(Hypertension. 2000;36:165.)
© 2000 American Heart Association, Inc.
Scientific Contributions |
Blood Pressure Is Associated With Body Mass Index in Both Normal and Obese Children
From the Department of Paediatrics, Queen Mary Hospital, The University of Hong Kong, China (Q.H., J.K.); Beijing Research Institute of Pediatrics, Bejing Children’s Hospital, China (Q.H., Z.Y.D); and The Clinical Trials Centre, Faculty of Medicine, The University of Hong Kong, China (D.Y-T.F., J.K.).
Correspondence to Johan Karlberg, MD, PhD, Department of Paediatrics, Queen Mary Hospital, The University of Hong Kong, Pokfulam, Hong Kong SAR, PR China. E-mail jpekarl{at}hkucc.hku.hk
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Abstract |
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Abstract—Obesity is associated with elevated blood pressure (BP) both in adults and children. Childhood obesity has become a severe health problem, especially during the last few decades. So far there has not been any large-scale study specifically focusing on the association between obesity and BP in early life. The aim of this study is to examine systematically the association between obesity and BP in preschool Chinese children in mainland China. In 1996, measurements of weight, height, and BP values were collected in a nationwide, case-control study of 748 boys and 574 girls who ranged in age from 0.1 to 6.9 years in 8 cities in mainland China. One obese child and 1 nonobese child were matched for gender and age. The BP differences of the mean-matched pair were
5
mm Hg for systolic blood pressure (SBP) and 4 mm Hg
for diastolic blood pressure (DBP)
(P<0.05); a higher value was noted in obese children.
The BP value of 19.4% children in the obese group and 7.0% children
in the nonobese group was higher than the 95th percentile value
(P<0.0001), which is defined as high BP by the Task
Force on Blood Pressure Control in Children. Both SBP and DBP were
significantly (P<0.05) positively related to body mass
index (BMI) values (P<0.05) for children in obese and
nonobese groups after adjustment for age, gender, and height. To be
specific, an increase of 1 BMI unit was associated with, on average, an
increase of 0.56 mm Hg and 0.54 mm Hg in SBP and DBP,
respectively, for obese children. In nonobese children, the increase in
SBP and DBP was 1.22 mm Hg and 1.20 mm Hg, respectively. An
increase in the BMI is conclusively associated with elevated SBP and
DBP in nonobese children. Furthermore, an increase in the adjusted BMI
was associated with an increase in SBP and DBP in obese and
nonobese children.
Key Words: obesity • blood pressure • children • race • body mass index
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Introduction |
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Children and adults who are obese have a significantly higher blood pressure (BP) than do those who are lean.1 Obesity represents an important risk factor for cardiovascular disease. Childhood obesity is often associated with the future development of hypertension.2 It is suspected that future hypertension may develop in children in whom BP is high for their respective age or body size.1 In addition, childhood obesity increases the risk of obesity in adulthood and is associated with cardiovascular disease risk factors such as hypertension, diabetes mellitus, and dyslipidemia.3 Thus the prevention of the onset of obesity during youth may be important in reducing the risk of coronary heart disease in later life.
The prevalence of obesity is increasing in both developed and developing countries.4 In China, childhood obesity has become a severe health problem, especially during the past 10 years. The prevalence of obesity in preschool children (with obesity defined as a height-adjusted weight of >120% of the National Center for Health Statistics [NCHS] mean value), was 2.2% in boys and 1.9% in girls, according to a large, nationwide obesity epidemiological study undertaken in 1996. Moreover, the annual increment of the prevalence of obesity from 1986 to 1996 was 10.0% in boys and 8.7% in girls.5 It is therefore important to observe the BP change in relation to obesity in Chinese children when the target population for early intervention is selected. In this article, we examine the association between obesity and BP in preschool Chinese children in mainland China.
