Saturday, July 18, 2009

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Body mass index ( BMI )

The body mass index (BMI), or Quetelet index, is a controversial statistical measurement which compares a person's weight and height. BMI provided a simple numeric measure of a person's "fatness" or "thinness".

The formulae universally used in medicine produce a unit of measure of kg/m2. BMI is calculated by dividing the subject's mass by the square of his or her height, typically expressed either in metric or US "Customary" units:

Metric: BMI = kilograms ( weight ) / meters2 ( height )

US/Customary and imperial: BMI = lb ( weight ) * 703 / in2 ( height )

Note : lb is the subject's weight in pounds and in is the subject's height in inches.

The most commonly used definitions, established by the WHO in 1997 and published in 2000, provide the values listed bellow

BMI Classification
< 18.5underweight
18.5–24.9 normal weight
25.0–29.9 overweight
30.0–34.9 class I obesity
35.0–39.9 class II obesity
>40.0 class III obesity

Body fat percentage

Body fat percentage is total body fat expressed as a percentage of total body weight. It is generally agreed that men with more than 25% body fat and women with more than 33% body fat are obese. Body fat percentage can be estimated from a person's BMI by the following formula:

Bodyfat% = (1.2 * BMI) + (0.23 * age) − 5.4 − (10.8 * gender)

Note : gender is 0 if female and 1 if male

This formula takes into account the fact that body fat percentage is 10 percentage points greater in women than in men for a given BMI. It recognizes that a person's percentage body fat increases as they age even if their weight remains constant. The results of this formula have an accuracy of 4%.

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