligible to participate). 85.1% of parent's questionnaires were returned. 62.5% of the eligible children participated in the case-control study (case: control ratio = 1:1.8). Early life health information was obtained from 292 children.

Conclusion:

The present study has the potential to provide important information about multiple outcomes and exposures related to restricted intrauterine growth and metabolic abnormalities.

Keywords:
Obesity; Fetal growth retardation; Metabolic diseases; Child; Adolescent; Birth weight.

INTRODUCTION

The increasing prevalence of obesity worldwide has required continuous efforts of control and prevention because, in addition to the set of cardiovascular risk factors such as systemic arterial hypertension (SAH), dyslipidemia, and insulin resistance, it increases morbidity and mortality rates of the population¹1. World Health Organization (WHO). Population-based prevention strategies for childhood obesity: report of a WHO forum and technical meeting. Geneva: WHO; 2009. p. 15-7..

The hypothesis of fetal programming postulates that the environment found before birth or during childhood modulates physiological control and homeostasis that may result in increased susceptibility to chronic noncommunicable diseases (NCDs) throughout one's life²2. Barker DJP. Fetal Origins of Coronary Heart Disease. BMJ 1995; 311(15): 171-74.. Weight history in childhood is also associated with overweight and metabolic syndrome³3. Victora CG, Sibbritt D, Horta BL, Lima RC, Cole T, Wells J. Weight gain in childhood and body composition at 18 years of age in Brazilian males. Acta Pædiatr 2007; 96(2): 296-300.^,44. Nader PR, O'Brien M, Houts R, Bradley R, Belsky J, Crosnoe R, Friedman S, Mei Z, Susman EJ; National Institute of Child Health and Human Development Early Child Care Research Network. Identifying risk for obesity in early childhood. Pediatrics 2006; 118(3): e594-601.^,55. Lawlor DA, Smith GD, Clark H, Leon DA. The associations of birth weight, gestational age and childhood BMI with the type 2 diabetes: findings from the Aberdeen Children of the 1950s cohort. 2006. Disponível em: http://www.springerlink.com/content/v0u667xr5gn72434/. (Acesso em 21 de setembro de 2006).
http://www.springerlink.com/content/v0u6... ^,66. Bismarck-Nasr EM, Frutuoso MFP, Gamabardella AMD. Efeitos tardios do baixo peso ao nascer. Rev Bras Desenvolv Hum 2008; 1(18): 98-103., but with differences between populations. At the beginning of the century, the hypothesis of "economic phenotype"⁷7. Hales CN, Barker DJ, Clark PM, Cox LJ, Fall C, Osmond C, et al. Fetal and infant growth and impaired glucose tolerance at age 64. BMJ 1991; 303(6809): 1019-22. considered that malnourished children had to adapt in utero to save energy and, if they were to be exposed to an environment with a greater energy supply, they would be at a greater risk of developing obesity and other chronic diseases. To prevent these diseases, it is important to act at critical periods of life, so that it is possible to avoid the "programming" of future obese individuals and to prevent exposure to other risk factors that can be more harmful if the individual had intrauterine malnutrition⁸8. Victora CG, Barros FC, Horta BL, Martorell R. Short-term benefits of catch-up growth for small-for-gestational-age infants. Int J Epidemiol 2001; 30(6): 1325-30..

There are gaps in knowledge about fetal programming, especially in developing countries. In addition, postnatal growth and perinatal socioeconomic context must be analyzed when studying one's life course. The study "Association between birth weight and prevalence of overweight, obesity, and hypertension in school-age children in Niterói, RJ," based on the hypothesis of fetal programming, aimed to investigate associations between the proxies of intrauterine growth restriction (IUGR), the components of the metabolic syndrome, and the thickness of carotid arteries considering the growth and socioeconomic trajectories.

The main methodological aspects of this study are presented below.

METHODOLOGY

A survey on the nutritional status and blood pressure of students aging 6 to 14 years was conducted, and their perinatal information and data from their first 2 years of age were recovered. Overweight students, sorted as cases, and a sample of all participants in the survey were selected for the case-control study. Chart 1 lists the procedures in all steps.

