Assessment of overweight and obesity in epidemiological studies of disease

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Definitions for classifying and reporting healthy weight, overweight, and obesity in populations have historically been based on measures of weight and height rather than clinical measures of adiposity (5,6). Although weight is the simplest anthropometric index of excess body fat, it does not distinguish between lean body mass (comprised primarily of muscle, bone, and extracellular water) and adipose tissue (5). Thus, measures of weight adjusted for height provide a better approximation of the proportion or total amount of adipose tissue in the body than does weight alone.

Since the 1980s, indices of weight adjusted for height have gained favor because they provide a single estimate of adiposity regardless of height and can be easily compared across studies and across populations. By far the most widely used weight-for-height measure is the body mass index (BMI, also called Quetelet's index), which is defined as weight (in kilograms) divided by height (in meters squared) (5). The assumption underlying the BMI (and all other such indices) is that true adiposity is unrelated to height. Indeed, among the many indices of weight-for-height that have been proposed, the correlation with height has generally been lowest for BMI (5).

Standards defining healthy weight, overweight, and obesity have evolved over time and reflect existing knowledge of and assumptions about the relation of weight to disease outcomes. Historically, weight-for-height standards prepared by the Metropolitan Life Insurance Company provided "ideal" and "desirable" gender-specific weight ranges for each inch of height based on actuarial data (7). Standards based on BMI have been reported for the US adult population since 1980 in the Dietary Guidelines for Americans (8). Widely accepted current standards based on BMI criteria for overweight and obesity are recommended by the World Health Organization (WHO) (9) and supported by other advisory committees and expert panels to federal agencies (1,8). The WHO cutpoints for BMI and their corresponding interpretations are shown in Table 1. Although the exact cut-points are somewhat arbitrary, this BMI classification scheme was derived largely from observational and epidemiological studies of BMI and disease end points and thus reflects the relationship of BMI to morbidity and mortality (1,9). The cut-point for the underweight category is based on adverse health consequences of malnutrition in developing countries (9).

Weight and height can be self-reported and thus are more easily determined in epidemiological studies of morbidity and mortality than measured weight and height or clinical measures of adiposity. Even though some systematic error exists in self-reported weight and height (weight tends to be underestimated and height overestimated), self-reported data are highly correlated with measured weight and height (r = 0.8 to >0.9) (5) and are sufficiently accurate to establish associations with diseases known to be related to obesity in epidemiological studies (5,10,11). However, prevalence estimates of overweight and obesity based on self-reported data tend to be lower than those based on measured values (12).

Many studies have found moderate to strong correlations (r = 0.6 to 0.9) between BMI and densitometry estimates of body fat composition in adult populations (5). The validity of BMI as a measure of adiposity is further supported by its association with obesity-related risk factors such as blood triglycerides, total cholesterol, blood pressure,

Table 1

Cut-Points of BMI for Classification of Weight (9)

Table 1

Cut-Points of BMI for Classification of Weight (9)

BMI (kg/m2)

World Health Organization

<18.5 kg/m2

Underweight

18.5-24.9 kg/m2

Normal range

25.0-29.9 kg/m2

Grade 1 overweight

30.0-39.9 kg/m2

Grade 2 overweight

>40.0 kg/m2

Grade 3 overweight

BMI, body mass index.

BMI, body mass index.

and fasting glucose (5). BMI may be a less valid indicator of adiposity among the elderly, who tend to have a shift of fat from peripheral to central sites with a concomitant increase in waist-to-hip ratio (WHR) at the same level of BMI (13). For such populations, and with increasing evidence of health risks associated with abdominal (visceral) fat, two measures of central adiposity, the WHR and, more recently, waist circumference, have been commonly used in epidemiological studies.

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