Eating disorders are complex mental health illnesses with many potential signs, symptoms, and contributing factors. BMI is derived from dividing the mass of a person by their height. It has long been a tool for defining what is assumed to be “healthy” body weight, as well as “obesity,” and for diagnosing an eating disorder.
However, mounting evidence discredits using BMI to measure health challenges the current paradigm of health and disputes current criteria in the Diagnostic and Statistical Manual for anorexia nervosa.
The idea that there is a normal weight or ideal body weight is dangerous and can lead to severe anorexia nervosa and other eating disorders.
Using BMI to Screen for Anorexia
BMI is traditionally one of the main diagnostic criteria that medical and mental health practitioners define as the severity of an eating disorder. Current beliefs about anorexia hold that the lower a person’s BMI, the more potential medical complications can arise. Those with extremely low BMI, and therefore classified as having extreme anorexia, have a higher risk for more serious medical complications, such as irregular heart rate, low bone mineral density, increased liver function, anemia, edema, and low blood sugar. 
However, studies have shown that the levels of anorexia severity as defined by BMI do not accurately indicate the severity of the disorder.  There are many other factors involved in the onset of anorexia and its progression. Low BMI should, therefore, not be used as a measure for diagnosing the disorder or classifying its severity.
What Is BMI?
BMI, or body mass index, divides weight in kilograms by the square of height in meters as a screening method for an arbitrarily determined weight category–and, by extension, according to current assumptions conflating weight and health, your health. Created in the 19th century, the concept of BMI was first intended to measure the “average” or “ideal” man based on data gathered from white European men—a very narrow population sample. 
It’s important to remember that a “healthy” BMI range is not an indication of healthy weight, as each individual is different and has a different body.
Is BMI a Good Metric?
There are several reasons why BMI is not a good metric to measure health in general.  Many factors influence BMI that a simple height-weight ratio does not take into account, including:
- Sex: Women tend to have more body fat than men
- Ethnicity: Different racial and ethnic groups can have varying levels of body fat
- Age: Older people often have higher levels of body fat than younger people
- Muscle mass, body composition, bone density, and fat distribution: These all vary depending on sex, race, and ethnicity. Plus, athletes often have a low level of body fat but can be heavier due to more muscle mass and, therefore, have a higher BMI
But perhaps most importantly, people in higher-weight bodies can have anorexia and life-threatening complications. Yet, they often are not diagnosed or misdiagnosed because of the common misconception that only thin people can have anorexia. And thanks to our fatphobic diet and fitness-focused culture, what are praised as being “healthy” habits can lead to eating disorders and go undetected until someone experiences life-threatening complications. 
All weight gain isn’t necessarily unhealthy weight gain.
You simply can’t measure how “healthy” a person is just by looking at them or by comparing them to a “normal” BMI.
Actual metabolic indicators should be used to assess a person’s health. They include blood sugar levels, blood pressure, cholesterol levels, and any vitamin and mineral deficiencies that can indicate malnourishment and cause serious medical complications.
Does a Low BMI Represent a Sign of Anorexia?
While a low BMI can be a sign of anorexia for some people, there are other signs to consider as well. For example, someone with anorexia will regularly restrict their food and calorie intake, be obsessed with food and weight, and have a distorted body image. This means that they will often think they are fat no matter what they look like and even if they are at a low body weight.
Additional signs of anorexia can include behavioral concerns, such as:
- Social withdrawal and isolation
- Unusual eating habits
- Skipping meals
- Talking often about being fat and food choices
- Refusing to eat in public or with others
- Making excuses to not eat
Emotional signs of anorexia include:
What to Do if You Suspect Anorexia
If you suspect someone you love may have anorexia, early intervention, and treatment are essential for a quicker recovery with fewer potential medical and mental health complications.
With remote treatment options, getting help for anorexia is now easier than ever.
It’s important to research anorexia, know the signs, and understand the potentially contributing factors, as well as treatment options. Because those who suffer from anorexia are often in denial about their condition and how serious it can be, it’s essential to inform yourself so you can have a safe, supportive, and private conversation with your loved one. The goal is to help them realize they have a serious illness that will not go away on its own and to seek professional treatment as soon as possible.
Treatment programs can combine medical interventions to help stabilize serious complications that require monitoring with behavioral therapies and counseling to manage the behavioral and psychological aspects of an eating disorder. It is challenging but possible to sustain a long and healthy recovery.
- A Healthy Lifestyle: WHO Recommendations. (May 2010). World Health Organization (WHO).
- Don’t use body mass index to determine whether people are healthy, UCLA-led study says. (2016). UCLA Health.
- Gibson D, Watters A, Cost J, et al. (2020). Extreme anorexia nervosa: medical findings, outcomes, and inferences from a retrospective cohort. Journal of Eating Disorders; 8(25).
- Evaluation of the DSM-5 Severity Indicator for Anorexia Nervosa. (May 2017). European Eating Disorders Review.
- About Adult BMI. (June 2022). Centers for Disease Control and Prevention (CDC).
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- Disordered Eating & Dieting. (n.d.) National Eating Disorders Collaboration. Retrieved March 25, 2023.
- Allen K, Mountford V, Brown A, Richards K, Grant N, Austin A, Glennon D, & Schmidt U. (2020). First episode rapid early intervention for eating disorders (FREED): From research to routine clinical practice. Early Intervention in Psychiatry; 14(5):625–630.