First step: survey

The survey was conducted from June to December 2010. For the prevalence of overweight and obesity to be expected in 15% in the age group from 6 to 14 years, with a margin of error of 5%, and a confidence level of 99.9%, the estimated sample size was 544 students. Two municipal elementary schools, all located in areas with lower risk of violence and enrolled in the Family Health Program (PMF) at least for 14 years, were selected.

Heights and weights were measured, respectively, using a portable stadiometer with 0.5 cm proximity and a calibrated digital scale with maximum capacity for 150 kg and 100 g accuracy, using the standard techniques²⁵25. Gordon CC, Chumlea WC, Roche AF. Stature recumbent length and weight. In: Lohman TG, Roche AF, Martorell R. Anthropometric standardization reference manual. Champaign Illinois: Human Kinetics Books; 1988. p. 3-8.. Height was measured twice and the measurement would be taken again in case of difference greater than 0.5 cm. Based on Z-scores of body mass index (BMI: kg/height in m²⁾ for age and gender, calculated by WHOANTROPLUS program, students were classified as underweight (Z <-2.00), eutrophic (-2.00 ≤ Z ≤ +1.00), overweight (+1.01 ≤ Z ≤ +2.00), and obese (Z >+2.00)²⁶26. De Onis M, Onyango AW, Borghi E, Siyan A, Nishida C, Siekmann J. Development of a WHO growth reference for school-aged children and adolescents. Bull World Health Organ 2007; 85: 660-7..

Blood pressure was measured twice, with a 5-minute interval, using digital equipment. If the differences between measurements of systolic blood pressure (SBP) and/or diastolic blood pressure (DBP) was greater than or equal to 5 mmHg, a third measurement would be performed. In this situation, the measure with the less accurate value would be dismissed. The final blood pressure measurement was the value obtained by calculating the mean of both SBP and DBP measurements. BP measurements below the 90th percentile (P90) of SBP or DBP distributions were classified as normal, between P90 and P95 high normal, and P95 high above, according to gender and age¹⁶16. National High Blood Pressure Education Program Working Group on Hypertension Control in Children and Adolescents. Pediatrics. 2004; 114(S2): 555-76..

Self-filling questionnaires containing 125 questions about lifestyle habits were given to students of the age 10-14 years in their class schedule (Chart 1). Caregivers were given a self-filling questionnaire developed by the researchers, containing 43 questions about pregnancy, birth, and growth of their children (Chart 1), which could be completed at school or at home.

Second step: case-control study

Concomitantly to the survey, the case-control study sample was selected, as the control group was a random sample of participants, with a maximum interval of 15 days between the two steps.

Cases were students who were classified as overweight (Z >+1.00) in the survey. To determine prevalence ratios, the control group consisted of a random sample of students participating in the survey, including cases stratified by age (6 to 9/10 to 14) and school. Case control ratio observed was of 1:2. When the population forming the sample is the total study population, odds ratio results in the prevalence ratio, namely:

The chance in total study population - (a/c) ÷ [(a + b)/(c + d)] - was estimated by a random²⁷27. Szklo M. Population-based cohort studies. Epidemiol Rev 1998; 20: 81-90. sample, as shown in Chart 2.

Students were scheduled to attend meetings preferably accompanied by their mother, and they should fast for 12 hours, with physical inactivity, and, for girls, they should not be in their menstrual period. New anthropometric and BP measurements were taken without knowing those collected from the survey. Caregivers also had their weight and height measured. Two electric bioimpedance measurements were taken.

Blood samples were collected and processed on the Clinical Research Unit (UPC), and also stocked up as two plasma aliquots in freezer.

The thickness of intima and media layers of the carotid (in millimeters) performed bilaterally were calculated as the mean measurement in three locations: internal carotid artery, common carotid artery, and carotid bifurcation²⁴24. Lande MB, Carson NL, Roy J, Meagher C. Effects of childhood primary hypertension on carotid intima media thickness: a matched controlled study. Hypertension 2006; 48: 40-4..

