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(Circulation. 1999;99:541-545.)
© 1999 American Heart Association, Inc.
Clinical Investigation and Reports |
From the Division of Cardiology, Department of Pediatrics, University of Cincinnati College of Medicine and Children's Hospital Medical Center, Cincinnati, Ohio (S.R.D., J.A.M., P.K., T.R.K.), and the Department of Cardiology, The Cleveland Clinic Foundation, Cleveland, Ohio (D.L.S.).
Correspondence to Stephen R. Daniels, MD, PhD, Division of Cardiology, Children's Hospital Medical Center, 3333 Burnet Ave, Cincinnati, OH 45229.
| Abstract |
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Methods and ResultsThis was a cross-sectional study of 127 children and adolescents 9 to 17 years of age. Dual-energy x-ray absorptiometry was used to measure total and regional fat mass. The dependent variables were fasting lipid and lipoprotein concentrations, systolic and diastolic blood pressures, and left ventricular mass. There were significant (P<0.05) univariate correlations between fat distribution and log triglycerides (r=0.27), log HDL cholesterol (r=-0.23), systolic blood pressure (r=0.26), and left ventricular mass (r=0.37). Multiple regression analysis showed that the significant independent correlates for triglycerides and HDL cholesterol were age and fat distribution; for systolic blood pressure, height and fat distribution; and for left ventricular mass, height, race, sex, and fat distribution.
ConclusionsThese results demonstrate that fat distribution is a more important independent correlate of cardiovascular risk factors than percent body fat in children and adolescents. Greater deposition of central fat (an android fat pattern) is associated with less favorable plasma lipid and lipoprotein concentrations, blood pressure, and left ventricular mass.
Key Words: obesity risk factors cholesterol blood pressure
| Introduction |
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Most previous studies of fat distribution have used indirect anthropometric methods, such as skin-fold thickness, circumferences, and waist-to-hip ratio, to estimate the pattern of fat distribution. The technique of dual-energy x-ray absorptiometry (DEXA) has been shown to provide a direct, accurate, and precise measure of lean body mass and total fat mass. This method has been validated against a range of established techniques, including underwater weighing.5 This method also allows quantification of fat mass in anatomically defined regions of interest,6 which allows more precise evaluation of the impact of fat distribution.
The purpose of this study was to evaluate the effect of adiposity and fat distribution on established cardiovascular risk factors in children and adolescents. The dependent variables were lipid and lipoprotein concentrations, systolic and diastolic blood pressures, and left ventricular mass.
| Methods |
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Dual-Energy X-Ray Absorptiometry
DEXA measurements were performed with a Hologic Inc
1000/W device for quantification of bone mineral density, lean
mass, and fat mass. This method uses 2 beams of low-energy x-rays that
are collected by the external detector after attenuation by the body
tissue through which they have passed. Soft tissue is resolved by use
of mass attenuation coefficients derived from tissue equivalent
standards for fat-free and fat tissue. DEXA has been shown to provide
accurate and precise measurements of bone mineral content, fat-free
mass, and fat mass in subjects over a wide range of ages and body
size.6 7 8 DEXA has been validated in adults and children
against the hydrodensitometry method, which has previously been
established as the most valid measurement of lean body mass and fat
mass.5 9 To evaluate the effects of total body adiposity,
percent body fat was used. Percent body fat was calculated as total
body fat mass divided by total body mass times 100. To evaluate the
effects of fat distribution, 4 regions of interest were manually
determined.10 These regions included the subscapular,
waist, hip, and thigh regions. The regions were defined by anatomic
bony landmarks. The height of each region was equivalent and was
defined as one third the distance from the top of the iliac crest to
the knee. The waist region was placed on the iliac crest, with the
subscapular region placed on top of that. The hip region was placed at
the middle of the pelvis, with the thigh region just below that. The
width of each region was adjusted to include all soft tissue in that
region. Body fat distribution was calculated as the fat mass in the
subscapular region plus that in the waist region divided by the fat
mass in the hip region plus that in the thigh region.
