Written by Enric Lledo, MD.
Body Mass Index (BMI) is a simple calculation—weight in kilograms divided by height in meters squared (kg/m²)—with a bit of a complicated history. However, it is a cornerstone of health assessment worldwide. It is commonly used to classify individuals’ weight status into one of four categories:
- underweight,
- normal weight,
- overweight, or
- obesity, which now has three subclasses.
Historically, BMI has been used to guide clinical decisions and public health policies. Its appeal has been considered to include:
- its ease of use,
- low cost, and
- ability to provide a standardized measure for large-scale epidemiological tracking of both malnutrition and obesity.
Therefore, in clinical practice, BMI has traditionally been the first step in evaluating nutritional status. However, despite its ubiquity and utility at the population level, BMI has significant limitations when applied to individual diagnosis.
Healthy Human Variability
BMI does not account for differences in body composition between males and females. Men typically have higher muscle mass at a given BMI compared to women. This can lead to a misclassification—men may be considered healthy or overweight when they are actually overly lean, even undernourished, with substantial muscle density. This limitation may mislead providers or patients themselves in the evaluation of conditions that include body weight as a sign or symptom of illness, such as eating disorders.
Similarly, athletes – both male and female – often have high muscle mass and low body fat. They may be categorized as overweight or obese by BMI standards. Conversely, athletes engaging in disordered eating may lose significant muscle mass without dropping below standard BMI thresholds, thereby masking malnutrition.
Standard BMI cutoffs do not account for ethnic differences in body composition, which can lead to missed diagnoses. For example, Asian populations may experience metabolic complications at lower BMIs than White populations. Black individuals may have higher bone density and muscle mass at the same BMI as White individuals.
Variability within a Clinical Population
There are also limitations to BMI when considering specific disorders, including the eating disorder anorexia nervosa.
The diagnosis of anorexia nervosa has historically been related to a low BMI threshold. However, leading experts in the field have emphasized that BMI fails to capture the clinical complexity of eating disorders, particularly their psychological, behavioral, and medical heterogeneity. Many individuals with severe restrictive eating behaviors do not land at weights – or BMI’s – considered low by population norms. Yet, these people experience profound medical and psychological consequences when they are underweight for their body’s needs — a presentation now termed atypical anorexia nervosa.
This presentation of anorexia nervosa is associated with medical instability and psychological distress equal to or greater than those with classic anorexia nervosa. Unfortunately, an overreliance on BMI may mean delayed or missed diagnosis. Studies suggest that restrictive eating disorders may be better described as a spectrum across weight ranges. Across the ranges, there are similar patterns of medical compromise and functional impairment regardless of whether admission BMI meets traditional underweight thresholds.
In addition, remission criteria for eating disorders have historically focused on weight restoration. Yet this approach that is insufficient for atypical anorexia nervosa. This is because these patients are presenting at normal or above-normal weights despite significant weight loss. Though this diagnostic category was recognized for the first time in 2013, with the publication of an updated diagnostic manual (DSM-5), more than a decade later, there is still a persistent lack of consensus regarding standardized diagnostic and remission criteria for atypical anorexia nervosa. This problem affects both clinical management and research comparability.
BMI, Access to Care, and Stigma
BMI-based thresholds may also function to restrict access to care. Individuals who do not meet underweight criteria are often denied appropriate treatment despite severe illness. And overreliance on BMI leads to missed or delayed diagnoses for individuals who do not appear underweight but are experiencing severe restrictive behaviors and medical complications.
This may be especially salient for children and adolescents, males, athletes, and people from non-Western backgrounds whose body composition norms differ from standard references.
Using BMI as a gatekeeper for diagnosis also perpetuates stigma. The focus on thinness as illness severity by insurers (and unfortunately, sometimes by misinformed providers) reinforces harmful stereotypes about who develops eating disorders and discourages help-seeking among those who do not fit these profiles.
Overcoming Barriers to BMI
Applying additional diagnostic methods when considering clinical presentations is critical. This can help providers put BMI information into a broader, more clinically useful context. There are several ideas of what other metrics can help.
Currently, experts advocate for a multidimensional approach to include:
- Growth curves (tracking individual growth trajectories provides context for interpreting weight changes, especially in youth)
- Body composition analysis (techniques that offer direct measures of fat mass versus lean mass, addressing one of BMI’s core shortcomings)
- Metabolic measurements and vital signs (such as blood pressure, heart rate variability, electrolyte levels, hormonal profiles, and other laboratory markers that can reveal malnutrition-related complications even when BMI appears normal)
- Clinical judgment and functional assessment (including strength loss, psychological impairment, history of rapid weight loss/suppression, amenorrhea/reproductive dysfunction, cognitive symptoms, and social withdrawal provides a fuller picture of illness severity beyond anthropometrics alone)
The severity of eating disorder symptoms, including disturbances in eating behavior, body image concerns, and attitudes about weight and diet, and the impact of these symptoms on daily functioning do not completely correlate with BMI. Nor does treatment outcome. It’s therefore essential that advocacy and educational efforts focus on building awareness about the pitfalls of overreliance on BMI.
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