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Methods |
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Subjects
The children included in this study were selected from a large, nationwide epidemiologic study of obesity in 208,513 Han Chinese children who ranged in age from 0.1 to 7.0 years. The nationwide study was undertaken in 11 major cities in China during 1996, and the population evaluated has been described in detail by Ding et al.5 Our study uses the same definition of obesity as that used by Ding et al,5 ie, a weight that exceeds the standard weight adjusted for height, age, and gender by >20%,6 or equivalently, a height-adjusted weight of >120% of the NCHS mean value. Eight of the 11 cities (Haerbing, Yingkou, Beijing, Chengdu, Chongqing, Changsha, Guiyang, and Nanchang) consented to participate in our study. The first 3 of the 8 participating cities are located in north China, and the other 5 cities are in the south. Children in whom a disease was detected on the day of investigation and those with physical or mental developmental problems were excluded.
Individual child health records were used to match each identified obese child with another randomly selected nonobese child from the same child healthcare center. The children were matched by gender and by age group (in years); ie, birth to 0.9, 1.0 to 1.9, 2.0 to 2.9, 3.0 to 3.9, 4.0 to 4.9, 5.0 to 5.9, and 6.0 to 6.9 years of age. This study was approved by the ethics committee of the Academic Committee of Beijing Children’s Hospital, Beijing, PR China, in 1996.
Data Collection
Our study team consisted of 10 professional medical staff from 6
child healthcare institutes and 2 medical universities in the 8 cities
listed previously. Study forms were distributed to the study team at a
prestudy coordinating meeting held in Chongqing in May 1996. Between
September 1996 and February 1997, data collection was performed by the
study team in the 8 cities through the local child healthcare system.
The parents of the obese and nonobese children had the details of the
study explained to them. All children whose parents gave verbal consent
to participate were included in the study. The participating children
were brought to a local child healthcare center by their parents to
meet our study team. Parents were asked questions from the study form
concerning the activities and feeding patterns of their child as well
as information about themselves, such as their education level, their
attitude about obesity, and their current weight and height on the day
of investigation.
Measurement
All measurements were performed according to a standardized
protocol developed for this study during the prestudy coordinating
meeting. Weight, height, and BP of all subjects were measured by our
study team in a standardized way7 with the equipment
available at the local child healthcare center on the day of
investigation. In children younger than 2 years of age, the supine
length was measured: The child lay fully extended with heels touching a
baseboard while a movable headboard was brought down to touch his or
her head. In children older than 2 years of age, height was measured by
a stadiometer attached to the wall. All height measurements were
rounded down to the nearest 0.5 cm. For each child in the supine
position, BP was measured twice on the right arm with an appropriately
sized cuff, and the average value was recorded on the study form.
Diastolic blood pressure (DBP) was defined via Korotkoff
Sound 4. Both systolic blood pressure (SBP) and DBP were
recorded to the nearest 2 mm Hg.
The recorded birth weight values of the children studied were taken from the individual growth charts that were kept in the local child healthcare center. The body mass index (BMI), defined as weight divided by height squared (kg/m2), was also computed for all children studied.
All completed study forms were then sent to the study coordinating center at the Beijing Research Institute of Pediatrics. Forms for a total of 748 boys and 574 girls in 8 cities in early 1997 were collected. Most of the forms were used, except for 36 matched pairs after data quality control. The main reason for the exclusion of forms was that of missing values of the weight and height of the parents and/or the information related to their attitude about obesity.
Quality of Data
In the 1970s, China constructed its own child healthcare network
system, which is known as the "3-level network," ie, village,
county, and city. Several nationwide epidemiological studies and other
research projects concerning child healthcare problems have been
conducted successfully through this network,8 9 which
contains listings for thousands of medical staff and hundreds of child
healthcare centers and institutes. Weight and height values for each
child from birth to 7 years of age are taken annually, and the values
are plotted on a growth chart for each individual. Any extreme values
caused by measurement error are identified and corrected
immediately.
The present case-control study was organized by the Chinese Society of Pediatrics and the Chinese Society of Child Health Care, which represent the 2 most important academic societies in the field of pediatric research in China. All staff in our study team have a strong background in both clinical and research work. All staff members were also involved in the nationwide epidemiological study on the obesity of children undertaken in 1996.5 For further quality control assurance, 2 prestudy meetings were organized to standardize the methods and equipment for measuring weight, height, and BP, which minimized the influence of measurement errors.
