Editing Body mass index (section)

==Limitations==
[[File:Correlation between BMI and Percent Body Fat for Men in NCHS' NHANES 1994 Data.PNG|right|thumb|upright=1.7|This graph shows the correlation between body mass index (BMI) and body fat percentage (BFP) for 8550 men in [[National Center for Health Statistics|NCHS]]' [[National Health and Nutrition Examination Survey|NHANES]] 1994 data. Data in the upper left and lower right quadrants suggest the limitations of BMI.<ref name="RomeroCorral2008">{{cite journal | vauthors = Romero-Corral A, Somers VK, Sierra-Johnson J, Thomas RJ, Collazo-Clavell ML, Korinek J, Allison TG, Batsis JA, Sert-Kuniyoshi FH, Lopez-Jimenez F | title = Accuracy of body mass index in diagnosing obesity in the adult general population | journal = International Journal of Obesity | volume = 32 | issue = 6 | pages = 959–966 | date = June 2008 | pmid = 18283284 | pmc = 2877506 | doi = 10.1038/ijo.2008.11 }}</ref>]]
The medical establishment<ref>{{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=July 8, 2007 |access-date=15 December 2013 |url-status=live |archive-url=https://web.archive.org/web/20131216071225/http://www.nhlbi.nih.gov/health/public/heart/obesity/lose_wt/risk.htm#limitations |archive-date=16 December 2013 }}</ref> and statistical community<ref>{{cite journal | vauthors = Kronmal RA | year = 1993 | title = Spurious correlation and the fallacy of the ratio standard revisited | journal = Journal of the Royal Statistical Society | volume = 156 | issue = 3| pages = 379–392 | doi = 10.2307/2983064 | jstor = 2983064 }}</ref> have both highlighted the limitations of BMI. 

=== Racial and gender differences ===
Part of the statistical limitations of the BMI scale is the result of Quetelet's original sampling methods.<ref name=":1">{{Cite book |last=Strings |first=Sabrina |url=http://worldcat.org/oclc/1256003500 |title=Fearing the black body : the racial origins of fat phobia |year=2019 |publisher=New York University Press |isbn=978-1-4798-9178-8 |oclc=1256003500}}</ref> As noted in his primary work, ''A Treatise on Man and the Development of His Faculties'', the data from which Quetelet derived his formula was taken mostly from Scottish Highland soldiers and French [[Gendarmerie]].<ref name=":0" /> The BMI was always designed as a metric for European men. For women, and people of non-European origin, the scale is often biased. As noted by sociologist Sabrina Strings, the BMI is largely inaccurate for black people especially, disproportionately labelling them as overweight even for healthy individuals.<ref name=":1" />{{Verify source|date=January 2025}} A 2012 study of BMI in an ethnically diverse population showed that "adult overweight and obesity were associated with an increased risk of mortality ... across the five racial/ethnic groups".<ref>{{cite journal | title = Body mass index and mortality in an ethnically diverse population: the Multiethnic Cohort Study | journal = The Health Board | date = 9 September 2023 | pmc = 4494097 | volume = 27 | issue = 7 | pages = 489–497 | doi = 10.1007/s10654-012-9695-5 | pmid = 22644110 | vauthors = Park SY, Wilkens LR, Murphy SP, Monroe KR, Henderson BE, Kolonel LN }}</ref>

===Scaling===
The BMI depends upon weight and the ''square'' of height. Since mass increases to the ''third power'' of linear dimensions, taller individuals with exactly the same body shape and relative composition have a larger BMI.<ref>{{cite journal | vauthors = Taylor RS | title = Letter to the editor | journal = Paediatrics & Child Health | volume = 15 | issue = 5 | pages = 258 | date = May 2010 | pmid = 21532785 | pmc = 2912631 | doi=10.1093/pch/15.5.258}}</ref> BMI is proportional to the mass and inversely proportional to the square of the height. So, if all body dimensions double, and mass scales naturally with the cube of the height, then BMI doubles instead of remaining the same. This results in taller people having a reported BMI that is uncharacteristically high, compared to their actual body fat levels. In comparison, the [[Ponderal index]] is based on the natural scaling of mass with the third power of the height.<ref>{{cite web | vauthors = Bonderud D | title = What is the Ponderal Index? | work = The Health Board | date = 9 September 2023 | url = https://www.infobloom.com/what-is-the-ponderal-index.htm}}</ref>

