# Body mass index

The body mass index ( BMI [ ˈbɒdi mæs ˈɪndɛks ]) - also body mass index ( KMI ), body mass number ( KMZ ) or Quetelet-Kaup index  - is a measure for evaluating a person's body weight in relation to their height . It was developed by Adolphe Quetelet in 1832 and by Ignaz Kaup after the First World War .

The BMI relates the body mass (English mass , colloquially weight ) to the square of the body height . The BMI is only a rough guide, as it does not take into account stature and gender, nor the individual composition of a person's body mass from fat and muscle tissue .

## calculation

The body mass index is calculated as follows:

${\ displaystyle {\ mathit {BMI}} = {\ frac {m} {l ^ {2}}}}$,

where the body mass (in kilograms ) and the body height (in meters ) are indicated. The BMI is given in the unit kg / m². ${\ displaystyle m}$${\ displaystyle l}$

## interpretation

According to the obesity classification of the World Health Organization (WHO), values ​​for people of normal weight are between 18.5 kg / m² and 25 kg / m². From 30 kg / m² people are considered obese and in need of treatment, in Germany 25% of the population, 9% of the population have the medical diagnosis of obesity and are therefore treated on an outpatient basis.

Weight classification in adults based on the BMI (according to WHO, as of 2008):

Weight classes depending on body mass and height (according to the BMI information below)
body weight category BMI (kg / m²) percent
survival is not possible <10.0 −54%
Underweight acute danger to life <12 −45%
Severe underweight grade II <13 −40%
Severe underweight grade I. < 13 , 0- 16 −26%
Increasing organic complications
Severe underweight
<16 −26%
Moderately underweight < 16 , 0- 17 −22%
Anorectic weight <17.5 −20%
Slightly underweight < 17 , 0- 18.5 −15%
Normal weight Normal weight < 18.5 - 25 100% = 21.75
Obesity Pre-obesity < 25 , 0- 30 + 15%
Obesity in need of treatment Grade I obesity < 30 , 0- 35 + 38%
Obesity grade II < 35 , 0- 40 + 61%
Obesity grade III <00.0 ≥ 40 + 84%
World records 148-155

Age and gender play an important role in the interpretation of the BMI. Men usually have a higher proportion of muscle mass in total body mass than women. That is why the lower and upper limits of the BMI value classes are slightly higher for men than for women. According to the German Nutrition Society, the normal value for men is in the range of 20 to 25 kg / m², while for women it is in the range of 19 to 24 kg / m². For seniors, the value may be higher with the years. Reason: Small fat reserves help the body to survive longer illnesses. From the age of 65, a BMI of up to 29 kg / m² is therefore still desirable.

The Broca index is also used to assess underweight , for example in the case of anorexia . The diagnostic criteria for anorexia provide a BMI of ≤ 17.5 kg / m² in adults, and a BMI below the 10th percentile of age in children and adolescents  .

### In children and adolescents

The BMI can also be used as a measure of healthy development in children and adolescents. The BMI is calculated using the same formula as the BMI of adults, but for children under 25 months of age, the length when lying down is used instead of the height when standing. This can be up to 0.7 cm longer than the standing height, which is why the normal BMI values ​​here in the tables show a characteristic kink. The child's BMI is compared in tables with the data of other children of the same age. The World Health Organization publishes BMI tables for boys and girls. A child with more than +1 standard deviation SD (corresponding to a BMI of over 25 in an adult) is considered overweight , as obese with more than +2 SD (corresponding to a BMI of over 30 in an adult). There are corresponding tables from the WHO for children under five years of age.

For Germany, the age percentiles published by the Working Group on Obesity in Children and Adolescents (AGA) are recommended for assessing the BMI. Current height and weight data of girls and boys aged 0 to 18 from different regions of Germany serve as reference values. A child is considered overweight if its BMI is higher than 90% (90th percentile of age) of its peers, and obese if its BMI is above the 97th percentile of age. A child is underweight if only 10% (10th percentile of age) or less have a lower BMI; a BMI below the 3rd age percentile is considered severely underweight.

The problem with this calculation base is that it would also shift the definition of undernourishment if the nutritional status of children in a society changes as a whole, for example if many children are undernourished due to a famine, or if there are many overweight children. If, according to the definition, exactly 15% of all children are always overweight, one cannot say, for example, that 25% of all children are overweight.

The limit values ​​of an appropriate BMI relate strongly to the child's level of development. For example, the rapid growth in length in the initial phase of puberty and the like are shown. If a child goes through these development phases earlier or later than the average, the BMI may be too high or too low, depending on the age group, despite normal weight.

## statistics

Color according to the above classification into underweight, normal weight, pre-obesity and the three degrees of obesity.

