Pressão arterial elevada e obesidade abdominal em adolescentes

OBJETIVO: Analisar a associacao entre obesidade abdominal e pressao arterial elevada em adolescentes da rede publica de ensino da regiao metropolitana da cidade de Londrina (PR). METODOS: Constituiu-se uma amostra de 656 adolescentes com idades entre dez e 13 anos. Foram realizadas analises antropometricas de massa corporal, estatura e circunferencia de cintura, alem da afericao da pressao arterial de repouso. Para analise e comparacao dos dados, foram aplicados Mann-Whitney e o teste t de Student. Tambem foram analisadas as possiveis associacoes entre pressao arterial e circunferencia de cintura por meio do teste do qui-quadrado. RESULTADOS: Foram identificadas associacoes entre obesidade abdominal e pressao arterial elevada em ambos os sexos (RP 2,7; IC95% 1,8-4,2). Alem disso, verificou-se que, independentemente do grupo etario, a obesidade abdominal associa-se com valores mais elevados de pressao arterial. CONCLUSOES: A obesidade abdominal esta associada a ocorrencia de aumento da pressao arterial em adolescentes.


Introduction
Some decades ago, infectious diseases were the leading cause of death worldwide.More recently however, chronic degenerative diseases have accounted for the majority of deaths in the population (1) .Several risk factors may be associated with these conditions, such as excess body fat, diabetes mellitus, inadequate dietary habits, insufficient physical activity, and hypertension (2) .
According to some studies (3) , the second most prevalent risk factor in adult populations is overweight, led only by sedentary lifestyle.Individuals with a body mass index (BMI) far above the normal range usually have higher blood pressure (BP) and higher rates of frank hypertension (4) .Abdominal adiposity poses a particular risk to health, as it is subject to higher rates of lipolysis and is located in close proximity to major blood vessels (5) .
This pathological association between body fat distribution and BP is especially important in pediatric populations, as anthropometric indicators of obesity, such as waist circumference (WC), are considered good alternatives in children and adolescents in view of their ease of measurement and status as strong predictors of all-cause and cardiovascular mortality (6) .On the other hand, there is little information on this topic in the Brazilian pediatric literature.
Therefore, the aim of this study was to evaluate the association between abdominal obesity and high blood pressure (HBP) in adolescent students of public schools in the metropolitan area of Londrina, in the northern region of the state of Paraná, Brazil.

Method
The sample comprised elementary school students of both genders, between the ages of 10 and 13 years, recruited from the pool of regularly enrolled students of public schools in the municipality of Londrina, state of Paraná, Brazil.For sample selection purposes, the city was divided into six regions: north, south, east, west, center, and outskirts.One school from each region was then selected by means of a random numbers table and the number of participants to be recruited from each school was calculated on the basis of the proportion of students required from each region.Had any of the chosen schools failed to meet the required proportion, one more school from the region would have been selected, but this procedure was not necessary.
Sample size was calculated for an expected HBP prevalence rate of 10% (according to data reported in the literature (7) , a tolerable error rate of 3%, and 80% statistical power.Thus estimated, the required sample size was 364 subjects.However, as the present study considered each class of each school as a cluster, we increased the projected sample size by 50% to correct for design effect, yielding a sample size of 573 adolescents.A further 20% was added to account for possible sampling loss during the course of the experiment, and the final minimum sample size was thus defined as 656.A total of 671 subjects were assessed in the study, thus meeting this requirement.It should be noted that the study sample was restricted to adolescents enrolled at the chosen schools who did not have any preexisting diagnosis of metabolic disease and were not taking antihypertensive agents for blood pressure control.
Prior to the study, all subjects and their parents or legal guardians were made aware of the objectives of the study and provided written informed consent for participation.The study was approved by the Universidade Estadual de Londrina Research Ethics Committee (accredited by the Brazilian Ministry of Health) and conducted in accordance with National Health Council Resolution 196/96 on human subject research.
All anthropometric measurements were obtained with participants wearing light clothing and no shoes.Body mass was measured using an electronic scale with 150 kg capacity and 100 g resolution, and height, with a portable stadiometer capable of measuring up to 200 cm and precise to 0.1 cm.The body mass index (BMI) was calculated by dividing body mass in kilograms by the square of the height in meters.Waist circumference (WC) was defined as the smallest circumference between the iliac crest and lowest rib, as measured using a nonstretching tape measure with mm increments.The cutoff points used for definition of abdominal obesity were those recommended by Taylor et al (8) .
Blood pressure was measured with an automatic oscillometric sphygmomanometer (Omron HEM-742) previously validated for use in adolescents (9) .Cuff size and placement followed American Heart Association recommendations (10) .Subjects were kept at rest in a seating position for 5 minutes prior to the first BP measurement.A second measurement was obtained 1 minute after the first, and the mean of these two measurements was used to determine systolic (SBP) and diastolic blood pressure (DBP).Participants were classified according to the National High Blood Pressure Education Program criteria (11) .Therefore, those with BP measurements above the 95 th percentile were classified as having HBP.
The Shapiro-Wilk test was used to assess the normality of data.Descriptive statistics were used for characterization of the sample (means, standard deviations, proportions, and frequencies), and the Student t test for independent samples was used to assess any possible gender differences in the variables of interest.The Mann-Whitney U was used for comparison between the mean BP values of subjects with and without abdominal obesity in each age range.The chi-square test was used to determine potential associations between BP and WC.The significance level was set at p<0.05.All statistical analyses were performed in the SPSS 17.0 software package.

