Body mass index(BMI) is a value derived from themass(weight) andheightof a person. The BMI is defined as thebody massdivided by thesquareof thebody height, and is universally expressed inunitsof kg/m2, resulting from mass inkilogramsand height inmetres.
The BMI may be determined using a table or chart which displays BMI as a function of mass and height using contour lines or colours for different BMI categories, and which may use other units of measurement (converted to metric units for the calculation).
The BMI is a convenientrule of thumbused to broadly categorize a person asunderweight,normal weight,overweight, orobesebased on tissue mass (muscle, fat, and bone) and height. That categorization is the subject of some debate about where on the BMI scale the dividing lines between categories should be placed. Commonly accepted BMI ranges are underweight (under 18.5kg/m2), normal weight (18.5 to 25), overweight (25 to 30), and obese (over 30).
A graph of body mass index as a function of body mass and body height. The dashed lines represent subdivisions within a major class.
BMIs under 20 and over 25 have been associated with higher all-causes mortality, with the risk increasing with distance from the 20–25 range.
History
Obesity and BMI
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".The modern term "body mass index" (BMI) for the ratio ofhuman body weightto squared height was coined in a paper published in the July 1972 edition of theJournal of Chronic DiseasesbyAncel Keysand 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".
The interest in an index that measuresbody fatcame with observed increasing obesity in prosperousWesternsocieties. Keys explicitly judged BMI as appropriate forpopulationstudies and inappropriate for individual evaluation. Nevertheless, due to its simplicity, it has come to be widely used for preliminary diagnoses.Additional metrics, such as waist circumference, can be more useful.
The BMI is universally 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) must be applied. When the term BMI is used informally, the units are usually omitted.
BMI provides a simple numeric measure of a person'sthicknessorthinness, 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 averagebody composition.For such individuals, the value recommendations as of 2014are as follows: a BMI from 18.5 up to 25kg/m2may indicate optimal weight, a BMI lower than 18.5 suggests the person isunderweight, a number from 25 up to 30 may indicate the person isoverweight, and a number from 30 upwards suggests the person isobese.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
BMI formula
The BMI (Body Mass Index) in (kg/m2) is equal to the mass in kilograms (kg) divided by the square height in meters (m):
BMI(kg/m2)= mass(kg)/ height2(m)
The BMI (Body Mass Index) in (kg/m2) is equal to the mass in pounds (lbs) divided by the square height in inches (in) times 703:
BMI(kg/m2)= mass(lb)/ height2(in)× 703
Children (aged 2 to 20)
BMI for age percentiles for boys 2 to 20 years of age.
BMI for age percentiles for girls 2 to 20 years of age.
BMI is used differently for children. It is calculated in the same way as for adults, but then compared to typical values for other children of the same age. Instead of comparison against fixed thresholds for underweight and overweight, the BMI is compared against thepercentilesfor children of the same sex and age.
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.
Recent studies in Britain have indicated that females between the ages 12 and 16 have a higher BMI than males of the same age by 1.0kg/m2on average.
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 ethnic groups, populations, and descent have different associations between BMI, percentage of body fat, and health risks, with a higher risk oftype 2 diabetes mellitusandatheroscleroticcardiovascular disease at BMIs lower than theWHOcut-off point for overweight, 25kg/m2, although the cut-off for observed risk varies among different populations. The cut-off for observed risk varies based on populations and subpopulations both in Europe and Asia.
United States
In 1998, the U.S.National Institutes of Healthand theCenters for Disease Control and Preventionbrought U.S. definitions in line withWorld Health Organizationguidelines, lowering the normal/overweight cut-off from BMI 27.8 to BMI 25. This had the effect of redefining approximately 29 million Americans, previouslyhealthy, tooverweight.
This can partially explain the increase in theoverweightdiagnosis in the past 20 years, and the increase in sales of weight loss products during the same time.WHOalso recommends lowering the normal/overweight threshold for South East Asian body types to around BMI 23, and expects further revisions to emerge from clinical studies of different body types.
The U.S. National Health and Nutrition Examination Survey of 2015-2016 showed that 71.6% of American men and women had BMIs over 25.Obesity—a BMI of 30 or more—was found in 39.8% of the US adults. A survey in 2007 showed 63% of Americans are overweight or obese, with 26% in the obese category (a BMI of 30 or more). As of 2014, 37.7% of adults in the United States were obese, categorized as 35.0% of men and 40.4% of women; class 3 obesity (BMI over 40) values were 7.7% for men and 9.9% for women.
| Body Mass Index values for males and females aged 20 and over, and selected percentiles by age: United States, 2011–2014. | ||||||||||
| Age | Percentile | |||||||||
| 5th | 10th | 15th | 25th | 50th | 75th | 85th | 90th | 95th | ||
| Men BMI (kg/m2) | ||||||||||
| 20 years and over (total) | 20.7 | 22.2 | 23.0 | 24.6 | 27.7 | 31.6 | 34.0 | 36.1 | 39.8 | |
| 20–29 years | 19.3 | 20.5 | 21.2 | 22.5 | 25.5 | 30.5 | 33.1 | 35.1 | 39.2 | |
| 30–39 years | 21.1 | 22.4 | 23.3 | 24.8 | 27.5 | 31.9 | 35.1 | 36.5 | 39.3 | |
| 40–49 years | 21.9 | 23.4 | 24.3 | 25.7 | 28.5 | 31.9 | 34.4 | 36.5 | 40.0 | |
| 50–59 years | 21.6 | 22.7 | 23.6 | 25.4 | 28.3 | 32.0 | 34.0 | 35.2 | 40.3 | |
| 60–69 years | 21.6 | 22.7 | 23.6 | 25.3 | 28.0 | 32.4 | 35.3 | 36.9 | 41.2 | |
| 70–79 years | 21.5 | 23.2 | 23.9 | 25.4 | 27.8 | 30.9 | 33.1 | 34.9 | 38.9 | |
| 80 years and over | 20.0 | 21.5 | 22.5 | 24.1 | 26.3 | 29.0 | 31.1 | 32.3 | 33.8 | |
| Age | Women BMI (kg/m2) | |||||||||
| 20 years and over (total) | 19.6 | 21.0 | 22.0 | 23.6 | 27.7 | 33.2 | 36.5 | 39.3 | 43.3 | |
| 20–29 years | 18.6 | 19.8 | 20.7 | 21.9 | 25.6 | 31.8 | 36.0 | 38.9 | 42.0 | |
| 30–39 years | 19.8 | 21.1 | 22.0 | 23.3 | 27.6 | 33.1 | 36.6 | 40.0 | 44.7 | |
| 40–49 years | 20.0 | 21.5 | 22.5 | 23.7 | 28.1 | 33.4 | 37.0 | 39.6 | 44.5 | |
| 50–59 years | 19.9 | 21.5 | 22.2 | 24.5 | 28.6 | 34.4 | 38.3 | 40.7 | 45.2 | |
| 60–69 years | 20.0 | 21.7 | 23.0 | 24.5 | 28.9 | 33.4 | 36.1 | 38.7 | 41.8 | |
| 70–79 years | 20.5 | 22.1 | 22.9 | 24.6 | 28.3 | 33.4 | 36.5 | 39.1 | 42.9 | |
| 80 years and over | 19.3 | 20.4 | 21.3 | 23.3 | 26.1 | 29.7 | 30.9 | 32.8 | 35.2 | |
