Body Mass Index (BMI)

<h1 id="XVKA-what-is-bmi" class="content-header content-header-h1">What is BMI</h1>
BMI, or Body Mass Index, is a simple formula using a person's height and weight to calculate a number which is supposedly representative of their level of body fat.
The formula itself is:
<ul>
<li>Weight (kg) / Height (m)2</li>
</ul>
or
<ul>
<li>(Weight (lbs) * 703) / Height (in)2</li>
</ul>
And the numerical value fits into the following categories:
<ul>
<li>Less than 18.5 is seen as underweight</li>
<li>18.5-24.9 is seen as average weight and is usually a target range</li>
<li>25-29.9 is seen as overweight</li>
<li>30 and above is seen as obese</li>
</ul>
In the obese category, it is further divided into 30-34.9 (Class I Obesity), 35-39.9 (Class II Obesity) and 40+ (Morbid Obesity). The first two classes are seen as at-risk populations whereas health complications associated with obesity are assumed in a state of Morbid Obesity.
<h2 id="Mg4E-obesity-as-defined-by-alternate-means" class="content-header content-header-h2">Obesity as defined by alternate means</h2>
BMI is not the only measurement of obesity, although it is the most commonly used.
Many studies report obesity rates as a function of body fat percentage (BF%), which is more accurate but requires special tools to assess; even then, the tools may not be highly accurate. 
The standard for obesity as defined by body fat percentage is greater than 25% body fat for men and greater than 35% for females; this is the World Health Organization's (WHO) reference standard made in 1995.<a href="#ref-1" id="ref-link-1" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;[No authors listed. &lt;a href="https://www.ncbi.nlm.nih.gov/pubmed/8594834" target="\_blank" rel="noopener noreferrer"&gt;Physical status: the use and interpretation of anthropometry. Report of a WHO Expert Committee&lt;/a&gt;.&lt;/p&gt;World Health Organ Tech Rep Ser (1995)&lt;/div&gt;'>[1]</a> A more recent (2009) proposal from the American Society of Bariatric Physicians suggest lowering the values to 25% and 30% for men and women, respectively, and these values are used in some studies.<a href="#ref-2" id="ref-link-2" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;&lt;a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3317663/" target="\_blank" rel="noopener noreferrer"&gt;Measuring Adiposity in Patients: The Utility of Body Mass Index (BMI), Percent Body Fat, and Leptin&lt;/a&gt;.&lt;/p&gt;&lt;/div&gt;'>[2]</a><a href="#ref-3" id="ref-link-3" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;Okorodudu DO, et al. &lt;a href="https://www.ncbi.nlm.nih.gov/pubmed/20125098" target="\_blank" rel="noopener noreferrer"&gt;Diagnostic performance of body mass index to identify obesity as defined by body adiposity: a systematic review and meta-analysis&lt;/a&gt;.&lt;/p&gt;Int J Obes (Lond) (2010)&lt;/div&gt;'>[3]</a>
<h1 id="M8jr-uses-of-bmi" class="content-header content-header-h1">Uses of BMI</h1>
BMI tends to be used in large-scale population research and surveys as it can be calculated from height and weight, either self-reported or taken quickly and non-invasively by a researcher.
It has the benefits of being quick, easy to calculate, and a measure of body fat in which most of the population will consent to (unlike calipers which are invasive due to touching skin with cold metal objects, and hydrostatic weighing which dunks people in water without oxygen in their lungs).
An exception to this is the NHANES series of studies, a large scale series of studies conducted in the US that assesses persons by calipers.
<h1 id="iTLb-accuracy-of-bmi-as-it-pertains-to-individuals" class="content-header content-header-h1">Accuracy of BMI as it pertains to individuals</h1>
<h2 id="bPZY-youth" class="content-header content-header-h2">Youth</h2>
In a study of 1,676 young girls (aged 5-16) it was found that although ethnicity differences existed in body fat that 89.9-92.4% of girls were accurately diagnosed with BMI.<a href="#ref-4" id="ref-link-4" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;Duncan JS, Duncan EK, Schofield G. &lt;a href="https://www.ncbi.nlm.nih.gov/pubmed/19786389" target="\_blank" rel="noopener noreferrer"&gt;Accuracy of body mass index (BMI) thresholds for predicting excess body fat in girls from five ethnicities&lt;/a&gt;.&lt;/p&gt;Asia Pac J Clin Nutr (2009)&lt;/div&gt;'>[4]</a> Results from NHANES 1999-2004 (three different NHANES surveys) found that 86.9%-89.1% of youth between the ages of 5-18 (both genders) were accurately diagnosed with BMI when compared against skin-fold calipers.<a href="#ref-5" id="ref-link-5" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;Laurson KR, Eisenmann JC, Welk GJ. &lt;a href="https://www.ncbi.nlm.nih.gov/pubmed/21961608" target="\_blank" rel="noopener noreferrer"&gt;Body Mass Index standards based on agreement with health-related body fat&lt;/a&gt;.&lt;/p&gt;Am J Prev Med (2011)&lt;/div&gt;'>[5]</a>
<h2 id="xtl7-adults" class="content-header content-header-h2">Adults</h2>
A cross-sectional study of 13,601 subjects in the US<a href="#ref-6" id="ref-link-6" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;&lt;a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2877506/" target="\_blank" rel="noopener noreferrer"&gt;Accuracy of Body Mass Index to Diagnose Obesity In the US Adult Population&lt;/a&gt;.&lt;/p&gt;&lt;/div&gt;'>[6]</a> compared BMI against BIA (Bioelectrical Impedence Analysis).
