A number of systems have been used to define and classify obesity. The body mass index (BMI), which is also known as the Quetelet index, is currently in use.The BMI is calculated as weight in kilograms divided by height in meters squared (kg/m²). Calculated BMI values are available in various chart and graphic forms, such as the one shown in Figure 43-1. According to the National Heart, Lung, and Blood Institute (1998), a normal BMI is 18.5 to 24.9 kg/m²; overweight is a BMI of 25 to 29.9 kg/m²; and obesity is a BMI of 30 kg/m² or greater. According to Freedman and colleagues (2002), obesity is further categorized as class I (BMI: 30 to 34.9 kg/m²), class II (BMI: 35 to 39.9 kg/m²), and class III (BMI: 40-plus kg/m²).

PREVALENCE. By the end of 2000, Pleis and colleagues (2003) reported that 34 percent of adult American individuals were overweight. Another 27 percent were obese, an increase of 75 percent compared with 1980 statistics. Thus, at the end of 2000, more than 50 percent of adults in the United States were either overweight or obese. Prevalence data for women are shown in Figure 43-2. Mokdad and associates (2003) reported that 2.8 percent of women and 1.7 percent of men were extremely obese (class III), with a BMI of 40 kg/m² or more. There is a disparate prevalence of obesity in indigent individuals in this country (Drewnowski and Specter, 2004).

As previously stated, childhood obesity is also a serious problem. Using the definition of overweight as at or above the 95 percentile for age and ethnicity, the prevalence of obesity in children has increased two- to threefold in the United States from 1971 to 1999 (National Center for Health Statistics, 1999; Ogden and co-workers, 2002). Moreover, the problem is worldwide, even in third-world countries (Ebbeling, 2002). In adolescents, the increasing prevalence of obesity is associated with declining physical activity (Kimm and associates, 2002).

METABOLIC SYNDROME. In some people, obesity interacts with inherited factors and leads to the onset of insulin resistance. This metabolic abnormality in turn is responsible for altered glucose metabolism and a predisposition to type 2 diabetes mellitus. In addition, it causes a number of subclinical abnormalities that predispose to cardiovascular disease and accelerate its onset. The most important among these are type 2 diabetes, dyslipidemia, and hypertension. These conditions, when clustered together with other insulin resistance-related subclinical abnormalities, are referred to as the metabolic syndrome (Abate, 2000). Virtually all obese women with hypertension demonstrate elevated insulin levels. The levels are even higher in women with excessive fat in the abdomen - an apple shape, compared with fat in the hips and thighs - a pear shape (American College of Obstetricians and Gynecologists, 2003). In fact, Gus and associates (2004) recently reported that for women, a waist circumference over 88 cm was more predictive of hypertension than a BMI greater than 30 kg/m².

The criteria used by the National Institutes of Health (2001) to define the metabolic syndrome are shown in Table 43-1. Drugs that increase sensitivity to insulin, such as metformin and glitazones, also cause a decrease in blood pressure levels (Chen and colleagues, 1996).

Prevalence. Because metabolic syndrome is newly defined, prevalence data have been calculated from a study in progress. Ford and colleagues (2002) did a follow-up study between 1988 and 1994 of 8814 men and women enrolled in the Third National Health and Nutrition Survey (NHANES III). Using the criteria shown in Table 43-1, there was an overall prevalence of the metabolic syndrome in 24 percent of women and 22 percent of men. As expected, prevalence increased with age. For 4549 women, prevalence was about 6 percent in those 20 to 29 years of age; 14 percent in those 30 to 39 years of age; 20 percent in those 40 to 49 years of age; and greater than 30 percent for women older than 50 years of age.