However, many taller people are not just "scaled up" short people but tend to have narrower frames in proportion to their height.<ref>{{cite journal | vauthors = Sperrin M, Marshall AD, Higgins V, Renehan AG, Buchan IE | title = Body mass index relates weight to height differently in women and older adults: serial cross-sectional surveys in England (1992-2011) | journal = Journal of Public Health | volume = 38 | issue = 3 | pages = 607–613 | date = September 2016 | pmid = 26036702 | pmc = 5072155 | doi = 10.1093/pubmed/fdv067 }}</ref> [[Carl Lavie]] has written that "The B.M.I. tables are excellent for identifying obesity and body fat in large populations, but they are far less reliable for determining fatness in individuals."<ref>{{cite web | vauthors = Brody JE | date = 31 August 2010 |url=https://www.nytimes.com/2010/08/31/health/31brod.html|title=Weight Index Doesn't Tell the Whole Truth|work=[[The New York Times]] |url-status=live| archive-url=https://web.archive.org/web/20170501153306/http://www.nytimes.com/2010/08/31/health/31brod.html |archive-date=1 May 2017}}</ref>

For US adults, exponent estimates range from 1.92 to 1.96 for males and from 1.45 to 1.95 for females.<ref>{{cite journal | title = Weight-height relationships and body mass index: some observations from the Diverse Populations Collaboration | journal = American Journal of Physical Anthropology | volume = 128 | issue = 1 | pages = 220–229 | date = September 2005 | pmid = 15761809 | doi = 10.1002/ajpa.20107 | author1 = Diverse Populations Collaborative Group }}</ref><ref>{{cite journal | vauthors = Levitt DG, Heymsfield SB, Pierson RN, Shapses SA, Kral JG | title = Physiological models of body composition and human obesity | journal = Nutrition & Metabolism | volume = 4 | pages = 19 | date = September 2007 | pmid = 17883858 | pmc = 2082278 | doi = 10.1186/1743-7075-4-19 | doi-access = free }}</ref>

===Physical characteristics===
The BMI overestimates roughly 10% for a large (or tall) frame and underestimates roughly 10% for a smaller frame (short stature). In other words, people with small frames would be carrying more fat than optimal, but their BMI indicates that they are ''normal''. Conversely, large framed (or tall) individuals may be quite healthy, with a fairly low [[body fat percentage]], but be classified as ''overweight'' by BMI.<ref>{{cite web|url=http://www.medicalnewstoday.com/articles/265215.php|title=Why BMI is inaccurate and misleading|work=Medical News Today|date=25 August 2013|url-status=live|archive-url=https://web.archive.org/web/20150723131349/http://www.medicalnewstoday.com/articles/265215.php|archive-date=2015-07-23}}</ref>

For example, a height/weight chart may say the ideal weight (BMI 21.5) for a {{convert|5|ft|10|in|m|order=flip|adj=mid|-tall}} man is {{convert|150|lb|kg|order=flip}}. But if that man has a slender build (small frame), he may be overweight at {{cvt|150|lb|kg|disp=or|order=flip}} and should reduce by 10% to roughly {{cvt|135|lb|kg|disp=or|order=flip}} (BMI 19.4). In the reverse, the man with a larger frame and more solid build should increase by 10%, to roughly {{cvt|165|lb|kg|disp=or|order=flip}} (BMI 23.7). If one teeters on the edge of small/medium or medium/large, common sense should be used in calculating one's ideal weight. However, falling into one's ideal weight range for height and build is still not as accurate in determining health risk factors as [[waist-to-height ratio]] and actual body fat percentage.<ref>{{cite web|url=http://www.medicalnewstoday.com/articles/255712.php|title=BMI: is the body mass index formula flawed?|work=Medical News Today|url-status=live|archive-url=https://web.archive.org/web/20150723095030/http://www.medicalnewstoday.com/articles/255712.php|archive-date=2015-07-23}}</ref>