### Germany

Distribution in the population
aged 18 and over
( 2017 microcensus )
BMI total Men Women
average 26.0 26.7 25.1
<18.5 02.0% 00.8% 03.3%
18.5-25.0 45.3% 37.2% 53.6%
25.0-30.0 36.4% 44.0% 28.5%
≥ 30.0 16.3% 18.1% 14.6%

Proportion of overweight residents aged 18 and over
(microcensus)
BMI year total Men Women
≥25 1999 48% 56% 40%
2003 49% 58% 41%
2005 50% 58% 42%
2009 51% 60% 43%
2013 52% 62% 43%
2017 53% 62% 43%

### Austria

Distribution in the population aged 20 and over
(1999 microcensus)
BMI Men Women
<18.5 00.9% 03.3%
18.5-25.0 35.7% 66.3%
25.0-30.0 54.3% 21.3%
≥ 30.0 9.1% 9.1%

Distribution in the population aged 15 and over
(2007 microcensus)
BMI Men Women
<18.5 01.3% 03.7%
18.5-25.0 44.2% 55.0%
25.0-30.0 42.5% 28.6%
≥ 30.0 12.0% 12.7%

### Switzerland

Proportion of overweight population aged 15 and over
BMI year total
≥25 1992 30.3%
1997 34.9%
2002 37.0%
2007 37.3%

### Age Dependency (USA)

Body mass index for men and women over 20 years of age (USA, 2007–2010)
Coloring according to the above classification into normal weight, pre-obesity and the three degrees of obesity
Age Percentiles
5 10 15th 25th 50 75 85 90 95
Men's BMI - kg / m²
average 20.7 22.2 23.2 24.7 27.8 31.5 33.9 35.8 39.2
20-29 years 19.4 20.7 21.4 22.9 25.6 29.9 32.3 33.8 36.5
30–39 years 21.0 22.4 23.3 24.9 28.1 32.0 34.1 36.2 40.5
40-49 years 21.2 22.9 24.0 25.4 28.2 31.7 34.4 36.1 39.6
50–59 years 21.5 22.9 23.9 25.5 28.2 32.0 34.5 37.1 39.9
60–69 years 21.3 22.7 23.8 25.3 28.8 32.5 34.7 37.0 40.0
70–79 years 21.4 22.9 23.8 25.6 28.3 31.3 33.5 35.4 37.8
from 80 years 20.7 21.8 22.8 24.4 27.0 29.6 31.3 32.7 34.5

Age Percentiles
5 10 15th 25th 50 75 85 90 95
Women BMI - kg / m²
average 19.5 20.7 21.7 23.3 27.3 32.5 36.1 38.2 42.0
20-29 years 18.8 19.9 20.6 21.7 25.3 31.5 36.0 38.0 43.9
30–39 years 19.4 20.6 21.6 23.4 27.2 32.8 36.0 38.1 41.6
40-49 years 19.3 20.6 21.7 23.3 27.3 32.4 36.2 38.1 43.0
50–59 years 19.7 21.3 22.1 24.0 28.3 33.5 36.4 39.3 41.8
60–69 years 20.7 21.6 23.0 24.8 28.8 33.5 36.6 38.5 41.1
70–79 years 20.1 21.6 22.7 24.7 28.6 33.4 36.3 38.7 42.1
from 80 years 19.3 20.7 22.0 23.1 26.3 29.7 31.6 32.5 35.2

## mortality

The relationship between mortality and BMI is controversial among scientists.

A meta-analysis of studies published up to 2012 came to the conclusion that overweight people are 6% less likely to die than people of normal weight, even though a higher BMI correlates with illnesses. One speaks here of the obesity paradox . Other scientists accused Flegal of not having given sufficient consideration to aspects such as smoking or diseases . Smoking harms health and reduces weight at the same time.

A study of healthy whites who had never smoked found that people with a BMI between 20 and 25 had the lowest mortality rate. Flegal and others contradicted this: sorting out large groups leads to statistical errors.

A meta-analysis by the Global BMI Mortality Collaboration came to the conclusion that - if you exclude smokers, the chronically ill and those who died within the first 5 years of observation - a BMI between 20 and 25 has the lowest probability of death.

Of the 10 million data records, only 4 million were taken into account.

## Correction values ​​for missing limbs (amputation)

### Calculation method

If there is an amputation , the theoretical body mass must be calculated before calculating the BMI : ${\ displaystyle m _ {\ mathrm {t}}}$

${\ displaystyle m _ {\ mathrm {t}} = m \ cdot {\ frac {1} {1- \ sum k}}}$
Missing body part Correction value ${\ displaystyle k}$
hand 0.008
forearm 0.023
upper arm 0.035
foot 0.018
Lower leg 0.053
Thigh 0.116

### example

A woman weighs 56 kg and is 1.70 m tall. The woman's left lower leg was amputated, which is why the correction values ​​for a lower leg and a foot (logically removed by amputation of the lower leg) must be used. Your theoretical body weight is calculated as follows:

${\ displaystyle m _ {\ mathrm {t}} = 56 \, \ mathrm {kg} \ cdot {\ frac {1} {1- (0 {,} 053 + 0 {,} 018)}} \ approx 60 { ,} 28 \, \ mathrm {kg}}$
This mass can then be used in the normal BMI formula:
${\ displaystyle {\ frac {60 {,} 28 \, \ mathrm {kg}} {(1 {,} 7 \, \ mathrm {m}) ^ {2}}} \ approx 20 {,} 86 \, {\ frac {\ mathrm {kg}} {\ mathrm {m} ^ {2}}}}$

## history

The BMI was developed in 1832 by the Belgian mathematician Adolphe Quetelet . The designation body mass index (BMI) comes from an article by Ancel Keys published in 1972 . Keys only recommended the BMI for the statistical comparison of populations, not for assessing the obesity of individuals. The BMI gained importance through its use by US life insurers, who use this simple classification to calculate premiums for life insurance in such a way that additional risks from obesity are taken into account. The BMI has also been used by the World Health Organization since the early 1980s. The current BMI classification of the WHO has essentially existed since 1995.

In some German states (e.g. Baden-Württemberg and North Rhine-Westphalia ) the body mass index is used as a criterion for civil service in the public service. People with too high or too low a BMI are not civil servants. This regulation has been severely criticized on various occasions.

In anthropometric history and historical anthropology , the mean body mass index of population groups, similar to body size , is used as an indicator of standard of living. With the help of historical data, which was collected, for example, during recruiting, it is possible to look back at the past. Estimates of the BMI that were carried out on bones from archaeological contexts lead back to earlier times. From them it can be estimated that the average diet in the early Middle Ages of Europe was quite good.

## criticism

The use of the BMI to diagnose underweight or excess weight due to body fat based on fixed limit values ​​is very controversial. Because a relatively high body weight and thus a high BMI can also be caused by a lot of muscle mass, higher bone density , larger bone and joint diameters, larger shoulder width (for people of the same height in the decimeter range) and many other factors. This is particularly true of athletes. Well trained strength athletes without a lot of body fat have a high BMI due to their muscle mass alone. Endurance athletes (5 km run, 10 km run, marathon run) who took part in the 1960 Olympic Games in Rome had a BMI of 20-21, strength athletes (weightlifters, javelin throwers, hammer and discus throwers, shot putters) had one BMI from 26 to 29. Therefore, for the medical diagnosis of underweight and overweight, the standard of what is considered to be normal weight is adjusted if necessary. For paraplegics, for example, a reduction in the limit between normal and overweight from 30 kg / m² to 22 kg / m² was found.

## Other indices to better identify health risks

### Broca Index, Ponderal Index and Physique Development Index

In addition to the BMI, there are a number of other indices. The best known are the Broca Index and the Ponderal Index . The Build-development index of Wutscherk should even be suitable for a biological age determination.

According to an eight-year study by the Munich Ludwig Maximilians University with over 11,000 test subjects, the waist-to-height ratio (WtHR) is more suitable for assessing health risks , as more precise conclusions are drawn here can be drawn on the harmful abdominal fat percentage.

### Area Mass Index and Body Adiposity Index

In contrast to the BMI, the Area Mass Index (AMI) represents the ratio of body mass (coll .: body weight) to the actual body surface, whereby the body surface depends on the individual physique (stature) and gender of a person.

The body adiposity index (BAI) is another method with which the body fat percentage is to be calculated or estimated. This index, which became popular in 2011, takes into account not only body length but also hip circumference with the formula:

BAI = (hip circumference in cm) / (body length in m) 1.5 - 18.

A study by the Institute for Nutritional Research Potsdam-Rehbrücke and the Medical Clinic IV of the University of Tübingen in 2012 came to the result that the BAI is inferior to the BMI in terms of its informative value and the BMI, on the other hand, is more closely related to the body fat distribution - especially in men. The measured waist circumference, on the other hand, according to the same study, is even more meaningful about the percentage of body fat than the BMI or the BAI. The BMI was also superior to the BAI when it came to assessing the risk of diabetes, although the waist circumference was again even more meaningful. According to the study, the BAI is therefore not an alternative to the BMI, but measuring the waist circumference as a supplement to determining the BMI is useful.

The waist-to-hip ratio , which was originally introduced primarily as a body-aesthetic measure, as well as the measure of the body surface area according to the Mosteller formula should also enable an estimation of the body fat percentage. So-called vital analyzes are to be regarded as more commercially oriented .

### Body Shape Index (BSI)

The Body Shape Index ( BSI or ABSI ) is supposed to predict health risks better than the BMI by including the particularly harmful belly fat in the calculation. The ABSI-z value, which compares one's own value with the average values ​​of the population (in the USA) and thus determines an above or below average risk, is particularly meaningful. Diseases associated with increased belly fat include: B. heart attack, high blood pressure, stroke and arteriosclerosis. This measurement method is not suitable for pregnant women. The method developed in the USA is said to apply to black and white, but not to Mexican ethnic groups.

### Waist-to-Height Ratio

The waist-to-height ratio (WtHR 'waist-to-height ratio') describes the relationship between waist size and body size . Compared to the BMI, it should make a better statement about the distribution of body fat and thus allow greater informative value with regard to the health relevance of overweight.

## Statutory minimum BMI for professional models

To combat anorexia and the resulting deaths among models and other fashion-interested people, some countries have introduced a minimum BMI for professional models, the value of which must be checked by a doctor on a regular basis. In France and Spain this minimum value is 18 kg / m², in Israel and Italy 18.5 kg / m².

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