Ultrasound diagnosis of hepatic steatosis was based on the presence of hyperechoic image of the hepatic parenchyma compared to the renal parenchyma in the medium axillary line²³23. Saverymuttu SH, Joseph AE, Maxwell JD. Ultrasound scanning in the detection of hepatic fibrosis and steatosis. Br J Med 1986; 292: 13-5..

Later, interviews were conducted separately, addressing lifestyle habits with both caregivers and students aged 10 to 14 years (Chart 1). The self-filling questionnaires and interviews took 40 minutes on an average.

The students participated in two meetings (one at the school and another at the UPC), and the caregivers attended only the meeting at the UPC.

Third step: secondary data

Information about pregnancy, childbirth, breast-feeding, food introduction, and growth until 2 years of age of the participants was obtained from medical records at the health units in Niterói and from the lists of the Information System Database on Live births (SINASC), from 1996 to 2004. At this stage, weight data from the first 2 years of life were obtained to evaluate the association between growth recovery (catch-up) of participants, who were small for gestational age (GA), and their nutritional status and metabolic markers in the current age.

Intrauterine malnutrition proxies

Data on birth weight (BW) and GA were obtained from different sources to ensure the quality of information.

When BW and accurate GA were available, IUGR was set as BW below the 10th percentile expected for GA, specific for each gender²⁸28. Adams MM, Alexander GR, Kirby RS. Birth Weight, Gestational Duration, and Fetal Growth. In: Adams MM, Alexander GR, Kirby RS, Wingate MS, editors. Perinatal Epidemiology for Public Health Practice. New York: Springer; 2009. p. 225-71.. When categorical GA was available, regardless of presentation of BW, the simplified definition of IUGR was used, restricted to non-premature: BW less than 2.500 g at 37 weeks or more of pregnancy²⁹29. Kiely JL, Brett KM, Yu S, Rowley DL. Low birth weight and intrauterine growth retardation. In: Wilcox LS, Marks HS, eds. From Data to Action: CDC's Public Health Surveillance for women, Infants and Children. Washington, DC: US Department of Health and Human Services; 1994. p. 185-2029..

Alternatively, BW controlled by GA (both continuous variables) may be used.

The choice of intrauterine malnutrition proxy depends on completeness and reliability of the information about BW and GA, the type of respective variables (categorical or continuous), and the study population chosen for the analysis, survey or case-control study.

Quality control

We conducted a pilot study in June and July 2010. No protocol changes were necessary or significant differences were found between the population of the pilot and the main studies, so we opted by adding the sample to the pilot study, thus increasing statistical power.

The field team was composed of health professionals trained, monitored, and evaluated regularly by leading researchers. The quality of anthropometric measurements was evaluated according to the protocol by Norton and Olds³⁰30. Norton K, Olds T. Antropométrica: um livro sobre medidas corporais para o esporte e cursos da área de saúde. Porto Alegre: Artmed; 2005. and through concordance analysis between measurements made in the first two steps. Imaging tests were performed according to the procedures of the study Metabolic Alterations Reggio Calabria Adolescents (M.A.RE.A.).

Data were processed by double input and stored in databases developed at the Census and Survey Processing System Program³¹31. Census and Survey Processing System - CSPRO, version 4.0. Washington: International Programs Center, U.S. Census Bureau; 2010.. Stata software, version 12.0, was used.

Ethical considerations

The study was approved by the Ethics Committee of Institute for Collective Health Studies (77-2008).

RESULTS

In total, 1,040 schoolchildren enrolled in the school for the year 2010 were eligible for the sample. The study included 795 children (76.4%) whose caregivers signed the informed consent form (Figure 1)(f2).

*Not selected for the second step.

Figure 1: Flowchart with number of participants in each step of the study "Association between birth weight and prevalence of overweight, obesity, and hypertension in schoolchildren from Niterói, RJ".

The questionnaire return rate sent to children's caregivers was 85.1% (n = 676).

