Obesity
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.
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