BMI defined 21% of men and 31% of women as obese, and BIA indicated 50% of men and 61% of women. Results from this study should be taken with a bit of scrutiny, as BIA is a measure of body fat with high variability based on hydration status.
A smaller scale study (1,691 persons) using DEXA scans (seen as a valid body fat measuring device) found that there was a 34.7% discrepancy between BMI and DEXA for women and 35.2% for men.<a href="#ref-7" id="ref-link-7" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;Kennedy AP, Shea JL, Sun G. &lt;a href="https://www.ncbi.nlm.nih.gov/pubmed/19360011" target="\_blank" rel="noopener noreferrer"&gt;Comparison of the classification of obesity by BMI vs. dual-energy X-ray absorptiometry in the Newfoundland population&lt;/a&gt;.&lt;/p&gt;Obesity (Silver Spring) (2009)&lt;/div&gt;'>[7]</a> However, BMI appeared to misclassify women as less fat as they were by DEXA; notable misclassifications include 20.3% of women being obese via BMI while DEXA showed 37.1%, 24.8% of men being obese via BMI compared with 38.4% of men being obese via DEXA. These results have been replicated in which persons in the normal BMI range were actually obese according to body fat percentage (20% of men, 9.2% of females) and more persons in the overweight BMI range were actually obese by body fat percentage (67.2% of men, 84.2% of females).<a href="#ref-8" id="ref-link-8" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;Peltz G, et al. &lt;a href="https://www.ncbi.nlm.nih.gov/pubmed/20737611" target="\_blank" rel="noopener noreferrer"&gt;The role of fat mass index in determining obesity&lt;/a&gt;.&lt;/p&gt;Am J Hum Biol (2010)&lt;/div&gt;'>[8]</a> High obesity rates in this study may be partially explained by socioeconomic issues, as it comprised Mexican persons (n=538) living in the southern USA. Finally, another study utilizing DEXA on a sample size of 1,393 persons found that 26% of persons were classified obese by BMI while 64% of persons were obese by DEXA; a misclassification rate (false negatives) of 25% for men and 48% for women was noted.<a href="#ref-2" id="ref-link-2" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;&lt;a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3317663/" target="\_blank" rel="noopener noreferrer"&gt;Measuring Adiposity in Patients: The Utility of Body Mass Index (BMI), Percent Body Fat, and Leptin&lt;/a&gt;.&lt;/p&gt;&lt;/div&gt;'>[2]</a>
This trend of BMI underreporting obesity has been replicated in a small (82) sample of active police officers,<a href="#ref-5" id="ref-link-5" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;Laurson KR, Eisenmann JC, Welk GJ. &lt;a href="https://www.ncbi.nlm.nih.gov/pubmed/21961608" target="\_blank" rel="noopener noreferrer"&gt;Body Mass Index standards based on agreement with health-related body fat&lt;/a&gt;.&lt;/p&gt;Am J Prev Med (2011)&lt;/div&gt;'>[5]</a> and for child-bearing aged women where BMI suggests 36.9% of these persons are obese and the WHO standard of 35% body fat indicates an obesity rate of 63.1%.<a href="#ref-9" id="ref-link-9" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;&lt;a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2886596/" target="\_blank" rel="noopener noreferrer"&gt;Accuracy of current body mass index obesity classification for white, black and Hispanic reproductive-age women&lt;/a&gt;.&lt;/p&gt;&lt;/div&gt;'>[9]</a>
One meta-analysis on the subject suggests that BMI fails to classify half of persons with excess body fat, reporting them as normal or overweight despite having a body fat percentage classifying them as obese.<a href="#ref-3" id="ref-link-3" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;Okorodudu DO, et al. &lt;a href="https://www.ncbi.nlm.nih.gov/pubmed/20125098" target="\_blank" rel="noopener noreferrer"&gt;Diagnostic performance of body mass index to identify obesity as defined by body adiposity: a systematic review and meta-analysis&lt;/a&gt;.&lt;/p&gt;Int J Obes (Lond) (2010)&lt;/div&gt;'>[3]</a>
<h1 id="TtDj-summation" class="content-header content-header-h1">Summation</h1>
<blockquote>
If you are normal weight or overweight according to BMI (18.5-29.9) there is still a chance you are actually obese, and thus is primarily due to low levels of lean mass (muscle, water, and glycogen).
If you are obese according to BMI, you are most likely obese according to body fat percentage as well. When sampling from the general population, over 95% of men and 99% of women identified as obese by BMI were obese via body fat levels.<a href="#ref-6" id="ref-link-6" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;&lt;a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2877506/" target="\_blank" rel="noopener noreferrer"&gt;Accuracy of Body Mass Index to Diagnose Obesity In the US Adult Population&lt;/a&gt;.&lt;/p&gt;&lt;/div&gt;'>[6]</a>
Outliers to this dataset, those who have enough lean mass to be classified as obese by BMI but not by body fat percentage, are far and few in society. These persons would normally be highly active athletes or dedicated 'weekend warriors', and it is unlikely sedentary persons or those with infrequent exercise habits would be these outliers.
</blockquote>

