Body mass index

The body mass index (BMI), or Quetelet index, is a statistical measurement which compares a person's weight and height. Though it does not actually measure the percentage of body fat, it is a useful tool to estimate a healthy body weight based on how tall a person is. Due to its ease of measurement and calculation, it is the most widely used diagnostic tool to identify obesity problems within a population. However it is not considered appropriate to use as a final indication for diagnosing individuals. [cite web
last = Zamore
first = Dulce ok...right..
date = 2005-05-05
accessdate = 2008-08-21
title = Do You Really Need to Lose Weight?
publisher = Medicinenet.com
url = http://www.medicinenet.com/script/main/art.asp?articlekey=56149&page=2
] It was invented between 1830 and 1850 by the Belgian polymath Adolphe Quetelet during the course of developing "social physics" [cite journal
quotes = yes
last=Eknoyan
first=Garabed
authorlink=
year=2008
month=Jan
title=Adolphe Quetelet (1796-1874)--the average man and indices of obesity
journal=Nephrol. Dial. Transplant.
volume=23
issue=1
pages=47–51
publisher = | location = yeah yeah
pmid = 17890752
doi = 10.1093/ndt/gfm517
bibcode = | oclc =| id = | url = | language = | format = | accessdate = | laysummary = | laysource = | laydate = | quote =
] .

Body mass index is defined as the individual's body weight divided by the square of their height. The formulas universally used in medicine produce a unit of measure of kg/m2:

Applications

Statistical device

The Body Mass Index is generally used as a means of correlation between groups related by general mass and can serve as a vague means of estimating adiposity. The duality of the Body Mass Index is that, whilst easy-to-use as a general calculation, it is limited in how accurate and pertinent the data obtained from it can be. Generally, the Index is suitable for recognising trends within sedentary or overweight individuals because there is a smaller margin for errors.Jeukendrup, A & Gleeson, M. (2005) "Sports Nutrition" Human Kinetics]

This general correlation is particularly useful for consensus data regarding obesity or various other conditions because it can be used to build a semi-accurate representation from which a solution can be stipulated, or the RDA for a group can be calculated. Similarly, this is becoming more and more pertinent to the growth of children, due to the majority of their exercise habits. [Barasi, M. E (2004) "Human Nutrition - a health perspective"]

The growth of children is usually documented against a BMI-measured growth chart. Obesity trends can be calculated from the difference between the child's BMI and the BMI on the chart. However, this method again falls prey to the obstacle of body composition: many children who primarily grow as endomorphs would be classed as obese despite body composition. Clinical professionals should take into account the child's body composition and defer to an appropriate technique such as densitometry e.g. Dual energy X-ray absorptiometry, also known as DEXA or DXA.

Clinical practice

BMI has been used by the WHO as the standard for recording obesity statistics since the early 1980s. In the United States, BMI is also used as a measure of underweight, owing to advocacy on behalf of those suffering with eating disorders, such as anorexia nervosa and bulimia nervosa.Fact|date=April 2007

BMI can be calculated quickly and without expensive equipment. However, BMI categories do not take into account many factors such as frame size and muscularity. The categories also fail to account for varying proportions of fat, bone, cartilage, water weight, and more.

Despite this, BMI categories are regularly regarded as a satisfactory tool for measuring whether sedentary individuals are "underweight," "overweight" or "obese" with various qualifications, such as: Individuals who are not sedentary being exempt - athletes, children, the elderly, the infirm, and individuals who are naturally endomorphic or ectomorphic (i.e., people who don't have a medium frame).

One basic problem, especially in athletes, is that muscle is denser than fat. Some professional athletes are "overweight" or "obese" according to their BMI - unless the number at which they are considered "overweight" or "obese" is adjusted upward in some modified version of the calculation. In children and the elderly, differences in bone density and, thus, in the proportion of bone to total weight can mean the number at which these people are considered "under"weight should be adjusted downward.

Medical underwriting

In the United States, where medical underwriting of private health insurance plans is widespread, most private health insurance providers will use a particular high BMI as a cut-off point in order to raise insurance rates for or deny insurance to higher-risk patients, thereby ostensibly reducing the cost of insurance coverage to all other subscribers in a 'normal' BMI range. The cutoff point is determined differently for every health insurance provider and different providers will have vastly different ranges of acceptability. Many will implement phased surcharges, in which the subscriber will pay an additional penalty, usually as a percentage of the monthly premium, for each arbitrary range of BMI points above a certain acceptable limit, up to a maximum BMI past which the individual will simply be denied admissibility regardless of price. This can be contrasted with group insurance policies which do not require medical underwriting and where insurance admissibility is guaranteed by virtue of being a member of the insured group, regardless of BMI or other risk factors that would likely render the individual inadmissible to an individual health planFact|date=April 2008.

Limitations and shortcomings

The medical establishment has generally acknowledged some shortcomings of BMI. [cite web|url=http://www.nhlbi.nih.gov/health/public/heart/obesity/lose_wt/risk.htm#limitations|title=Aim for a Healthy Weight: Assess your Risk|publisher=National Institutes of Health|date=2007-07-08] Because the BMI is dependent only upon weight and height, it makes simplistic assumptions about distribution of muscle and bone mass, and thus may overestimate adiposity on those with more lean body mass (e.g. athletes) while underestimating adiposity on those with less lean body mass (e.g. the elderly).

