MEASURING OBESITY
The IOTF proposes that adult cut-off points be linked to BMI centiles for children to determine child cut-off points. For clinical assessment of obesity, indirect measures such as waist circumference may be used in addition to the BMI.
By Sioksoan Chan-Cua, MD
Contributor
The prevalence of obesity has been rising worldwide not only among adults but also among children, affecting both their health and quality of life. One disturbing trend is that the adverse medical consequences of the obesity epidemic are occurring at an earlier age.
There is an increasing prevalence of cardiovascular risk factors in overweight children and adolescents. In the Bogalusa Heart Study, for example, about 60 percent of overweight children between the ages of five and 10 years were found to have one cardiovascular risk factor (hypertension, dyslipidemia) and about 20 percent have two or more risk factors.
Studies have shown that childhood obesity significantly contributes to the development of the metabolic syndrome (syndrome X) and premature atherosclerosis while type 2 diabetes mellitus is now commonly diagnosed among overweight adolescents.
What is obesity?
Obesity refers to excess body fat. Defining obesity enables us to predict health risks and make comparisons between populations.
Fatness or adiposity may be measured directly or indirectly. The direct measures of body composition provide an estimation of total body fat mass and various components of fat-free mass.
Such techniques include underwater weighing, magnetic resonance imaging (MRI), computed tomography (CT), dual energy X-ray absorptiometry (DEXA) and bioelectrical impedance analysis (BIA). However, these are relatively expensive and impractical to use routinely.
The indirect methods are anthropometric measurements including weight, weight for height, waist and hip circumferences, skin-fold thickness and indices derived from measured height and weight such as body mass index (BMI = weight in kilograms divided by height in meters squared).
For practical reasons, the definition of obesity has usually been based on weight and height measurements; waist circumference and BMI are the most convenient way of measuring relative adiposity in clinical setting and population studies.
BMI is the most widely used method to screen for obesity and to classify the degree of obesity. Among adults, the World Health Organization (WHO) classifies BMI over 25kg/m2 as overweight and over 30 kg/m2 as obesity.
These markers provide common benchmarks for assessment, but the risks of disease in populations can increase progressively from lower BMI levels. Among Asians, for example, health risks begin at a BMI lower than 25 and BMI of 23 or above is already considered overweight.1 According to the International Obesity Task Force (IOTF), Asians usually have a higher level of body fat, and a lower level of muscularity for every level of BMI compared with Caucasians.
BMI in childhood changes substantially with age.2 In children, the age and gender-specific BMI charts are used. Various cut-off criteria have been proposed based on reference populations and different statistical approaches.
At birth the median is as low as 13 kg/m2; it increases to 17 kg/m2 at age one, decreases to 15.5 kg/m2 at age six, then increases to 21 kg/m2 at age 20. Clearly, a cut-off point related to age is needed to define child obesity based on the same principle at different ages, for example, using reference centiles.
In the United States, the 85th and 95th centiles of BMI for age and sex based on nationally representative survey data have been recommended as cut-off points to identify overweight ("risk of overweight") and obesity ("overweight").
For wider international use, this definition raises this question: Why base it on data from the US and why use the 85th or 95th centile?
The IOTF proposes that these adult cut-off points be linked to BMI centiles for children to determine child cut-off points. The ITOF describes the development of age- and sex-specific BMI cut-off points for overweight and obesity in children using dataset-specific centiles linked to adult cut-off points.
For clinical assessment of obesity, indirect measures such as waist circumference may be used in addition to the BMI. Researchers are encouraged to collect data on waist circumference in childhood and adolescence and further study is needed to validate the waist-for-age cut-off points associated with health risks in childhood and adolescence.
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