Body Mass Index (BMI)

By Douglas Bunnell, Ph.D, FAED, CEDS


The Body Mass Index (BMI) is a mathematical ratio of weight to height. First defined by a mathematician for use in establishing “normal” weight ranges for European males, it is generally used by statisticians and researchers to estimate general trends of weight in populations and groups.1  Yet, because it is a simple calculation, and relatively easy and inexpensive to collect the data necessary to make the calculation, BMI has now become the most widely used measure of individual weight status. 

Use with Individuals

The BMI was not designed to be used as a marker of an individual’s weight or health status. Yet, despite its’ original purpose as a general population statistic, BMI has become a commonly used measure in healthcare, weight-loss programs and eating disorder treatment programming to track changes in body weight and is often used to assess whether individuals are categorized as “underweight”, “overweight” or “obese.” 


Using BMI as an assessment for individuals is highly problematic. Because BMI only measures the ratio of weight to height it cannot account for other factors that influence weight and body composition. Most notably, BMI cannot account for an individual’s degree of musculature or bone density, which further diminishes its validity as a measure of weight status.2 

BMI is commonly used in medical practice to categorize individuals’ weight status and their personal risk for medical consequences thought to be associated with excess weight. These categories use arbitrary cut-offs scores to define different levels of risk despite ambiguous research on the actual risks associated with these different categories. These risk categories do not take race, ethnic and gender factors into account which further limits the usefulness of this measure for women, BIPOC, and gender non-binary individuals.3 Furthermore, since BMI is a unreliable marker of an individual’s health, it’s widespread use continues to reinforce weight categories that perpetuate weight stigmatization.

Despite its use in general medical practice, BMI has very little usefulness as a measure of any individual’s medical status. It should not be used as the basis for conclusions about medical risk or as a stand-alone treatment goal. 

Use of BMI in the Treatment of People with Eating Disorders

BMI is still used as a general and easily calculated marker of low weight for individuals being treated for anorexia nervosa. Other types of information, particularly an individual’s weight history, are thought to be more reliable in assessing someone’s nutritional status. BMI may be a useful initial screening assessment. For instance, a BMI below 18.5 is commonly used as a threshold for clinically significant low weight and should signal the need to do a more thorough assessment of the individual’s nutritional status. That assessment can involve medical tests and the review of that individual’s weight and growth trajectories. Many providers working with children, adolescents and young adults use pediatric growth charts to assess a patient’s weight changes over time in order to evaluate the significance of low BMI.

For low BMI patients who require nutritional rehabilitation and weight gain, BMI may be used as one consideration in tracking weight gain and in establishing weight gain goals. Once again though, BMI should not be used as a stand-alone measure of weight gain goals. These goals should involve a more thorough and personal assessment of each individual’s weight history, growth and medical status. 

Insurance companies typically use BMI to assess whether patients meet criteria for medical necessity for higher levels of treatment such as residential and partial hospital programs. While this use can be problematic, your treatment provider may need to track BMI in order to secure insurers’ authorizations for ongoing treatment. 

Questions for your treatment provider regarding BMI

  • What conclusions are you making about my personal medical status on the basis of my BMI?
  • What specific medical conditions are you addressing with your treatment recommendations?
  • What treatments for these conditions would you prescribe for someone with a lower BMI? 
  • What other ways are you measuring my health status besides my BMI? 
  • Are there specific markers of cardiac or diabetes risk?


[1] Blackburn, H., & Jacobs, D., Jr (2014). Commentary: Origins and evolution of body mass index (BMI): continuing saga. International journal of epidemiology, 43(3), 665–669.

[2] Rothman K. J. (2008). BMI-related errors in the measurement of obesity. International journal of obesity (2005), 32 Suppl 3, S56–S59.

[3] Richmond, T. K., Thurston, I., Sonneville, K., Milliren, C. E., Walls, C. E., & Austin, S. B. (2015). Racial/ethnic differences in accuracy of body mass index reporting in a diverse cohort of young adults. International journal of obesity (2005), 39(3), 546–548.