body mass index

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n. (Abbr. BMI)

A measurement of the relative percentages of fat and muscle mass in the human body, in which mass in kilograms is divided by height in meters squared and the result used as an index of obesity.

Variant: body mass index

An index of obesity, given as the ratio (weight in kg)/(height in m)². A person with an index greater than 30 is officially obese.

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BMI

An index of fatness and obesity. The weight (in kg) divided by the square of height (in m). The acceptable (desirable) range is 20-25. Above 25 is overweight, and above 30 is obesity. BMI below the lower end of the acceptable range indicates undernutrition and wasting. Also called Quetelet's index. See also weight, ideal.

BMI; Quetelet index

An index of weight for height, calculated as:

BMI = weight in kg/ (height in metre)²

Measurements are for a subject barefooted and without clothes. You can use the above equation to calculate your own BMI (if measurements are made in pounds and inches, divide pounds by 2.2 to convert to kilograms, and divide inches by 39.4 to convert to metres).

The BMI has been used as a guideline for defining whether a person is overweight because it minimizes the effect of height, but it does not take into consideration other important factors, such as age and body build. The BMI has also been used as an indicator of obesity on the assumption that the higher the index, the greater the level of body fat. However, this assumption is not always true. Highly muscular people, such as body-builders, may have a high BMI but low fat content. Nevertheless, for most people, the BMI is a good way of determining a range of acceptable weights (figure 16).


Figure 16 Chart depicting body weight to height ratio

Most authorities use the following guidelines:

BMI	CONDITION
<20	underweight (may need to gain weight)
20-25	advisable range
25-30	overweight (some weight loss may be beneficial to health)
30-40	obese (need to lose weight)
>35	severely obese (urgent need to lose weight; advised to consult doctor)

The risk of developing diseases associated with obesity (e.g. high blood pressure and diabetes), do not appear to occur until the BMI exceeds 27, then there is a gradual increase in risk as the BMI increases. In practical terms, this means that the average woman, 1.63 metres (5 foot 4 inches) in height, should weigh between 52.7 and 67 kg (8 stone 4 pounds to 10 stone 8 pounds), but there is no health risk until she weighs more than 71.3 kg (11 stone 3 pounds). When the BMI exceeds 35, the risk of premature death is doubled.

health The ratio of a person's mass in kilograms to the square of their height in metres. Providing a rough correction for mass relative to height, it is approximately 20 for a ‘trim’ person, 25 for overweight, 30 for obese.

BMI; Quetelet index

An estimate of body composition used extensively as an indicator of obesity. BMI = weight in kg/ (height in metre)². The index does not correlate well with stature, but has a strong positive correlation with skinfold measurements. The assumption that the index is always a good reflection of body fat is not always true. Lean, but heavy individuals, such as weight-lifters, may have a high BMI, but they are not obese.

Tanita Body Mass Scale Index