Anorexia is a complex eating disorder with many potential signs, symptoms, and contributing factors. Body mass index (BMI) is derived from dividing the mass of a person by their height. It has long been a tool for defining healthy body weight as well as obesity and for diagnosing an eating disorder.
BMI should not be used as a sole method of indicating anorexia, however. The mental disorder will involve emotional and behavioral aspects as well as physical ones.
BMI & Anorexia
A healthy BMI is defined by the World Health Organization as between 18.5 and 24.9 kg/m2 for adults ages 20 and older. Anything below this can indicate that someone is underweight, and anything above this is considered overweight to obese.
BMI is traditionally one of the main ways that medical and mental health practitioners define an eating disorder.
BMI is used as a diagnostic criterion for anorexia and its severity based on the following:
- Mild anorexia: BMI greater than or equal to 17.5 kg/m2.
- Moderate anorexia: BMI between 16 and 16.99 kg/m2.
- Severe anorexia: BMI between 15 and 15.199 kg/m2.
- Extreme anorexia: BMI less than 15 kg/m2.
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
- Low blood sugar
Studies have shown that the levels of anorexia severity as defined by BMI are not actually indicative of the severity of the disorder, however. There are many other factors involved in the onset of anorexia and its progression. Low BMI should therefore not be the only measure for diagnosing the disorder or classifying its severity.
What Is BMI?
BMI, or body mass index, divides your weight in kilograms by the square of your height in meters as a screening method for your weight category. A BMI that is considered healthy can indicate an ideal weight, while one that is above or below the healthy range is either overweight or underweight.
It is important to understand that while BMI can be used to screen for obesity, being underweight, or the presence of an eating disorder, it does not directly measure body fat. BMI is merely one tool of many that can be used to determine a person’s overall health.
Is BMI a Good Metric?
There are several factors that can influence BMI and make it a less accurate metric. For example, BMI does not directly correlate to body fat indexes and does not take into account factors such as these:
- 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.
- Athleticism: Athletes often have a low level of body fat but can be heavier due to more muscle mass and therefore have a higher BMI.
BMI can indicate a person’s level of malnutrition and risk for significant medical conditions, and it can help to screen for anorexia. It should also be recognized that people with anorexia are not always underweight and can have what is considered a healthy weight or even be overweight.
Anorexia encompasses many emotional, behavioral, and physical attributes beyond just body mass index and potential malnutrition. BMI therefore should not be used as the sole classification for a diagnosis of an eating disorder. Other factors should be considered as well.
Does a Low BMI Represent a Sign of Anorexia?
A low BMI can be a sign of anorexia, but 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 underweight.
Additional signs of anorexia can include behavioral concerns, such as the following:
- 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 the following:
- Mood changes
- Fear of getting fat
- Negative self-view
What to Do if You Suspect Anorexia?
If you suspect anorexia, early intervention and treatment are essential for a more robust and quicker recovery with fewer potential medical and mental health complications.
Research anorexia, know the signs, and understand the potential contributing factors as well as treatment options. Then, have a safe and private conversation with your loved one. The goal is to help them see the issue and seek professional treatment.
Treatment programs can combine medical interventions to help stabilize a person’s nutrition and physical self with behavioral therapies and counseling to manage the behavioral and psychological aspects of an eating disorder. It is important to understand the root causes of anorexia to improve outcomes and sustain a long and healthy recovery.
- World Health Organization (WHO). (May 2010). A Healthy Lifestyle: WHO Recommendations.
- Harrington BC, Jimerson M, Haxton C, Jimerson DC. (2015). Initial evaluation, diagnosis, and treatment of anorexia nervosa and bulimia nervosa. Am Fam Physician; 91(1):46-52.
- Gibson D, Watters A, Cost J, Mascolo M, Mehler PS. (2020). Extreme Anorexia Nervosa: Medical Findings, Outcomes, and Inferences from a Retrospective Cohort. Journal of Eating Disorders, 8(25).
- Machado PPP, Grilo CM, Crosby RD. (2017). Evaluation of the DSM-5 Severity Indicator for Anorexia Nervosa. European Eating Disorders Review, 25(3):221-223.
- Centers for Disease Control and Prevention (CDC). (June 2022). About Adult BMI.
- Jones M. National Eating Disorders Association. (2022). Why Early Intervention for Eating Disorders Is Essential.