of Obesity in Adults
F. XAVIER PI-SUNYER
Obesity is of public health concern because of its association with medical complications that lead to increased morbidity and mortality (see Chapter 76). Both the National Institutes of Health and the World Health Organization define obesity as a body mass index (BMI) of 30 kg/m2 or greater and overweight as a BMI of 25–30 kg/m2. The most common complications associated with obesity are insulin resistance, diabetes mellitus, hypertension, dyslipidemia, cardiovascular disease, gallstones and cholecystitis, respiratory dysfunction, and increased incidence of certain cancers.
There is a strong positive correlation between the average weight in a population and the presence of type 2 (non-insulin-dependent) diabetes mellitus. In a male population divided into groups with BMIs of 25.0 to 26.9, 29 to 30, and greater than 35, the risk for diabetes (compared to a population with BMIs less than 21) increases 2.2-, 6.7-, and 42fold, respectively. The severity of the obesity is a determinant, as is the length of time obesity has been present.
The pathogenesis of the diabetes is related to the insulin resistance caused by the obesity, which tends to increase in severity as BMI rises. Primarily sensitive to the action of insulin are muscle, adipose, and liver tissue. As excess fat accumulates in the body, the ability of insulin to act at the cellular level is impaired. The cellular effect is manifested both at the insulin receptor and postreceptor levels. There is a downregulation of insulin receptors, with a decrease in both the number of receptors at the membrane surface and the affinity of the insulin for the receptors. These changes lower the ability of circulating insulin to bind to the receptors and to initiate the signals that lead to the many intracellular actions of insulin.