New research presented at ENDO 2023, the annual meeting of the Endocrine Society, suggests that many individuals with “normal” body mass indexes (BMI) may still be classified as obese. This study adds to a growing body of evidence that highlights the limitations of using BMI as a sole measure of obesity. While BMI is widely used to determine obesity and overweight status based on a person’s height and weight, it may not provide an accurate representation of an individual’s body fat composition.
Obesity is generally characterized by an excess accumulation of body fat. BMI, calculated by dividing weight in kilograms by height in meters squared, categorizes individuals with a BMI of 30 or above as obese and those with a BMI between 25 and under 30 as overweight. It is commonly employed by medical professionals to assess the risk of conditions such as Type 2 diabetes and heart disease and to determine eligibility for weight loss surgery or anti-obesity medications.
However, the recent study involving almost 10,000 U.S. adults, conducted between 2011 and 2018, suggests that BMI may underestimate the prevalence of obesity. While nearly 36% of the participants were classified as obese based on their BMI, an alternative measure of obesity using body fat percentage revealed that 74% of individuals had obesity. It is important to note that this study has not yet undergone peer review.
Dr. Aayush Visaria, an internal medicine resident at Rutgers Robert Wood Johnson Medical School and the lead researcher of the study, emphasized that many people with a normal BMI still have obesity. The American Medical Association is also taking steps to move away from relying solely on BMI for assessing health and obesity. The association now advises healthcare professionals to consider other measures, such as waist circumference, fat distribution, and the percentage of weight derived from fat, in addition to BMI.
The limitations of BMI arise from the fact that it does not differentiate between fat, muscle, bone, water, and organ weight. Therefore, individuals with the same BMI may have different body compositions. For instance, individuals with higher muscle mass may have a higher BMI but a healthier body composition. Muscle weighs more than fat, which can lead to a skewed BMI measurement for very muscular individuals.
To address these limitations, researchers suggest alternative measures like body fat percentage. This measurement can be obtained through techniques like a Dexa scan, which uses X-rays to distinguish between bone and soft tissue, including fat. Other methods include specialized scales, calipers, or wearable devices that estimate body fat percentage. However, these tools do not reveal where fat is stored in the body, which has implications for health. Visceral fat stored around the organs is associated with increased risks of conditions like Type 2 diabetes and heart disease, whereas fat stored around the legs may have protective effects.
Despite the imperfections of BMI, experts caution against completely discarding its use. Instead, they propose refining BMI cutoffs to include considerations for sex, age, and race. Fine-tuning BMI measurements based on specific data can enhance its accuracy and practicality in clinical care, particularly in resource-limited settings. Physicians need to understand the benefits and limitations of using BMI and explore complementary measures that can provide a more comprehensive assessment of body composition and associated health risks.