One recent study Romero-Corral et al. found that BMI-defined obesity was present in 19.1% of men and 24.7% of women, but that obesity as measured by bodyfat percentage was present in 43.9% of men and 52.3% of women. [Cite journal
author = A. Romero-Corral, V. K. Somers, J. Sierra-Johnson, R. J. Thomas, M. L. Collazo-Clavell, J. Korinek, T. G. Allison, J. A. Batsis, F. H. Sert-Kuniyoshi & F. Lopez-Jimenez
title = Accuracy of body mass index in diagnosing obesity in the adult general population
journal = International Journal of Obesity
volume = 32
issue = 6
pages = 959–956
year = 2008
month = June
doi = 10.1038/ijo.2008.11
pmid = 18283284
] Moreover, in the intermediate range of BMI (25-29.9), BMI failed to discriminate between bodyfat percentage and lean mass. The study concluded that "the accuracy of BMI in diagnosing obesity is limited, particularly for individuals in the intermediate BMI ranges, in men and in the elderly. . . . These results may help to explain the unexpected better survival in overweight/mild obese patients."

The exponent of 2 in the denominator of the formula for BMI is arbitrary. It is meant to reduce variability in the BMI associated only with a difference in size, rather than with differences in weight relative to one's ideal weight. If taller people were simply scaled-up versions of shorter people, the appropriate exponent would be 3, as weight would increase with the cube of height. However, on average, taller people have a slimmer build relative to their height than do shorter people, and the exponent which matches the variation best is between 2 and 3. An analysis based on data gathered in the USA suggested an exponent of 2.6 would yield the best fit. [ [http://www.math.utah.edu/~korevaar/ACCESS2003/bmi.pdf Power law fit to USA weight and height data] ] The exponent 2 is used instead by convention and for simplicity.

Some argue that the error in the BMI is significant and so pervasive that it is not generally useful in evaluation of health. [cite web|url=http://www.rockymounttelegram.com/featr/content/shared/health/stories/BMI_INDEX_0830_COX.html|title=Is obesity such a big, fat threat?|publisher=Cox News Service|date=2004-08-30|accessdate=2007-07-08] Due to these limitations, body composition for athletes is often better calculated using measures of body fat, as determined by such techniques as skinfold measurements or underwater weighing and the limitations of manual measurement have also led to new, alternative methods to measure obesity, such as the body volume index. However, recent studies of American football linemen who undergo intensive weight training to increase their muscle mass show that they frequently suffer many of the same problems as people ordinarily considered obese, notably sleep apnea. [Brown, David; "Linemen More Likely To Have Sleep Condition", in "The Washington Post", January 23, 2003] [ [http://www.healthonnet.org/News/HSN/601353.html Ex.NFL Linemen unusually prone to Heart Disease] ]

In an analysis of 40 studies involving 250,000 people, heart patients with normal BMIs were at higher risk of death from cardiovascular disease than people whose BMIs put them in the "overweight" range (BMI 25-29.9). [cite web|url=http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=16920472&query_hl=1&itool=pubmed_DocSum|title=Association of bodyweight with total mortality and with cardiovascular events in coronary artery disease: a systematic review of cohort studies.|publisher=Lancet|date=2006-08-19;368(9536):666-78|accessdate=2007-07-08] Patients who were underweight (BMI <20) or severely obese (BMI >35) did, however, have an increased risk of death from cardiovascular disease. The implications of this finding can be confounded by the fact that many chronic diseases, such as diabetes, can cause weight loss before the eventual death. In light of this, higher death rates among thinner people would be the expected result.Fact|date=July 2007

A further limitation relates to loss of height through aging. In this situation, BMI will increase without any corresponding increase in weight.

ee also

* Body volume index
* Waist-hip ratio
* Body fat percentage
* Body water
* Muscle
* Skeletal muscle
* Allometric law
* Ponderal index

References

External links

* U.S. National Center for Health Statistics [http://www.cdc.gov/growthcharts/ BMI Growth Charts for children and young adults] , BMI calculators for [http://apps.nccd.cdc.gov/dnpabmi/Calculator.aspx ages 2-19] and [http://www.cdc.gov/nccdphp/dnpa/bmi/adult_BMI/english_bmi_calculator/bmi_calculator.htm ages 20 and older] .
* [http://www.halls.md/bmi/bibliography.htm Collection of articles about the Body Mass Index]
* [http://www.chp.edu/clinical/03_weight_bmi.php Information on BMI and Children] via Childrens Hospital of Pittsburgh
* [http://junkfoodscience.blogspot.com/2007/11/fat-and-long-life-obesity-crisis-is.html Analysis of National Center for Health Statistics study on increased or decreased death rates for various BMIs]
* [http://www.healthyweightforum.org/eng/calculators/bmi-visual-graph/ The Body Mass Index in pictures]


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