Obesity: Encyclopedia II - Obesity - Definition

Obesity - Definition

Obesity is a concept that is being continually redefined. In humans, the most common statistical estimate of excess fat mass is the body mass index (BMI), calculated by dividing the weight by the height squared; its unit is therefore kg/m², although no actual surface is implied. The BMI was created in the 19th century by the Belgian statistician Adolphe Quetelet.

Interpretation of the BMI:

• A BMI over 25.0 kg/m² is categorized as overweight.
• A BMI over 30.0 kg/m² is categorized as obesity.
• A further threshold at 35.0 kg/m² is categorized as "severe obesity" (formerly known as morbid obesity).

The American Institute for Cancer Research considers a BMI between 18.5 and 25 to be an ideal target for a healthy individual (although several sources consider a person with a BMI of less than 20 to be underweight).

The cut-off points between categories are occasionally redefined, and may indeed differ from country to country. In June 1998 the National Institutes of Health brought official U.S. category definitions into line with those used by the WHO, moving the American "overweight" threshold from BMI 27 to BMI 25. Thresholds are in principle designed to be "best estimates" concerning health risk at the time they are established and are also designed to ensure cohort uniformity in epidemiological studies. About 30,000,000 Americans moved from "ideal" weight to being 1–10 pounds (0.5–5 kg) "overweight". In 2000, WHO was advised to consider lowering the BMI threshold for overweight in Asians from BMI 25 to BMI 23, and for obesity in Asians from BMI 30 to BMI 25, due to epidemiological studies indicating that Asians suffer a greater number of obesity-related conditions at lower BMI; however, to date, WHO has not made any changes in recommendations. In addition, some clinicians suggest raising the BMI thresholds for those of African, African-American, and Polynesian descent because members of these groups have a greater ratio of lean body mass to fat at all body weights; the proposed thresholds for these groups are BMI 26 for overweight, and BMI 32 for obesity. To date, no major professional or medical organization has officially adopted this suggestion. In the future, a healthy BMI for a given individual may be defined to some extent by his ethnic or racial origin or gender.

In addition to the somewhat inexact process of establishing thresholds for overweight and obesity across diverse populations, the BMI has other weaknesses which limit its value as a preditive tool. In particular, the formula does not take into account differing ratios of adipose to lean tissue; nor does it distinguish between differing forms of adiposity, some of which may correlate more closely with cardiovascular risk (see central obesity). For example, a powerful athlete may be classified as obese by his BMI due to increased musculature, while the BMI may fail to flag increased risk in an elderly person with low lean mass and excess central adiposity. In individual cases, a BMI score alone is therefore inadequate as a predictive tool.

An alternative way to determine obesity is to assess percent body fat. Doctors and scientists generally agree that men with more than 25 percent body fat and women with more than 30 percent body fat are obese. However, it is difficult to measure body fat precisely. The most accepted method has been to weigh a person underwater. But underwater weighing is a procedure limited to laboratories with special equipment. Two simpler methods for measuring body fat are the skinfold test, in which a pinch of skin is precisely measured to determine the thickness of the subcutaneous fat layer; or bioelectrical impedance analysis, usually only carried out at specialist clinics.

In practice, in most examples of overweight that may designate risk, both doctor and patient can see "by eye" whether excess fat is a concern. In these cases, BMI thresholds provide simple targets all patients can understand. Doctors may also use a simple measure of waist circumference (which is a better predictor of complications such insulin resistance due to visceral fat^[1]).

Such clinical data is rarely available in the statistical raw materials required for large public health studies, however — whereas height and weight is commonly recorded. For this essential reason, the BMI's most valuable function is in epidemiological studies, where it remains the most commonly-used, approach allowing for cross-border, longitudinal, and other types of comparative analysis.

Adapted from the Wikipedia article "Definition", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki