BMI = body mass index; BRFSS = Behavioral Risk Factor Surveillance System;
NHANES = National Health and Nutrition Examination Survey
Until very recently in the history of human existence, obtaining enough food or survival was a major challenge, and regular physical activity was unavoidable. Technological advances and the built environment have resulted in a progressive increase in average body weight and body mass index (BMI) (calculated as weight in kilograms divided by the square of height in meters). Moreover, the prevalence of extreme obesity, defined as a BMI of 40 kg/m2 or greater, has markedly increased in the United States and throughout the world. This article focuses on the prevalence trends, etiology, and consequences of extreme obesity among US adults. Trends in bariatric surgery are also discussed.
DEFINITION AND CLASSIFICATION
Obesity is defined as an excess of body fat. Surrogate measures are used to
identify obesity because measuring body fat directly is impractical. Currently,
BMI is the standard used to classify obesity. Although BMI correlates directly
with body fat mass and health outcomes, it is not a perfect tool. An individual
with a high BMI and a large proportion of lean tissue (ie, muscle) could be
classified as being obese yet have a low percentage of body weight as fat.
The current classification of obesity is shown in Table 1. Overweight is defined as a BMI of 25.0 to 29.9 kg/m2 and obesity as a BMI of 30 or greater. Obesity is further divided into categories of class I (BMI, 30.0-34.9), class II (BMI, 35.0-39.9), and class III or extreme obesity (BMI, >=40).1 As BMI increases above the normal range, health risks increase, slowly at lower BMIs and more steeply with greater degrees of obesity. The term medically complicated obesity refers to a subset of obese individuals who also have health complications due to their obesity, for example, a patient with a BMI of 36 kg/m2 who has type 2 diabetes mellitus, hypertension, or obstructive sleep apnea. Increased waist circumference, reflective of abdominal subcutaneous and visceral fat, is associated with increased risk of metabolic risk factors for coronary heart disease. However, the importance of waist circumference measurements in determining health risks decreases with increasing BMI. In patients with BMI values of 35 kg/m2 or greater, waist circumference does not add to the level of risk determined by assessment of BMI alone.1