Data Analysis
The mean or median and the standard deviation (SD) of the BP and
BMI values of the children were computed by age group and gender. The
signed rank test and the paired t-test were used to compare
the difference of the central tendency of BMI and BP between the paired
samples (obese or nonobese) by age group and gender. The 95th
percentile of BP, which was recommended by the Task Force on
Blood Pressure Control in Children,10 was used as the
cutoff point for children with "high normal BP." A multiple linear
regression model (Model 1) was applied to examine whether BP was
associated with BMI, age, gender, and height in the obese and nonobese
groups considered separately and together. Another multiple regression
model (Model 2) was used in which the BMI was replaced by an adjusted
BMI value. The adjusted BMI value was computed as the difference
between the BMI value of an individual child and the median BMI value
in a child of the same gender and age of the Swedish BMI reference
values (Q. He, K. Albertsson-Wikland, and J. Karlberg,
unpublished data, 1999). Such an adjustment is usually applied to
children because of the possible effects of gender and age; height is,
for instance, commonly converted into SD scores. All significance tests
were based at the 0.05 level of significance, and all analyses
were performed with the Statistical Analysis Software (SAS)
6.10 on a PC platform.11
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Results |
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The BMI distribution in our sample is shown in Figure 1, in which the BMI is used to define obesity. The percentile values of the BMI curves (P3, P50, and P90) in Figure 1 were chosen from recently derived Swedish BMI percentile curves (Q. He, K. Albertsson-Wikland, and J. Karlberg, unpublished data, 1999). As shown in Figure 1, both boys and girls in our obese group have a BMI value of > the 90th percentile of the Swedish BMI curves.
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) subjects
in our study. The BMI values of the centile curve (P3, P50, and P90)
were taken from a large, population-based longitudinal growth study of
Swedish children (Q. He, K. Albertsson-Wikland, and J. Karlberg,
unpublished data, 1999).
The mean/median and SD of the BMI and BP are given in Table 1, Table 2,
and Table 3 by age group and
gender. Median differences in the BMI between the matched pairs
were statistically significant (P<0.05) in all age groups
and genders (Table 1). In Table 2 and Table 3, the
BP differences of mean matched pairs were significantly
(P<0.05) different from zero, with an averaged higher value
of 5 mm Hg in SBP and 4 mm Hg in DBP in the obese
group compared with those values in the nonobese group.
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BP (either SBP or DBP) in 91 of 469 children (19.4%) in the obese
group was > the 95th percentile value, in comparison with 33 of
469 children (7.0%) in the control (
2 test,
P<0.0001).
Two different multiple linear regression models were applied to each of the 2 BP values in the obese and nonobese groups separately (Table 4). Gender, age, and height were the common independent variables in the 2 models. The adjusted BMI value was significantly related (P<0.05) to both SBP and DBP in the obese and nonobese groups. An increase of 1 U of BMI was associated with, on average, a 0.56 mm Hg increase in SBP and a 0.54 mm Hg increase in DBP in the obese children and with a 1.22 mm Hg increase in SBP and a 1.20 mm Hg increase in DBP in the nonobese children. The BMI showed similar significant effects on the BP of the nonobese children, but little or no significant effect was found in the obese group.
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The regression analysis of the total sample indicated that the
effects of age and BMI on BP were significant (P<0.05), but
the effects were not significantly different (P>0.05)
between the obese and nonobese groups or according to gender.
Specifically, an increase of 1 BMI unit was associated with, on
average, a 1.11 mm Hg increase in SBP and a 1.05 mm Hg
increase in DBP. For the full sample range of BMI values of 6 U,
the BMI "effect" on BP was 6 mm Hg in both SBP and DBP.
In addition, we also examined whether the parental BMI (recall data), the socioeconomic status of the family, or the birth weight of the child was related to the child’s BP by multiple regression analysis. However, no statistically significant (P>0.05) associations were found.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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One finding of this case control study was that the mean difference of both SBP and DBP between the matched pairs was found to be significant (P<0.05) in early life. Moreover, there was a clear impact of childhood BMI on BP in the nonobese children. An increase in adjusted BMI was associated with an increase in SBP and DBP in obese and nonobese children.
