The limitations of using BMI for anorexia diagnosis

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)

BMI for anorexia: DSM-5

According to the DSM-5,  BMI criteria for 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.

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.

What can we use instead of BMI for anorexia nervosa diagnosis?

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. 

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.