Blood Pressure
Blood pressure was measured in the right arm with the subject
sitting quietly by use of the methodology described by the Second NHLBI
Task Force on Blood Pressure Control in Children.11 Blood
pressure was measured by trained examiners who had received 16 hours of
instruction and were certified for blood pressure measurement as part
of the quality control process for a multicenter
investigation.12 Measurements were made by auscultation
with a mercury-column sphygmomanometer and a cuff appropriately sized
for the arm size of the subject. The onset of the first Korotkoff phase
was used to determine systolic blood pressure, and the onset of
the fifth Korotkoff phase was used to determine diastolic
blood pressure. Three blood pressure measurements were taken. The
average of the 3 measurements was used in the analysis.
Lipids and Lipoproteins
Blood was drawn from subjects after a 12-hour fast. Lipid
profiles were measured in the Lipid Laboratory of the Department of
Internal Medicine at the University of Cincinnati, which is an
NHLBI-CDCstandardized laboratory. Analyses were performed on
a Hitachi 705 analyzer with enzymatic procedures for
measurement of cholesterol and triglycerides
and triglyceride blanking, and the modified Lipid Research
Clinic method was used for measurement of HDL cholesterol.
The LDL cholesterol level was calculated by use of the
Friedewald equation.13
Left Ventricular Mass
Echocardiographic examination was performed with
subjects in the supine position. Studies were performed with
2-dimensional guided M-mode echocardiography.
Measurements of the left ventricle were made at end
diastole according to the methods of the American Society
of Echocardiography.14 Left
ventricular mass was calculated as previously
described.15 Left ventricular mass index was
calculated as left ventricular mass (grams) divided by
height (meters)2.7, as recommended by De Simone
et al.16
Statistical Analysis
Descriptive statistics, including mean and SD for continuous
variables and proportions for categorical variables, were
calculated. Appropriate transformations were performed for continuous
variables that were not normally distributed. The dependent
variables for this study were established
cardiovascular risk factors, including lipid and
lipoprotein concentrations, blood pressure, and left
ventricular mass. Univariate relationships
between body size variables and risk factors were assessed with
correlation analysis. In these analyses, the values for
lipids and lipoproteins were included after logarithmic transformation.
To evaluate the independence of correlates of risk factor
variables, stepwise multiple linear regression analysis was
used. In these analyses, both the percent body fat and fat
distribution were allowed to enter the model as independent
variables. A value of P<0.05 indicated statistical
significance.
| Results |
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Univariate correlation coefficients between body fat
measures and cardiovascular risk factors are
presented in Table 2
.
Triglyceride concentration was related to body fat
distribution, and HDL cholesterol was inversely associated
with body fat distribution. Greater truncal fat distribution was
associated with higher triglycerides and lower HDL
cholesterol. HDL cholesterol was also inversely
related to the total percent body fat and the amount of fat in the
android segments. LDL cholesterol was not associated with
any of the measures of body fat. Systolic blood pressure was
related to both the total amount of fat and fat distribution, whereas
diastolic blood pressure was related to the percent body
fat but not fat distribution. Left ventricular mass was
strongly correlated with both percent body fat and fat
distribution.
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Results of the multiple regression analysis for each of the
dependent variables are presented in Table 3
. Fat distribution was a significant
independent predictor of both plasma triglyceride and HDL
cholesterol levels. There was a direct relationship between
the android/gynoid fat distribution and triglyceride
concentration and an inverse relationship with plasma HDL
cholesterol concentration. In these models, the fat
distribution and age and fat distribution explained a relatively small
but significant proportion of the variance of triglyceride
and HDL cholesterol levels, respectively. Android/gynoid
fat distribution was also a significant correlate of systolic
blood pressure and left ventricular mass. Fat distribution
was not a significant predictor of diastolic blood
pressure. In the multiple regression analyses, fat distribution
was always a stronger correlate of the cardiovascular
risk factors than percent body fat, which is an overall measure of
obesity. Height was a significant independent correlate of
systolic and diastolic blood pressures and left
ventricular mass. Race was a correlate of
diastolic blood pressure and left ventricular
mass. The regression coefficient for race was negative in each model,
indicating that blacks had higher diastolic blood pressure
and left ventricular mass than whites. Sex was also a
correlate of left ventricular mass, with boys having
greater left ventricular mass than girls.