BMI is not the only measurement of obesity, although it is the most commonly used.Many studies report obesity rates as a function of body fat percentage (BF%), which is more accurate but requires special tools to assess; even then, the tools may not be highly accurate. The standard for obesity as defined by body fat percentage is greater than 25% body fat for men and greater than 35% for females; this is the World Health Organization's (WHO) reference standard made in 1995.<a href="#ref-1" id="ref-link-1" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;[No authors listed. &lt;a href="https://www.ncbi.nlm.nih.gov/pubmed/8594834" target="\_blank" rel="noopener noreferrer"&gt;Physical status: the use and interpretation of anthropometry. Report of a WHO Expert Committee&lt;/a&gt;.&lt;/p&gt;World Health Organ Tech Rep Ser (1995)&lt;/div&gt;'>[1]</a> A more recent (2009) proposal from the American Society of Bariatric Physicians suggest lowering the values to 25% and 30% for men and women, respectively, and these values are used in some studies.<a href="#ref-2" id="ref-link-2" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;&lt;a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3317663/" target="\_blank" rel="noopener noreferrer"&gt;Measuring Adiposity in Patients: The Utility of Body Mass Index (BMI), Percent Body Fat, and Leptin&lt;/a&gt;.&lt;/p&gt;&lt;/div&gt;'>[2]</a><a href="#ref-3" id="ref-link-3" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;Okorodudu DO, et al. &lt;a href="https://www.ncbi.nlm.nih.gov/pubmed/20125098" target="\_blank" rel="noopener noreferrer"&gt;Diagnostic performance of body mass index to identify obesity as defined by body adiposity: a systematic review and meta-analysis&lt;/a&gt;.&lt;/p&gt;Int J Obes (Lond) (2010)&lt;/div&gt;'>[3]</a>

