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The Limitations of Diagnosing Anorexia with BMI

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The Limitations of Diagnosing Anorexia with BMI

What is BMI?

Body mass index (BMI) is a way of measuring a person’s weight compared to their height to get a general sense of whether or not they are a “healthy” weight or are at risk for developing weight related health issues. It is important to note that at Within Health, we believe using BMI to measure health, such as looking at “anorexia BMI” is an outdated, and severely reductive practice, as weight does not always equate to health. This article intends to educate readers on BMI, as well as its limitations, recognizing that there is health at every size (HAES). 

How is BMI Measured?

BMI is measured by dividing someone’s weight in kilograms (kg) by their height in meters (m). (1) This number provides a rough estimate of body fat based on weight and height for adults. Historically, this was created to predict someone’s “ideal” weight and subsequent cardiovascular health. (2)

Clinicians who use BMI use the following categories. (2)

Underweight = <18.5

Normal weight = 18.5 - 24.9

Overweight = 25-29.9

Obese = >30

According to the DSM-5, a diagnosis of anorexia nervosa (AN) or “anorexia BMI” requires that someone have a BMI of less than 18.5. (3) However, many experts argue that BMI alone is a very poor indicator of health, and should not be part of the criteria involved in the diagnosis of AN.

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The History of BMI as a Metric for Estimating Healthy Weight

A Belgium mathematician named Lambert Adolphe Jacques Quetelet first developed the BMI equation in the 1800s. He was not a physician, nor did he claim the number would be any direct measurement of health. Studies have since then shown that squaring the height of an irregularly shaped object, let alone a human, is not the way to accurately determine someone’s health. He developed this metric for large population studies as a way to gauge rates of obesity. (4,5)

In 1972, BMI later appeared as a medical headline in the Journal of Chronic Diseases. The author Ancel Keys argued that BMI was not “fully satisfactory,” but merely a good rule of thumb in helping to determine our body mass and measurement, while still searching for its meaning in health, disease, and survival. Despite clearly stating the usefulness of BMI in population studies, but not in individual evaluation, medical business and practitioners adopted BMI as the standard. (4,5)

Life and health insurance companies later adopted this measurement as a way to determine someone’s risk for cardiovascular disease and how much coverage he or she should receive. They compared the height and weight of their policyholders to illustrate that those who are ‘overweight’ died earlier than those of ‘normal’ weight. This did not take into account adipose tissue percentage or distribution, which more recent studies have shown largely determine health outcomes more so than BMI alone. (6,7)

Healthcare providers track BMI on medical charts for population health metrics, insurance, and billing information. If someone has a low or high BMI, this adds an additional ICD-10 code that can be billed for, and alerts the provider to screen for other health issues.

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Why BMI Is Becoming Recognized as a Poor Indicator for Weight Related Health Conditions

In the National Health and Nutrition Examination Survey, researchers evaluated over 40,000 adults from 2005-2012. They looked at blood pressure, triglycerides, cholesterol, glucose, insulin resistance, and many other labs. In this study, according to BMI nearly 50% of the “overweight” individuals, 29% of obese individuals and even 16% of obesity type 2 individuals were metabolically healthy. In contrast, over 30% of “normal” weight individuals were cardio-metabolically unhealthy. The study therefore concluded that using BMI categories as the main indicator of health creates an estimated 74,936,678 US adults misclassified as cardio metabolically unhealthy or cardio metabolically healthy.” (8)

BMI does not measure body fat directly, nor does it account for sex, ethnicity, muscle mass, or age. A BMI measurement cannot be used on children, as they grow tall during puberty at rates that may not directly match their weight. (9) A classic example is a weight lifter, who has a very high percentage of muscle. Muscle weighs slightly more than fat, thus weight lifters may have “high BMIs” but actually be very healthy from a cardio-metabolic standpoint. 

Thus, most clinicians now feel that, while BMI ranges are helpful, to give a general sense of a person’s risk for either malnutrition or obesity, it is important to keep in mind for inpatient treatment that numerous studies have illustrated how BMI often oversimplifies and thus misclassifies overall health.

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What Can We Use Instead of BMI to Diagnose Anorexia Nervosa?

If someone meets the DSM-5 criteria regarding the biopsychosocial factors, then he or she should be evaluated for an eating disorder, regardless of their exact BMI. 

