Obesity can be defined as an excess of body fat. A sur-rogate marker for body fat content is the body mass index (BMI), which is calculated as:
BMI = Weight in kg/Height m2
In clinical terms, a BMI between 25 and 29.9 kg/m2 is called overweight, and a BMI greater than 30 kg/m2 is called obese. BMI is not a direct estimate of adiposity and does not take into account the fact that some indi-viduals have a high BMI due to a large muscle mass. A better way to define obesity is to actually measure the percentage of total body fat. Obesity is usually defined as 25 per cent or greater total body fat in men and 35 per cent or greater in women. Although percentage of body fat can be estimated with various methods, such as measuring skin-fold thickness, bioelectrical impedance, or underwater weighing, these methods are rarely used in clinical practice, where BMI is commonly used to assess obesity.
The prevalence of obesity in children and adults in the United States and in many other industrialized countries is rapidly increasing, rising by more than 30 per cent over the past decade. Approximately 64 per cent of adults in the United States are overweight, and nearly 33 per cent of adults are obese.
Obesity Results from Greater Energy Intake than Energy Expendi- ture.When greater quantities of energy (in the form offood) enter the body than are expended, the body weight increases, and most of the excess energy is stored as fat. Therefore, excessive adiposity (obesity) is caused by energy intake in excess of energy output. For each 9.3 Calories of excess energy that enter the body, approximately 1 gram of fat is stored.
Fat is stored mainly in adipocytes in subcutaneous tissue and in the intraperitoneal cavity, although the liver and other tissues of the body often accumulate sig-nificant amounts of lipids in obese persons.
It was previously believed that the number of adipocytes could increase substantially only during infancy and childhood and that excess energy intake in children led to hyperplastic obesity, associated with increased numbers of adipocytes and only small increases in adipocyte size. In contrast, obesity devel-oping in adults was thought to increase only adipocyte size, resulting in hypertrophic obesity. Recent studies, however, have shown that new adipocytes can differen-tiate from fibroblast-like preadipocytes at any period of life and that the development of obesity in adults is accompanied by increased numbers, as well as increased size, of adipocytes.An extremely obese person may have as many as four times as many adipocytes, each con-taining twice as much lipid, as a lean person.
Once a person has become obese and a stable weight is obtained, energy intake once again equals energy output. For a person to lose weight, energy intake must be less than energy expenditure.