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| Discussion |
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The association of increased body weight with elevated triglycerides and diminished HDL cholesterol has been described in both adults and children.17 18 These lipid and lipoprotein alterations have also been reported to be associated with measures of central obesity, such as waist-to-hip ratio in adults.19 20 21 Significant weight loss has been shown to result in decreased triglyceride and increased HDL cholesterol concentrations.22 23 The relationships between fat distribution and lipids and lipoproteins in the present study are similar to those found in adult subjects with the use of DEXA. Haarbo et al24 25 found positive associations between central fat and cholesterol, triglycerides, and LDL cholesterol and a negative association with HDL cholesterol in postmenopausal women. Walton et al10 studied healthy men 21 to 77 years of age and found a relationship of increasing android fat distribution and elevated serum triglycerides and decreased HDL C2 concentrations. They did not report an association between fat distribution and either LDL or HDL cholesterol. We found relationships of body fat distribution and triglycerides and HDL cholesterol in young subjects but did not find an association of fat distribution and LDL cholesterol.
From a clinical standpoint, there is often heterogeneity in the cardiovascular risk status in obese children and adolescents. In adults, the most typical lipid profile seen in obese individuals is increased fasting plasma triglyceride levels, reduced plasma HDL cholesterol, and marginally elevated plasma LDL cholesterol levels.26 27 In adult men and women, accumulation of abdominal fat is associated with high plasma triglyceride and low plasma HDL cholesterol levels.27
Previous studies in children have shown that there is a relationship between obesity and cardiovascular risk factors in populations of young subjects.28 We have previously shown the relationships between adiposity and blood pressure29 30 and left ventricular mass in children and adolescents.31 Some studies in young subjects have also evaluated the relationship of fat distribution and cardiovascular risk. These studies have generally supported the concept that fat distribution is related to cardiovascular risk factors.28 However, these studies have usually used less direct anthropometric measures, such as skin-fold thickness and waist and hip circumferences. These methods can be useful and convenient but are subject to variability because of differences in measurements or observer bias. DEXA has the advantage of direct measurement of fat and lean tissue mass and the ability to evaluate the differences in fat deposition by region. This allows more precise measurement of fat mass and a better understanding of the role of fat distribution.
The mechanism by which truncal fat deposition influences these cardiovascular risk factors is not completely understood. However, it is clear in adults that some metabolic alterations are related to more central fat deposition. Subjects with greater central fat have lower insulin sensitivity, resulting in higher circulating insulin levelsand elevated concentration and turnover of nonesterified fatty acids.32 Visceral adipocytes are less responsive to the action of insulin in obesity states. This insulin resistance leads to increased delivery of fatty acids to the liver. These fatty acids are a determinant of triglyceride production in the liver.33 Elevated insulin concentrations may also influence the activity of hepatic lipase, and insulin resistance may affect lipoprotein lipase, both of which are involved in the metabolism of HDL cholesterol.34 35 Reduced lipoprotein lipase and increased hepatic lipase activity result in decreased maturation and increased catabolism of HDL cholesterol, respectively. Rocchini36 has studied the relationship of insulin sensitivity to blood pressure in obese adolescents and found that increased circulating levels of insulin are related to blood pressure elevation. This may be due to an effect of insulin on excretion of sodium and water, leading to increased circulating blood volume.37 Other possible mechanisms include an adverse effect of insulin on sympathetic nerve activity or on the endothelium.38 The effect of insulin on left ventricular mass may involve the growth-promoting action of insulin or other metabolic and hemodynamic effects of insulin.36
In conclusion, we found that the more android or more central fat distribution is an important predictor of plasma triglycerides, HDL cholesterol, systolic blood pressure, and left ventricular mass in children and adolescents. Fat distribution appears to be a more important influence on cardiovascular risk factors in young subjects than overall adiposity. These findings, for which a direct measure of fat distribution by region was used, indicate that understanding the impact of central adiposity on cardiovascular risk will be important in the design of future studies and planning of clinical intervention strategies to lower cardiovascular risk for young people.
Received May 21, 1998; revision received September 24, 1998; accepted October 22, 1998.
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