What defines obesity? It's certainly not BMI, says American Medical Association

The American Medical Association (AMA) recently adopted a new policy that calls for de-emphasising the use of body mass index (BMI) in clinical practice. This shift has sparked discussions among healthcare professionals regarding the measurement and definition of obesity.

BMI, calculated by dividing body weight in kilograms by height in metres squared, has been widely used since the 1990s to estimate normal body weight and screen for obesity. However, there are significant limitations to using BMI as an indicator of health.

1. Inaccurate measurement of body fat percentage: While BMI correlates with body fat percentage at a population level, it does not directly measure an individual’s body fat. Factors such as age, muscle mass, sex, and race can result in different body fat percentages for individuals with the same BMI.
2. Lack of fat distribution information: BMI cannot provide information about how fat is distributed in the body, despite its importance in assessing health risks. Fat stored around internal organs poses higher health risks than fat distributed in the extremities.
3. Variability in health outcomes: Health outcomes associated with a specific BMI can vary based on factors such as race, sex, age, and physical fitness level. Some individuals with a higher BMI may have metabolically healthy obesity and lower health risks, while others with a lower BMI may still face health issues.

BMI has been commonly used by clinicians to determine whether weight loss is recommended for patients. However, recent evidence suggests that lifestyle-based weight loss programmes, such as diet and exercise, may not significantly improve cardiovascular events or overall quality of life. The only specific outcome that showed improvement was a reduced risk of developing diabetes.

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As the AMA shifts away from BMI, alternative measures are being recommended to assess the health risks of elevated body weight. These include measures such as body adiposity index, relative fat mass, waist-to-hip ratio, and waist circumference. However, these measurements may require additional clinic visits and can potentially contribute to stigma.

Further research

The AMA’s policy statement marks an important step in reducing the reliance on BMI as a general measure of health and a tool for obesity treatment. Further research is needed to identify more accurate and comprehensive ways to assess the health risks associated with elevated body weight.

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In conclusion, the AMA’s new policy signals a significant shift in the medical community’s perspective on BMI as a measure of obesity. While BMI has been widely used in the past, its limitations have prompted a reevaluation of its role in clinical practice. Healthcare professionals are now exploring alternative measures and calling for further research to better assess the health risks associated with elevated body weight.

The Conversation