Results
Table 1 shows the general characteristics of the study sample.Significant gender differences in mean WC and DBP values were found, with higher WC measurements in males and higher DBP in females.
An association between abdominal obesity and HBP was found in both genders (Table 2).The overall prevalence of HBP in subjects with abdominal obesity was 17.3 percentage points higher than in subjects with normal WC measurements.In girls, the difference was 19 percentage points, and in boys, 15.9 percentage points.The overall risk of HBP was 170% greater in the presence of abdominal obesity (197% greater in girls, 147% in boys).

Discussion
This study assessed the prevalence of, and association between, HBP and abdominal obesity in a sample of 10-to-13-year-old students of public schools in the municipality of Londrina, Paraná, Brazil.The overall prevalence of HBP was 12.2% (13.1% in boys and 11.5% in girls).Although the prevalence of HBP varies by country, the percentages found in this study are consistent to those reported elsewhere in the literature (12)(13)(14) , which have ranged from 1% to 22.8%.
A previous study of Brazilian children and adolescents (6) found a 1.7 percentage point reduction in the prevalence of hypertension when a second BP measurement was obtained 2 minutes after the first.In the present study, two standardized measurements were obtained at least 1 minute apart in an attempt to minimize this potential bias.Another important factor that should be taken into account is the use of different cutoffs for BP classification in different studies, leading to a lack of convergence in the results reported in the literature.
In addition to HBP, obesity has been considered one of the main risk factors for cardiovascular disease, particularly when there is a central distribution of adipose tissue.This is a worrisome finding, as the recent literature indicates that WC measurements have been increasing over the past decades in male and female adolescents alike (15) .Of the various anthropometric indicators available for classification of obesity, WC is considered a good option due to the ease of measurement and to the fact that, according to studies, it is a good predictor of all-cause and cardiovascular mortality (6) .Nevertheless, a recent systematic review (16) showed the scarcity of studies that have used WC as an indicator of abdominal obesity in Brazilian children and adolescents.
In the present study, the overall prevalence of abdominal obesity was 11.9%.This rate was higher in boys (14.3%) than in age-matched girls (9.8%).Other experiments conducted in Brazil using WC as an indicator of abdominal obesity reported prevalence rates of 6% (17) and 14.8% (18) in age ranges different from that assessed herein.
Although obesity is considered a grave public health issue (19) , few nationwide surveys have assessed the frequency and distribution of abdominal obesity in adolescents and its association with comorbidities such as hypertension.Therefore, another objective of the present study was to ascertain whether an association exists between abdominal obesity and HBP.The risk of HBP was approximately 2.5 times higher in girls with abdominal obesity than in their non-obese peers; in boys, the risk was nearly threefold.Considering both genders, the overall risk of HBP was 2.7 times greater in subjects with abdominal obesity.
The findings of this investigation are consistent with those previously published in the literature (7,20,21) .Although use of the BMI as a marker is routine in studies on the topic, these findings suggest that WC may be superior for identification of adolescents with HBP (21) , as WC measurement appears to provide a better estimate of intra-abdominal fat, whereas BMI seems more sensitive to total and subcutaneous adiposity (22)(23)(24) .Obesity appears to be associated with HBP, although the causes of this are not fully known.One of the many mechanisms possibly implicated in the influence of central obesity on BP in adolescents involves reduced insulin sensitivity, with subsequent development of compensatory hyperinsulinemia.Excess insulin secretion, which has been observed in obese adolescents and appears to be associated with the presence of intra-abdominal fat (25) , is believed to increase retention of sodium and, consequently, fluid; this would stimulate sympathetic activity, ultimately increasing blood pressure.Therefore, salt ingestion -which was not taken into account in the present study-may have been a confounder.
Another potential limitation of this study concerns BP measurement, which, while procured in a standardized manner and in duplicate, with a minimum interval of 1 minute between measurements, was assessed over the course of a single day and period.Moreover, the choice to include only students of public schools may have led to underestimation of the prevalence of obesity, which tends to be higher among students of private educational institutions (26) .Furthermore, the cross-sectional study design precludes establishment of a causal relationship.
On the basis of these results, we conclude that abdominal adiposity was associated with HBP in this sample of adolescents.Public policies geared at preventing obesity, including monitoring of body composition during the transition from childhood to adolescence and then to adulthood, as well as BP monitoring in this population, are necessary.

Figure 2 -
Figure 2 -Comparison of mean diastolic blood pressure according to waist circumference in subjects aged 10-13 years.

Figure 1 -
Figure 1 -Comparison of mean systolic blood pressure according to waist circumference in subjects aged 10-13 years.

Table 2 -
Prevalence and prevalence ratio (95% confidence interval) of high blood pressure according to waist circumference in a sample of adolescents, Londrina, Paraná, Brazil AO: abdominal obesity; WC: waist circumference.