According to the DSM-5, a diagnosis of anorexia nervosa requires having the following traits and behaviors: (3)

  • Intense fear of gaining weight with persistent behavior that interferes with weight gain, despite having a very low weight.
  • Severe restriction of energy intake that leads to a low body weight when in context of age, sex, development state, and physical health.
  • Poor insight on body weight on self-evaluation, persistent lack of recognition of the seriousness of his or her current body weight.
  • Amenorrhea (no longer having menstrual cycles) used to be a criteria in the DSM-4, but this has since been removed.

The guidelines further outline the following severity ranking based on BMI: (3)

  • Mild anorexia: 17 - 18.49 kg/m2
  • Moderate: 16 - 16.99 kg/m2
  • Severe: 15 - 15.99 kg/m2
  • Extreme: less than 15 kg/m2

Instead of using BMI, the following metrics may be better measure of weight and health: (6,12)

  • Use a tape measure to record waist circumference.
  • Waist-hip ratio. Measure the circumference at the level of the belly button and again at the thickest circumference of the thighs. Divide the waist/hip numbers (the units will not affect the result). 
  • Percent body fat.
  • Body composition scales, which are available at many health clinics and gyms.

External and internal (visceral) fat are absolutely necessary to support and protect the internal organs. It provides a buffer in the event someone becomes gravely sick and cannot eat for some time, or if someone experiences a serious high impact accident. (2)

In addition to seeking medical care, cognitive behavioral therapy and establishing a strong support system are key to recovering from anorexia nervosa. It is normal to sometimes feel overwhelmed by the BMI number, but treating the biopsychosocial factors are just as important when on the road to recovery.

At Within Health we offer virtual care for people with eating disorders, so they may be treated at home or wherever they need support. We work with our clients to accommodate their specific needs while treating them, and place thoughtful care into each and every session. If you’re interested in learning more about our program, call our team to get started.

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Frequently asked questions


  1. CDC: Adult BMI Calculator. 24 Nov, 2021. Accessed 20 Jan, 2022.
  2. The SuRF Report 2. The Surveillance of Risk Factors Report Series (SuRF). World Health Organization. 2005. p22. Accessed 20 Jan, 2022.
  3. Yager, J. (2020). Eating disorders: overview of epidemiology, clinical features, and diagnosis. UpToDate. Last updated Oct 8, 2020. Accessed Feb 24, 2022. 
  4. Keys A Karvonen; N Kimura; Taylor HL. Indices of relative weight and obesity. J Chronic Dis 1972;25:329–43. Reprinted Int J Epidemiol 2014. Accessed 20, Jan 2022. doi:10.1093/ije/dyu058.
  5. Blackburn H, Jacobs D (June 2014). "Commentary: Origins and evolution of body mass index (BMI): continuing saga.” International Journal of Epidemiology. 43 (3): 665–669. doi:10.1093/ije/dyu061.
  6. ​​D.C. Chan, G.F. Watts, P.H.R. Barrett, V. Burke. Waist circumference, waist-to-hip ratio and body mass index as predictors of adipose tissue compartments in men. QJM: An International Journal of Medicine, Volume 96, Issue 6, June 2003, Pages 441–447. Accessed 20 Jan, 2022.
  7. Flegal KM, Graubard BI, Williamson DF, Gail MH (April 2005). "Excess deaths associated with underweight, overweight, and obesity". JAMA. 293 (15): 1861–1867. doi:10.1001/jama.293.15.1861.
  8. Tomiyama, A., Hunger, J., Nguyen-Cuu, J. et al. Misclassification of cardiometabolic health when using body mass index categories in NHANES 2005–2012. Int J Obes 40, 883–886 (2016).
  9. Whitlock G, Lewington S, Sherliker P, Clarke R, Emberson J, Halsey J, Qizilbash N, Collins R, Peto R (March 2009). "Body-mass index and cause-specific mortality in 900 000 adults: collaborative analyses of 57 prospective studies". Lancet. 373 (9669): 1083–1096. doi:10.1016/S0140-6736(09)60318-4.
  10. Plevin, Rebecca. BMI: An unreliable scale that could alter your insurance premium. Impatient, KPCC. 10 Feb, 2016. Accessed 20 Jan, 2022. in children and teens. Medical News Today. 9 Nov, 2018. Accessed 20 Jan, 2022.
  11. Fetters, K Aleisha, MS, CSCS. 3 Ways to track your body composition that are better than BMI. US News and Health. 21 April, 2017. Accessed 20 Jan, 2022.
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