Obesity is a concept that is being continually redefined. In humans, the most common statistical estimate of obesity is the body mass index (BMI). Try our BMI Calculator.

A person with a BMI over 25.0 kg/m2 is considered overweight; a BMI over 30.0 kg/m2 is considered obese. A further threshold at 40.0 kg/m2 is identified as urgent morbidity risk. 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 BMI was created in the 19th century by the Belgian statistician Adolphe Quetelet. The cut-off points between categories are occasionally redefined, and may differ from country to country. In June 1998 the NIH brought official US 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” as a result.

The BMI cannot offer a complete diagnosis, in that it ignores fat distribution within the body (ie: Central Obesity: (or ‘apple-shaped’ or ‘masculine’ obesity), is when the main deposits of body fat are localised around the abdomen and the upper body.), 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. In practice, in most examples of overweight 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 – see Janssen et al, 2004); 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.

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.

For more overweight information:

Classification and Risks of Overweight and Obesity
Overweight and Obesity: What You Can Do

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