BMI tends to be used in large-scale population research and surveys as it can be calculated from height and weight, either self-reported or taken quickly and non-invasively by a researcher.It has the benefits of being quick, easy to calculate, and a measure of body fat in which most of the population will consent to (unlike calipers which are invasive due to touching skin with cold metal objects, and hydrostatic weighing which dunks people in water without oxygen in their lungs).An exception to this is the NHANES series of studies, a large scale series of studies conducted in the US that assesses persons by calipers.

In a study of 1,676 young girls (aged 5-16) it was found that although ethnicity differences existed in body fat that 89.9-92.4% of girls were accurately diagnosed with BMI.<a href="#ref-4" id="ref-link-4" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;Duncan JS, Duncan EK, Schofield G. &lt;a href="https://www.ncbi.nlm.nih.gov/pubmed/19786389" target="\_blank" rel="noopener noreferrer"&gt;Accuracy of body mass index (BMI) thresholds for predicting excess body fat in girls from five ethnicities&lt;/a&gt;.&lt;/p&gt;Asia Pac J Clin Nutr (2009)&lt;/div&gt;'>[4]</a> Results from NHANES 1999-2004 (three different NHANES surveys) found that 86.9%-89.1% of youth between the ages of 5-18 (both genders) were accurately diagnosed with BMI when compared against skin-fold calipers.<a href="#ref-5" id="ref-link-5" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;Laurson KR, Eisenmann JC, Welk GJ. &lt;a href="https://www.ncbi.nlm.nih.gov/pubmed/21961608" target="\_blank" rel="noopener noreferrer"&gt;Body Mass Index standards based on agreement with health-related body fat&lt;/a&gt;.&lt;/p&gt;Am J Prev Med (2011)&lt;/div&gt;'>[5]</a>

