Body mass index (BMI) is a value derived from the mass (weight) and height of a person. BMI is calculated as the body mass, in kilograms (kg), divided by the square of the body height, in square metres (m2); although the quotient has units of kilograms per square metre, BMI is most often reported normalized by 1 kg/m2, thus as a pure number.
Medical diagnostic methodBody mass index (BMI) is a value derived from the mass (weight) and height of a person. BMI is calculated as the body mass, in kilograms (kg), divided by the square of the body height, in square metres (m2); although the quotient has units of kilograms per square metre, BMI is most often reported normalized by 1 kg/m2, thus as a pure number.
The BMI is a convenient rule of thumb used to broadly categorize a person as based on tissue mass (muscle, fat, and bone) and height. Major adult BMI classifications are: underweight (under 18.5), normal weight (18.5 to 24.9), overweight (25 to 29.9), and obese (30 or more).[1] When used to predict an individual's health, rather than as a statistical measurement for groups, the BMI has limitations that can make it less useful than some of the alternatives, especially when applied to individuals with abdominal obesity, short stature, or high muscle mass.
BMI values under 20 and over 25 have been associated with higher all-cause mortality, with the risk increasing with distance from the 20–25 interval.[2]
History
Adolphe Quetelet, a Belgian astronomer, mathematician, statistician, and sociologist, devised the basis of the BMI between 1830 and 1850 as he developed what he called "social physics".[3] Quetelet himself never intended for the index, then called the Quetelet Index, to be used as a means of medical assessment. Instead, it was a component of his study of l'homme moyen, or the average man. Quetelet thought of the average man as a social ideal, and developed the body mass index as a means of discovering the socially ideal human person.[4] According to Lars Grue and Arvid Heiberg in the Scandinavian Journal of Disability Research, Quetelet's idealization of the average man would be elaborated upon by Francis Galton a decade later in the development of eugenics.[5]
The modern term "body mass index" (BMI) for the ratio of human body weight to squared height was coined in a paper published in the July 1972 edition of the Journal of Chronic Diseases by Ancel Keys and others. In this paper, Keys argued that what he termed the BMI was "if not fully satisfactory, at least as good as any other relative weight index as an indicator of relative obesity".[6][7][8]
The interest in an index that measures body fat came with observed increasing obesity in prosperous Western societies. Keys explicitly judged BMI as appropriate for population studies and inappropriate for individual evaluation. Nevertheless, due to its simplicity, it has come to be widely used for preliminary diagnoses.[9] Additional metrics, such as waist circumference, can be more useful.[10]
Calculation
B M I = mass kg height m 2 = mass lb height in 2 × 703 {\displaystyle \mathrm {BMI} ={\frac {{\text{mass}}_{\text{kg}}}{{{\text{height}}_{\text{m}}}^{2}}}={\frac {{\text{mass}}_{\text{lb}}}{{{\text{height}}_{\text{in}}}^{2}}}\times 703}
The BMI is expressed in kg/m2, resulting from mass in kilograms and height in metres. If pounds and inches are used, a conversion factor of 703 (kg/m2)/(lb/in2) is applied. (If pounds and feet are used, a conversion factor of 4.88 is used.) When the term BMI is used informally, the units are usually omitted.
BMI provides a simple numeric measure of a person's thickness or thinness, allowing health professionals to discuss weight problems more objectively with their patients. BMI was designed to be used as a simple means of classifying average sedentary (physically inactive) populations, with an average body composition.[11] For such individuals, the BMI value recommendations as of 2014[update] are as follows: 18.5 to 24.9 kg/m2 may indicate optimal weight, lower than 18.5 may indicate underweight, 25 to 29.9 may indicate overweight, and 30 or more may indicate obese.[9][10] Lean male athletes often have a high muscle-to-fat ratio and therefore a BMI that is misleadingly high relative to their body-fat percentage.[10]
The BMI may be determined first by measuring its components by means of a weighing scale and a stadiometer. The multiplication and division may be carried out directly, by hand or using a calculator, or indirectly using a lookup table (or chart).[12] The table displays BMI as a function of mass and height and may show other units of measurement (converted to metric units for the calculation).[a] The table may also show contour lines or colours for different BMI categories.
Categories
A common use of the BMI is to assess how far an individual's body weight departs from what is normal for a person's height. The weight excess or deficiency may, in part, be accounted for by body fat (adipose tissue) although other factors such as muscularity also affect BMI significantly (see discussion below and overweight).[13]
The WHO regards an adult BMI of less than 18.5 as underweight and possibly indicative of malnutrition, an eating disorder, or other health problems, while a BMI of 25 or more is considered overweight and 30 or more is considered obese.[1] In addition to the principle, international WHO BMI cut-off points (16, 17, 18.5, 25, 30, 35 and 40), four additional cut-off points for at-risk Asians were identified (23, 27.5, 32.5 and 37.5).[14] These ranges of BMI values are valid only as statistical categories.
Children and youth
BMI is used differently for people aged 2 to 20. It is calculated in the same way as for adults but then compared to typical values for other children or youth of the same age. Instead of comparison against fixed thresholds for underweight and overweight, the BMI is compared against the percentiles for children of the same sex and age.[15]
A BMI that is less than the 5th percentile is considered underweight and above the 95th percentile is considered obese. Children with a BMI between the 85th and 95th percentile are considered to be overweight.[16]
Studies in Britain from 2013 have indicated that females between the ages 12 and 16 had a higher BMI than males of the same age by 1.0 kg/m2 on average.[17]
International variations
These recommended distinctions along the linear scale may vary from time to time and country to country, making global, longitudinal surveys problematic. People from different populations and descent have different associations between BMI, percentage of body fat, and health risks, with a higher risk of type 2 diabetes mellitus and atherosclerotic cardiovascular disease at BMIs lower than the WHO cut-off point for overweight, 25 kg/m2, although the cut-off for observed risk varies among different populations. The cut-off for observed risk varies based on populations and subpopulations in Europe, Asia and Africa.[18][19]
Hong KongThe Hospital Authority of Hong Kong recommends the use of the following BMI ranges:[20]
