Obesity

Obesity is a condition in which the natural energy reserve of humans or other mammals, which is stored in fat tissue, is expanded far beyond usual levels to the point where it impairs health. Obesity in wild animals is relatively rare, but it is common in domestic animals like pigs and household pets who may be overfed and underexercised. In humans it is considered a major challenge to health.

Definition

Obesity is a concept that is being continually redefined. In humans, the most common statistical estimate of obesity 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.

Related Topics:
Body mass index - Kg - M² - 19th century - Belgian - Adolphe Quetelet

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Interpretation of the BMI:

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• A person with a BMI over 25.0 kg/m² is considered overweight.
• A BMI over 30.0 kg/m² denotes obesity.
• A further threshold at 40.0 kg/m² is identified as urgent morbidity risk (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).

Related Topics:
American Institute for Cancer Research - BMI - Underweight

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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. 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 future, healthy BMI for a given individual may be defined to some extent by his ethnic or racial origin.

Related Topics:
1998 - National Institutes of Health - WHO - 2000 - Asians - Epidemiological - African - African-American - Polynesian

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As a result of this somewhat arbitrary process, the BMI cannot offer a complete diagnosis, in that it ignores fat distribution within the body (see central obesity), and the relative fat-muscle-bone contributions to total body weight. A powerful athlete may be classified as obese by the BMI due to heavy musculature, while a false-normal may be diagnosed in the case of an elderly person with very low lean mass, which masks excess adiposity. On its own, a BMI score is therefore inadequate as a diagnostic tool.

Related Topics:
Central obesity - Body weight

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In practice, in most examples of overweightness that may be harmful to health, both doctor and patient can see "by eye" that fat is an issue. 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{{mn|Janssen|1}}); 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.

Related Topics:
Insulin resistance - Subcutaneous - Impedance

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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, BMI remains the most commonly-used approach for public health studies, and the most useful for cross-border, longitudinal, and other types of comparative analysis.

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