Accurate frame size calculators use several measurements (wrist circumference, elbow width, neck circumference, and others) to determine what category an individual falls into for a given height.<ref>{{cite web | vauthors = Lewis T| date = 22 August 2013 |url=http://www.livescience.com/39097-bmi-not-accurate-health-measure.html|title=BMI Not a Good Measure of Healthy Body Weight, Researchers Argue |work=LiveScience.com |url-status=live |archive-url= https://web.archive.org/web/20150721123646/http://www.livescience.com/39097-bmi-not-accurate-health-measure.html |archive-date=2015-07-21 }}</ref> The BMI also fails to take into account loss of height through ageing. In this situation, BMI will increase without any corresponding increase in weight.

===Muscle versus fat===
Assumptions about the distribution between muscle mass and fat mass are inexact. BMI generally overestimates [[adipose tissue|adiposity]] on those with leaner body mass (e.g., athletes) and underestimates excess adiposity on those with fattier body mass.

A study in June 2008 by Romero-Corral et al. examined 13,601 subjects from the United States' third [[National Health and Nutrition Examination Survey]] (NHANES III) and found that BMI-defined obesity (BMI ≥ 30) was present in 21% of men and 31% of women. Body fat-defined obesity was found in 50% of men and 62% of women. While BMI-defined obesity showed high [[Sensitivity and specificity|specificity]] (95% for men and 99% for women), BMI showed poor [[Sensitivity and specificity|sensitivity]] (36% for men and 49% for women). In other words, the BMI will be mostly correct when determining a person to be obese, but can err quite frequently when determining a person not to be. Despite this undercounting of obesity by BMI, BMI values in the intermediate BMI range of 20–30 were found to be associated with a wide range of body fat percentages. For men with a BMI of 25, about 20% have a body fat percentage below 20% and about 10% have body fat percentage above 30%.<ref name="RomeroCorral2008"/>

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 alternative methods to measure obesity, such as the [[Classification of obesity#Body Volume Index|body volume indicator]].<ref>{{Cite journal |last1=Tahrani |first1=Abd |last2=Boelaert |first2=Kristien |last3=Barnes |first3=Richard |last4=Palin |first4=Suzanne |last5=Field |first5=Annmarie |last6=Redmayne |first6=Helen |last7=Aytok |first7=Lisa |last8=Rahim |first8=Asad |date=2008-04-01 |title=Body volume index: time to replace body mass index? |url=https://www.endocrine-abstracts.org/ea/0015/ea0015p104 |journal=Endocrine Abstracts |language=en |volume=15 |issn=1470-3947}}</ref>

===Variation in definitions of categories===
It is not clear where on the BMI scale the threshold for ''[[overweight]]'' and ''[[obesity|obese]]'' should be set. Because of this, the standards have varied over the past few decades. Between 1980 and 2000 the U.S. Dietary Guidelines have defined overweight at a variety of levels ranging from a BMI of 24.9 to 27.1. In 1985, the [[National Institutes of Health]] (NIH) consensus conference recommended that overweight BMI be set at a BMI of 27.8 for men and 27.3 for women.

In 1998, an NIH report concluded that a BMI over 25 is overweight and a BMI over 30 is obese.<ref name = "CNN_1998" /> In the 1990s the [[World Health Organization]] (WHO) decided that a BMI of 25 to 30 should be considered overweight and a BMI over 30 is obese, the standards the NIH set. This became the definitive guide for determining if someone is overweight.

One study found that the vast majority of people labelled 'overweight' and 'obese' according to current definitions do not in fact face any meaningful increased risk for early death. In a quantitative analysis of several studies, involving more than 600,000 men and women, the lowest mortality rates were found for people with BMIs between 23 and 29; most of the 25–30 range considered 'overweight' was not associated with higher risk.<ref>{{cite journal | vauthors = Campos P, Saguy A, Ernsberger P, Oliver E, Gaesser G | title = The epidemiology of overweight and obesity: public health crisis or moral panic? | journal = International Journal of Epidemiology | volume = 35 | issue = 1 | pages = 55–60 | date = February 2006 | pmid =  16339599| doi = 10.1093/ije/dyi254 | doi-access =  }}</ref>