Obesity Epidemic

Definition of Overweight and Obesity

Obesity is commonly defined as severely overweight. This verbal definition is complemented by a quantitative specification of the boundaries between underweight, normal weight, overweight, and obesity. The World Health Organization (WHO) and most experts in this field refer to the body mass index (BMI) as a simple indicator relating the weight of an individual to his or her height. BMI is defined as the weight in kilograms (w), divided by the square of the height (h) in meters (kg/m2). To give an example in both standard and metric units: An adult who weighs 170 pounds (77 kg) and whose height is 6 feet (1.828 m) will have a BMI of 23.0:

BMI = 77 kg/(1.828 m)2 = 77/3.341584 = 23.0 kg/m2.

Is that underweight, normal, or overweight? Table 1 gives an answer to this question by referring to the weight classification of the WHO.

According to Table 1, the person in the previous example with a BMI of 23.0 is of normal weight. The same person would be classified as overweight and pre-obese (BMI = 25.1) with a weight of 185 pounds (83.9 kg) and as obese (BMI = 30.1) with a weight of 221 pounds (100.24 kg).

Anyone not familiar with their own BMI or with the metric units or without a calculator at hand can use one of the many online BMI calculators that make it easy to determine this index using both [Page 536]standard and metric units (see, for example, the calculator provided by the National Heart, Lung, and Blood Institute, 2009).

Medical Aspects of Overweight

Overweight and obesity are caused by an imbalance of energy intake (nutrition) and energy expenditure (physical activity). If the energy intake exceeds the energy expenditure, the body saves this energy in the form of fat, associated with a weight increase. If the energy intake is lower than the energy expenditure, the body takes the lacking energy from the fat, resulting in a weight loss. This physiological mechanism has been instrumental for survival in premod-ern societies with a very irregular supply of food and long periods of hunger. However, the same process promotes overweight and obesity in modern societies with a lack of physical activity and an oversupply of food.

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