Anorexia Nervosa (AN) is a serious psychiatric eating disorder involving the restriction of energy intake relative to requirements. This restriction leads to a significantly low body weight. Body Mass Index (BMI) serves as a standardized, measurable metric for assessing this weight status in adults. While BMI is a foundational tool in the assessment of AN, it is only one part of the overall diagnostic picture, and the classification of severity based on BMI helps determine the appropriate level of care.
Understanding Body Mass Index (BMI)
Body Mass Index is a simple numerical measurement used to categorize a person’s weight relative to their height. It is calculated by dividing a person’s weight in kilograms by the square of their height in meters. For adults, the World Health Organization (WHO) recognizes a BMI below 18.5 as being underweight.
BMI is a useful indicator in a medical setting, particularly for conditions involving low body weight, such as Anorexia Nervosa. However, its utility has limitations when assessing overall health because the calculation does not account for differences in muscle mass, bone density, or body fat distribution. For example, a highly muscular person might register as overweight despite having low body fat.
BMI Thresholds for Anorexia Nervosa Severity
The Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), specifies that Anorexia Nervosa requires restriction of energy intake leading to a significantly low body weight. For adults, this criterion is generally met when the BMI falls below 18.5 kg/m\(^2\).
The DSM-5 provides specific BMI ranges to categorize the severity of AN, which guides treatment planning and hospitalization decisions. The mild category is defined by a BMI that is greater than or equal to 17.0 kg/m\(^2\). A moderate classification corresponds to a BMI falling between 16.0 and 16.99 kg/m\(^2\).
The condition is considered severe when the BMI drops further, settling in the range of 15.0 to 15.99 kg/m\(^2\). The extreme category, which indicates the highest level of physical risk, is assigned when the BMI is less than 15.0 kg/m\(^2\).
Clinical Criteria Beyond BMI
While a low BMI is a necessary physical finding for an AN diagnosis, the disorder is fundamentally a mental illness. The diagnostic criteria include specific psychological and behavioral components, such as an intense fear of gaining weight or becoming fat, even when the individual is significantly underweight.
This fear is often accompanied by persistent behaviors that interfere with weight gain, such as excessive exercise or rigid dietary rules. Another criterion involves a disturbance in the way one’s body weight or shape is experienced, often manifesting as an undue influence of body shape or weight on self-evaluation.
Furthermore, a person with AN may exhibit a persistent lack of recognition of the seriousness of their current low body weight. This cognitive distortion highlights that the disorder is driven by a profound alteration in self-perception and not merely a desire to be thin.
Serious Health Risks Associated with Low BMI
The significantly low body weight associated with Anorexia Nervosa results in severe medical complications affecting nearly every organ system. The body enters a state of starvation, leading to metabolic adaptations that can be life-threatening.
Cardiovascular complications are of particular concern, including bradycardia (abnormally slow heart rate) and hypotension (low blood pressure). Electrolyte imbalances, such as low levels of potassium, sodium, or phosphate, can occur due to malnutrition, potentially precipitating dangerous cardiac arrhythmias. Low BMI also severely compromises skeletal health, frequently leading to osteopenia or osteoporosis, which increases the risk of fractures.
Additionally, prolonged malnutrition can lead to muscle wasting, renal failure, and brain structural changes. These physical dangers underscore why immediate medical intervention is often required once a low BMI is established in the context of Anorexia Nervosa.