The limitations of using BMI for anorexia diagnosis

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."

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

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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

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

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.