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The limitations of using BMI for anorexia diagnosis

The body mass index (BMI) is a calculation made by comparing someone's weight and height. It's sometimes used to help make medical diagnoses, particularly eating disorders like anorexia nervosa (AN) and bulimia nervosa (BN).

But over the years, the measurement has faced an increasing amount of pushback from members of the medical community, who believe it's too limited to truly account for the state of someone's health.

In the wake of this debate, it may be necessary to develop new and more accurate ways to assess someone for an eating disorder; determine their "ideal body weight"; or make medical recommendations based on their perceived state of health.

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to note that at Within Health, we believe using BMI to measure health, such as looking at “anorexia BMI” is an outdated, and severely reductive practice, as weight does not always equate to health.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).

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 minutes read
Last updated on 
July 12, 2023
July 12, 2023
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In this article
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What is BMI?

The body mass index is a diagnostic tool that measures the ratio between a person’s weight and height. The calculation is then compared to a chart—the index, made up of a range of results—to determine whether someone is considered "underweight," "normal weight," or "overweight."

According to the BMI, weight ranges are determined as follows:1

  • Underweight: BMI of <18.5
  • Normal weight: BMI of 18.5-24.9
  • Overweight: BMI of 25-29.9
  • Obese: BMI of 30 or higher

It's important to keep in mind, however, that these categories represent broad designations and do not account for the many other components that contribute to someone's overall health. For example, someone can be in the healthy BMI range but still exhibit other risk factors for medical complications.

How is the BMI used?

The BMI was developed in 1800s by a Belgian statistician named Lambert Quetelet, in his pursuit to find the dimensions of the "average man." He was looking to find a central point, in order to create a bell curve of common human measurements.3

It was nearly 100 years before the idea of health was tied to the measurement. In the 1970s, the concept was adapted to measure obesity among large populations. The World Health Organization began using the tool shortly thereafter in its tracking of public health, and its use soon became widespread.3,4

Still, while the measurement started off as a way to examine large groups of people, it's become more of a personal metric. Today, BMI is commonly used to estimate someone's body fat levels, aid in individual medical diagnoses, or act as a guidepost to monitor patients for certain medical conditions.

It's also become a favored statistic of health insurance companies and life insurance providers, which often include an individual's BMI in their risk assessment calculations.3 Though, some in the medical community are raising concerns over the measurement's use in evaluating individuals, arguing that it's too broad a tool to account for someone's overall health on its own.12


BMI and anorexia nervosa

Body mass index has also long been used in the diagnosis of eating disorders. The Diagnostic and Statistical Manual of Mental Disorders (DSM), the record of all officially-recognized mental health conditions, has previously designated specific BMI ranges for AN, BN, and other eating disorders.

In the DSM-5, the most recently updated version of the list, BMI is no longer considered a required criteria for being diagnosed with AN. But the measurement is used as a way to determine the severity of the condition, with certain BMI ranges corresponding to "severe anorexia nervosa," or cases that are considered "mild" or "moderate."2

But even with this change, there has been some pushback. At least one study brought into question the use of the measurement as a specifier for AN, concluding that body mass index made no substantial difference when considering the duration of illness, presence of certain clinical characteristics, or hospitalization outcomes for patients with the condition.8

woman looking at self in a mirror

Issues with BMI as a diagnostic tool

Many studies considering the link between BMI and anorexia nervosa will acknowledge that the measurement does seem to correlate with the intensity of treatment needed for AN patients, with those who have a very low BMI, or "extreme anorexia," typically requiring more malnutrition-related attention.13

But in terms of diagnosing anorexia nervosa—or other medical conditions—BMI is only one small piece of the puzzle, and it may actually be one of the least informative health metrics to follow.

While someone's BMI is sometimes used to estimate their body fat percentage, the measurement does not measure body fat directly. It fails to take into account the density of muscles and bones, which are more dense than fat, and can lead to misleading numbers, even in someone presenting with a normal BMI.9

The averages represented by the body mass index also dilute or fail to take into account age, sex, ethnicity, and other factors that may contribute to what is considered a healthy weight for someone. In particular, this can make it difficult to diagnose children or adolescents with AN, as growth spurts during puberty frequently place them outside of BMI averages.6

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Other ways to diagnose anorexia nervosa

Anorexia nervosa is often marked by extremely low body weight, but there are a number of other criteria listed by the DSM-5, including:2

  • An intense fear of gaining weight, with persistent behavior that interferes with weight gain.
  • Severe restriction of energy intake, relative to the needs for someone of that age, sex, development state, and physical health.
  • Poor insight on body weight, disturbed body image, and/or low sense of self-worth or self-esteem, based on these misconceptions

Someone presenting with these traits may not have anorexia nervosa—just as someone with a certain BMI may or may not have the condition—but they are strong indicators of a serious problem.

BMI alternatives

Low body weight is often considered a key factor in diagnosing AN, despite it not occurring in all cases. But instead of using BMI to classify "low" body weight, there are other methods to make that assessment, including:7

  • Using a tape measure to record someone's waist circumference.
  • Calculating someone's waist-hip ratio.
  • Finding someone's true body fat percentage.
  • Using body composition scales.
Remote treatment for anorexia nervosa

Regardless of how it presents, anorexia nervosa is a dangerous and potentially deadly condition. If you or a loved one are struggling, it's important to seek out help.

At Within Health, our multidisciplinary team of experts offers a variety of care to help treat eating disorders. We will work with you to accommodate your specific needs, and our virtual program means you can find treatment without having to leave home. If you’re interested in learning more about our program, call our team to get started.

Call (866) 293-0041

Disclaimer about "overeating": Within Health hesitatingly uses the word "overeating" because it is the term currently associated with this condition in society, however, we believe it inherently overlooks the various psychological aspects of this condition which are often interconnected with internalized diet culture, and a restrictive mindset about food. For the remainder of this piece, we will therefore be putting "overeating" in quotations to recognize that the diagnosis itself pathologizes behavior that is potentially hardwired and adaptive to a restrictive mindset.

Disclaimer about weight loss drugs: Within does not endorse the use of any weight loss drug or behavior and seeks to provide education on the insidious nature of diet culture. We understand the complex nature of disordered eating and eating disorders and strongly encourage anyone engaging in these behaviors to reach out for help as soon as possible. No statement should be taken as healthcare advice. All healthcare decisions should be made with your individual healthcare provider.


  1. Body Mass Index (BMI). Cleveland Clinic. Accessed June 2023. 
  2. Table 19DSM-IV to DSM-5 Anorexia Nervosa Comparison. Substance Abuse and Mental Health Services Administration. Accessed June 2023. 
  3. Nuttall F. Q. (2015). Body Mass Index: Obesity, BMI, and Health: A Critical Review. Nutrition Today, 50(3), 117–128. 
  4. 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.
  5. Brazier, Y. (2018, November 9). BMI in children and teens. Medical News Today. Accessed January 2022.
  6. Fetters, K. (2017, April 21). 3 Ways to track your body composition that are better than BMI. US News and Health. Accessed January 2022.
  7. Shmerling, R. (2023, May 5). How useful is the body mass index (BMI)? Harvard Medical School. Accessed June 2023. 
  8. Toppino, F., Longo, P., Martini, M., Abbate-Daga, G., & Marzola, E. (2022). Body Mass Index Specifiers in Anorexia Nervosa: Anything below the "Extreme"?. Journal of Clinical Medicine, 11(3), 542.
  9. Ricciotti, H. (2016, June 16). Is body mass index (BMI) still the best measure of body fat? Harvard Medical School. Accessed June 2023.


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Further reading

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