Of the 581 schoolchildren selected for the case-control study, 62.5% (n = 363) went to the University Hospital Antonio Pedro, resulting in a case: control ratio of 1:1.8 (Figure 1)(f2).

Records of 55.8% (n = 444) of schoolchildren measured in schools were located, of which 65.7% (n = 292) had their information about the first years of life collected.

DISCUSSION

In studies on fetal programming, the long interval between exposure and outcomes in the course of life increases the likelihood of loss to follow-up and confounding problems. Another limitation is the lack of information about both exposure and potential confounding factors³²32. Matthew W. Gillman, Epidemiological challenges in studying the fetal origins of adult Disease. Paediatr Perinat Epidemiol 2005; 19(S1): 1., outlined in this study to obtain the data of perinatal and up to 2 years of age available in medical records, in addition to information obtained through caregivers.

Sample size is one of the limitations of this study, which reduces the statistical power of the associations evaluated. As this is a sample composed of two schools, their representativeness is possibly restricted, particularly as to prevalence estimates, but not necessarily as to measures of association²⁷27. Szklo M. Population-based cohort studies. Epidemiol Rev 1998; 20: 81-90..

Given the gap between the main data (IUGR) and outcomes (6 - 14 years), one must consider the possibility of reporting bias, particularly with regard to validity of information concerning the early life of students. Medical records and data recovery from SINASC, both promising strategies for studies about fetal programming, aim to mitigate such problems.

Over the years, SINASC has proven to be an important epidemiological tool to improve the coverage and quality of information they provide. Registration of BW in SINASC features high reliability when compared with medical records data (intraclass correlation coefficient = 0.97), being an excellent source to identify babies with low BW³³33. Theme Filha MM, da Gama SGN, da Cunha CB, Leal MC. Confiabilidade do Sistema de Informações sobre Nascidos Vivos Hospitalares no Município do Rio de Janeiro, 1999-2001. Cad Saúde Pública 2004; 20 (Suppl 1): S83-91.. This study used different sources of proxy IUGR variables, with possibility of using markers such as SGA, restricted or not to those born at term²⁹29. Kiely JL, Brett KM, Yu S, Rowley DL. Low birth weight and intrauterine growth retardation. In: Wilcox LS, Marks HS, eds. From Data to Action: CDC's Public Health Surveillance for women, Infants and Children. Washington, DC: US Department of Health and Human Services; 1994. p. 185-2029.. The choice of markers entails greater or lesser error of measurements of the main data, in view of the different stages of information collection, that is, at birth (SINASC) and at the current time. Using only information about BW is another option³⁴34. Chen W, Srinivasan SR, Hallman DM, Berenson GS. The relationship between birth weight and longitudinal changes of blood pressure is modulated by beta-adrenergic receptor genes: the Bogalusa Heart Study. J Biomed Biotechnol 2010: 543514.^.

Data such as maternal education at birth and at present and BW, postnatal growth in childhood, or adolescence allow assessing relevant socioeconomic and nutritional pathways in the course of their life.

The methodology in the selection of controls in the second step should be highlighted, configuring a sample of the population recruited in the survey, which allows the calculation of prevalence ratios rather than odds ratio prevalence²⁷27. Szklo M. Population-based cohort studies. Epidemiol Rev 1998; 20: 81-90.. It is known that the odds ratio estimated when the outcome has a high prevalence (as in the case of overweight) can lead to an incorrect interpretation of association of magnitude because of built-in bias³⁵35. Szklo M, Nieto FJ. Epidemiology beyond the basics. 3rd ed. Ontario: Jones & Bartlett Learning; 2014..

Storage of blood samples enables the evaluation of risk factors related to obesity and other NCDs not considered initially.

CONCLUSION

Facing the global scenario of high load of NCDs-related disorders, early prevention measures are necessary. This study is expected to provide a basis for further research with larger samples and/or other designs in which associations will be evaluated between subclinical atherosclerosis measures and IUGR with metabolic factors. In short, this study will allow analysis of multiple exposures and outcomes related to IUGR, fetal programming, metabolic disorders, and risk factors for NCDs in an age group that is studied less in developing countries.

References

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