Childhood obesity and its consequences have been attracting more attention in the medical field because of the increasing prevalence worldwide and the long-term effects of childhood obesity in adults. Many studies have been performed to explore the association between childhood obesity and its short- and long-term health effects. It is, however, not easy to compare the findings of previous studies because of diversities in the study populations, such as the differences in sample size, age range, and ethnic group. In addition, various determinants of childhood obesity were used in previous studies, such as the 95th percentile value of the NCHS triceps skinfold thickness12 or a BMI specific centile value (90% or 95%) that was recently developed by the Centers for Disease Control and Prevention.13 As a result, the magnitude of the prevalence varies with different definitions of obesity.14 15 16 In this study, we define obesity as a weight that exceeded the standard weight adjusted for height, age, and gender6 by >20%, ie, a height-adjusted weight >120% of the NCHS mean value. This definition has also been used in several previous obesity prevalence studies.17 18 Other definitions of obesity, such as the BMI, could have been used. As shown in Figure 1, most of the boys and girls in our obese group had a BMI value > the 90th percentile of the Swedish BMI curves, and the vast majority of the nonobese group had a BMI value below the 90th percentile. As a result, the population could be separated effectively into obese and nonobese groups with either a height-adjusted weight cutoff point of 120% of the NCHS means or a BMI specific percentile, such as the 90th. Until now, there have been no BMI reference values constructed for mainland Chinese children. The Swedish BMI value, which we used as the reference in both Figure 1 and in the adjusted BMI in Model 2 (Table 4), was only recently constructed; it is based on a large, population-based longitudinal growth study (Q. He, K. Albertsson-Wikland, and J. Karlberg, unpublished data, 1999).
Many previous studies19 20 reported that obese children
had significantly higher BP than did nonobese children and that the
association between age and BP disappears after controlling for
weight.21 Pela et al22 reported that the
alterations on blood pressure of the obese children were detected by
ambulatory 24-hour monitoring and that higher SBP levels were observed
in 6- to 11-year-old obese children both during the day and the night.
Figueroa et al23 also reported that higher SBPs and DBPs
were found with the usual blood pressure check in a study of 5- to
11-year-old obese children. However, the association between BP and
obesity in preschool children has received less attention in previous
studies. It was assumed that obesity in the preschool years may not be
related to medical problems, or that health problems may not emerge
until an individual is overweight for many years, or that
obesity-related health problems may not become evident until
adolescence or adulthood.24 On the other hand, BP in
preschool children seemed to be relatively stable in comparison with BP
in children 
6 years. In our study, the SBP and DBP difference between
the matched pairs was found to be significant (P<0.05) for
both boys and girls (Table 2).
In our study, the BMI of children was positively related to SBP and DBP in both obese and nonobese groups. We also provided the results of a regression model that used the adjusted BMI (Table 4). The reason for using the adjusted BMI was that the same BMI value yielded different interpretations when it was less than or higher than the corresponding median value that changes nonlinearly with age (Figure 1).
Of particular importance were the findings that the BMI was positively
related to BP in the nonobese group of preschool children and that an
increase of 1 BMI unit was associated with an increase of 1
mm Hg in both SBP and DBP. The magnitude of the BMI effect on BP in
the nonobese group was approximately twice that in the obese group. It
has been reported in several intervention studies25 that
the treatment of obesity by weight loss decreases blood pressure
substantially in both hypertensive and normotensive subjects.
Further, a change in the weight of
children was associated with a change in the BP in the full range of
the population-based study.26 To the best of our
knowledge, this is the first time that an association between BMI and
BP in healthy, nonobese preschool children has been documented. This
important finding raises the question of how to define childhood
obesity: Should it be based on a functional outcome such as BP or on a
statistical cutoff point of a population-based BMI distribution?
Because the BMI is clearly associated with elevated SBP and DBP in
nonobese children, maintaining an ideal increase in the BMI in
childhood is important in preventing elevated BP.
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Acknowledgments |
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This study was supported by the Faculty of Medicine, The University of Hong Kong, Hong Kong SAR, and the Beijing Children’s Hospital, Beijing, PR China. We greatly appreciate the efforts of the staff who worked on the data collection and those of James Thorburn for his comments on the English scientific writing.
Received December 13, 1999; first decision January 6, 2000; accepted February 24, 2000.
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