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Obesity

Obesity is a medical condition characterized by excess adipose, or fat, tissue sufficient to cause several adverse health effects. With modernization have come increased access to food supplies and decreased necessity for physical activity in daily life. The resulting imbalance in caloric intake and expenditure has caused a dramatic increase in the rates of obesity, in both developed and developing countries. This new global epidemic brings with it an increased risk of many serious adverse health consequences, including heart disease and type 2 diabetes mellitus (T2DM).

Definition

Obesity is most commonly defined in terms of the body mass index (BMI), calculated by dividing an individual's body weight in kilograms by the square of his or her height in meters. Obesity is generally defined by a BMI of 30 kg/m2 or greater. The American Gastroenterological Association uses the following BMI values to categorize a person's adiposity: overweight (BMI 25.0–29.9 kg/m2), class I obesity (BMI 30.0–34.9 kg/m2), class II obesity (BMI 35.0–39.9 kg/m2), and extreme or class III obesity (BMI >40 kg/m2). A normal BMI falls in the range of 18.5–24.9 kg/m2while a BMI less than 18.5 kg/m2 is considered to be underweight. Absolute BMI cutoffs are not used in children aged 2 to 19. Rather, a child is considered to be overweight if his or her BMI is greater than the 95th percentile of BMI for age and gender. Similarly, he or she is considered to be obese if over the 99th percentile.

It is now recognized that the percentage of body weight that is adipose tissue and the distribution of this adipose tissue are more important for increased disease risk than the absolute value of BMI. A central (“apple-shaped”) distribution of adipose tissue inside the abdominal cavity is associated with a higher risk of diabetes than superficial (“pear-shaped”) adiposity around the hips and thighs. For this reason, a simple waist circumference or waist-to-hip ratio is used in some assessments of risk instead of BMI. Moreover, BMI can be misleading in some circumstances. For example, a lean person with increased muscle mass and little adiposity might appear to have a BMI in the obese range, despite having a low body fat percentage. Additionally, a certain BMI value may imply different risks in different ethnic groups. In some East Asian populations, a BMI of about 27 kg/m2 corresponds to the same degree of adiposity as a BMI of 30 kg/2in European populations. Nonetheless, BMI has been shown to have good correlation with body fat percentage in most cases and is still useful for epidemiologic studies. Bioelectrical impedance, in which body fat percentage is determined by passing a small electric current through the body, is also commonly used. More expensive methods of determining adiposity include computed tomography (CT) and magnetic resonance imaging (MRI).

Epidemiology

The recent trends in obesity are appropriately described as a global epidemic centered predominantly but not exclusively in more developed countries. More than 300 million people were obese in the year 2000. The most recent study from the U.S. Centers for Disease Control and Prevention (CDC) found that 17.1 percent of U.S. children and adolescents aged 2 to 19 were overweight and 32.2 percent of U.S. adults were obese. About two-thirds of the American adult population is obese or overweight; that is, the minority of the population has a healthy weight. The prevalence of obesity remained relatively stable in the United States from 1960 to 1980 and then experienced a sharp increase of 100 percent from 1976 to 1980 and 1999 to 2002 in all ages and ethnic groups—a rise of 15 percent of the population in the first survey to nearly 33 percent in the following one. Europe has similarly experienced an increase in prevalence over the last 10 years, most pronounced in Eastern Europe, certain Scandinavian countries, and the United Kingdom, where the rate of obesity was as high as 24 percent in women in 2005. In developed countries, BMI tends to vary inversely with socioeconomic status, but this relationship reverses in the developing countries of Africa, where the prevalence of obesity is highest in urban women with higher educational attainment. In general, women have higher rates of obesity than men, and the countrywide prevalence of obesity among women in almost all countries in Central and South America, North Africa, and the Middle East exceeds 30 percent. Although rates of overweight and obesity are increasing in East Asia, they remain remarkably low, being about 2.5 percent in Chinese women in 2005. Urban Indian women, on the other hand, have attained a prevalence of greater than 16 percent. Polynesia has the highest prevalence of obesity worldwide, reaching 80 percent among women in countries such as Nauru and Tonga.

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