A cross-sectional study of 13,601 subjects in the US<a href="#ref-6" id="ref-link-6" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;&lt;a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2877506/" target="\_blank" rel="noopener noreferrer"&gt;Accuracy of Body Mass Index to Diagnose Obesity In the US Adult Population&lt;/a&gt;.&lt;/p&gt;&lt;/div&gt;'>[6]</a> compared BMI against BIA (Bioelectrical Impedence Analysis).
BMI defined 21% of men and 31% of women as obese, and BIA indicated 50% of men and 61% of women. Results from this study should be taken with a bit of scrutiny, as BIA is a measure of body fat with high variability based on hydration status.A smaller scale study (1,691 persons) using DEXA scans (seen as a valid body fat measuring device) found that there was a 34.7% discrepancy between BMI and DEXA for women and 35.2% for men.<a href="#ref-7" id="ref-link-7" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;Kennedy AP, Shea JL, Sun G. &lt;a href="https://www.ncbi.nlm.nih.gov/pubmed/19360011" target="\_blank" rel="noopener noreferrer"&gt;Comparison of the classification of obesity by BMI vs. dual-energy X-ray absorptiometry in the Newfoundland population&lt;/a&gt;.&lt;/p&gt;Obesity (Silver Spring) (2009)&lt;/div&gt;'>[7]</a> However, BMI appeared to misclassify women as less fat as they were by DEXA; notable misclassifications include 20.3% of women being obese via BMI while DEXA showed 37.1%, 24.8% of men being obese via BMI compared with 38.4% of men being obese via DEXA. These results have been replicated in which persons in the normal BMI range were actually obese according to body fat percentage (20% of men, 9.2% of females) and more persons in the overweight BMI range were actually obese by body fat percentage (67.2% of men, 84.2% of females).<a href="#ref-8" id="ref-link-8" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;Peltz G, et al. &lt;a href="https://www.ncbi.nlm.nih.gov/pubmed/20737611" target="\_blank" rel="noopener noreferrer"&gt;The role of fat mass index in determining obesity&lt;/a&gt;.&lt;/p&gt;Am J Hum Biol (2010)&lt;/div&gt;'>[8]</a> High obesity rates in this study may be partially explained by socioeconomic issues, as it comprised Mexican persons (n=538) living in the southern USA. Finally, another study utilizing DEXA on a sample size of 1,393 persons found that 26% of persons were classified obese by BMI while 64% of persons were obese by DEXA; a misclassification rate (false negatives) of 25% for men and 48% for women was noted.<a href="#ref-2" id="ref-link-2" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;&lt;a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3317663/" target="\_blank" rel="noopener noreferrer"&gt;Measuring Adiposity in Patients: The Utility of Body Mass Index (BMI), Percent Body Fat, and Leptin&lt;/a&gt;.&lt;/p&gt;&lt;/div&gt;'>[2]</a>This trend of BMI underreporting obesity has been replicated in a small (82) sample of active police officers,<a href="#ref-5" id="ref-link-5" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;Laurson KR, Eisenmann JC, Welk GJ. &lt;a href="https://www.ncbi.nlm.nih.gov/pubmed/21961608" target="\_blank" rel="noopener noreferrer"&gt;Body Mass Index standards based on agreement with health-related body fat&lt;/a&gt;.&lt;/p&gt;Am J Prev Med (2011)&lt;/div&gt;'>[5]</a> and for child-bearing aged women where BMI suggests 36.9% of these persons are obese and the WHO standard of 35% body fat indicates an obesity rate of 63.1%.<a href="#ref-9" id="ref-link-9" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;&lt;a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2886596/" target="\_blank" rel="noopener noreferrer"&gt;Accuracy of current body mass index obesity classification for white, black and Hispanic reproductive-age women&lt;/a&gt;.&lt;/p&gt;&lt;/div&gt;'>[9]</a>One meta-analysis on the subject suggests that BMI fails to classify half of persons with excess body fat, reporting them as normal or overweight despite having a body fat percentage classifying them as obese.<a href="#ref-3" id="ref-link-3" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;Okorodudu DO, et al. &lt;a href="https://www.ncbi.nlm.nih.gov/pubmed/20125098" target="\_blank" rel="noopener noreferrer"&gt;Diagnostic performance of body mass index to identify obesity as defined by body adiposity: a systematic review and meta-analysis&lt;/a&gt;.&lt;/p&gt;Int J Obes (Lond) (2010)&lt;/div&gt;'>[3]</a>

Accuracy of BMI as it pertains to individuals

<blockquote>
If you are normal weight or overweight according to BMI (18.5-29.9) there is still a chance you are actually obese, and thus is primarily due to low levels of lean mass (muscle, water, and glycogen).
If you are obese according to BMI, you are most likely obese according to body fat percentage as well. When sampling from the general population, over 95% of men and 99% of women identified as obese by BMI were obese via body fat levels.<a href="#ref-6" id="ref-link-6" class="ref-link popover" data-tooltip='&lt;div&gt;&lt;p class="mb-3"&gt;&lt;a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2877506/" target="\_blank" rel="noopener noreferrer"&gt;Accuracy of Body Mass Index to Diagnose Obesity In the US Adult Population&lt;/a&gt;.&lt;/p&gt;&lt;/div&gt;'>[6]</a>
Outliers to this dataset, those who have enough lean mass to be classified as obese by BMI but not by body fat percentage, are far and few in society. These persons would normally be highly active athletes or dedicated 'weekend warriors', and it is unlikely sedentary persons or those with infrequent exercise habits would be these outliers.
</blockquote>

Body Mass Index (BMI)

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