Assessment of Obesity and Health Risks

Measurement Methods

Sophisticated adiposity assessment methodologies are generally used in research rather than large-scale screening. Technologies such as dual energy X-ray absorptiometry (DEXA), computerized tomography (CT) scanning, and air displacement-plethysmography (Bod Pod) all offer a high level of accuracy but are expensive and time consuming. Others methods, including underwater weighting and stable isotope dilution measurement, although safe, are invasive and inconvenient in clinical settings. Bioelectrical impedance analysis and the measurement of body dimensions, such as arm circumference and subscapular skin fold thickness, estimate body fatness and are easy to perform, but lack reliability and are inaccurate with obese persons making them less than optimal for measuring obesity across populations.

Body Mass Index (BMI)

Determined by the indirect calculation of weight in kilograms to the square of height in meters (kg/m2), BMI is a simple and convenient proxy measure for excess adiposity in clinical settings. Many health-related indices, such as mortality risk, heart disease, high blood pressure, and diabetes, have a graded and continuous correlation with BMI. BMI is highly correlated with the precise DEXA method of fat assessment and is thus the universal acceptance as the standard index for the definition of overweight and obesity. Three methods are used to determine BMI: the calculation of the individual's weight divided by the height in meters squared, the use of several Web-based computation sites, or comparison with published reference tables that plot BMI as height and weight intersect. In adults, regardless of sex or age, one set of BMI values is used to assess adiposity and health risk. Based on classification by the National Heart Lung and Blood Institute of the National Institutes of Health (NIH), a BMI under 25 is normal, 25–29.9 is overweight, and a BMI of 30 and above is obese.

BMI is particularly useful for large population surveys and for screening purposes; however, it may not always categorize individual risk well. Although BMI represents the degree of body fat, it does not distinguish between excess weight due to fat mass and nonfat mass such as muscle, edema, or bone. For example body builders have a low percentage of body fat, but their BMI may be in the overweight range because of their large lean muscle mass. Individuals with higher fat mass may be classified as normal BMI despite having a low bone density or muscle mass. Another limitation of BMI is that the relationship to body fatness for those of different gender, age, and ethnicity has not been firmly established. In a family study of 665 African American and Caucasian men and women over the age of 17, the relationship of BMI and fat mass was dependent on gender and age, particularly at lower BMI levels. For women, race was also a factor.

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