Obesity

Definition

Obesity is defined as increased body weight related to excess accumulation of body fat. The term commonly refers to a range of weight above that which is considered healthy for a given height. A simple and widely used method for estimating body fat, the body mass index (BMI), or Quetelet index, is calculated by dividing weight in kilograms by the square of height in meters (BMI = kg/μ2). BMI does not measure body fat directly but correlates with direct measures of body fat, such as underwater weighing and dual-energy X-ray absorptiometry (DXA).

The current BMI categories for adults 20 years and older published by the U.S. National Heart, Lung, and Blood Institute (NHLBI) of the National Institutes of Health is similar to the classification used by the World Health Organization (WHO) (σee Table 1). The primary BMI cutoff points for excess weight occur at 25, 30, and 40 kg/μ2. Obesity is defined as a BMI of 30 or more.

For children and adolescents, BMI is plotted on national growth charts developed by the U.S. Centers for Disease Control and Prevention (CDC) to obtain percentile rankings among children of the same sex and age in the United States. The CDC and the American Academy of Pediatrics (AAP) recommend the use of BMI to screen for overweight in children and adolescents aged 2 to 19 years. Health advocates prefer the term overweight over obese to avoid the potential stigma of the label in this age group. The CDC BMI-for-age weight status categories for children and adolescents are provided in Table 2.

As a simple and inexpensive method for measuring relative weight, BMI cannot distinguish between [Page 756]increased weight for height due to body fat from that attributable to fat-free mass (muscle, bone, and fluids). Thus, BMI may lead to overestimates of adiposity in athletes, for example. More direct methods of estimating body fat include skinfold thickness, ultrasound, computed tomography, and magnetic resonance imaging (MRI).

Among adults, the NHLBI guidelines recommend assessing two additional predictors of potential health risks associated with overweight and obesity, in addition to BMI. These are waist circumference, as abdominal fat is associated with greater health risk, and also other risk factors and comorbidities associated with obesity, such as high blood pressure and physical inactivity. Both absolute waist circumference and waist-to-hip ratio are used to assess central obesity, also known as ‘apple-shaped’ or ‘masculine’ obesity, in which the main fat deposits are stored around the abdomen and upper body.

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