Seventy-five percent of U.S. adults would be classified as obese under a new index that integrates body measurements with the traditional body mass index (BMI). This updated approach, introduced by the Lancet Diabetes and Endocrinology Commission in January 2023, seeks to more accurately reflect the complexities of obesity, a growing global health issue.
Researchers from a JAMA study published last week utilized this revised obesity index, which includes factors such as waist circumference, waist-to-hip ratio, and waist-to-height alongside BMI. This combination aims to provide a clearer picture of body fat distribution, distinguishing between fat and muscle mass. The findings revealed that four in five U.S. adults classified as overweight by BMI would be redefined as obese. Moreover, an additional 38% of individuals with a “healthy” BMI would also fall into the obese category under this new classification.
From 2021 to 2023, the U.S. Centers for Disease Control and Prevention reported a 40% obesity prevalence among U.S. adults based solely on traditional BMI measurements. The accuracy of the new index in reflecting the obesity epidemic remains a topic of discussion among health professionals. Dr. Maria Escobar Vasco, an endocrinologist and program director at UT Health San Antonio, expressed surprise at the high percentage, stating, “Obesity is certainly an epidemic, and we’re seeing rising incidence and prevalence of obesity, but 75% [seems high].”
Obesity rates in the United States have surged since the introduction of the BMI measurement in the early 1970s. This shift has sparked a broader dialogue regarding the multifaceted causes of obesity, societal stigma, and whether it should be categorized as a chronic disease or merely a risk factor for conditions like metabolic disorders and type 2 diabetes. Dr. Aaron King, a family medicine physician with the Baptist Health System, noted the ambiguity surrounding the definition of obesity. He remarked, “I think when you draw the line that’s always a question that is up for a little bit of debate. But I think if you’re asking the question, ‘Do I think that 75% of U.S. adults have or are at high risk for developing metabolic disease?’ I think what we’re finding in the data is yes, sadly.”
A consensus among researchers indicates that relying solely on BMI is insufficient for assessing healthy weight ranges. The traditional BMI formula calculates a person’s weight in kilograms divided by their height in meters squared, categorizing those with a BMI of 30 or higher as obese. This method fails to account for differences in body composition, such as the proportion of adipose tissue to muscle mass, bone density, and crucially, the distribution of body fat. Dr. King highlighted a common example: “You can have a weightlifter who is 10% body fat or less, and might have a BMI over 30, which will classify as obesity. But of course, all that weight is solid muscle, and that person likely doesn’t have any metabolic disease and certainly doesn’t have truncal obesity.”
Truncal obesity, which refers to excess abdominal fat, is particularly linked to serious health problems, including insulin resistance and cardiovascular disease. Conversely, fat stored in the hips, buttocks, and thighs has been found to offer protective benefits against metabolic and cardiovascular conditions. The Lancet Commission aims to address the shortcomings of BMI by incorporating additional measurements that highlight fat distribution, which can assist healthcare providers in identifying individuals at greater risk for disease.
Dr. Escobar emphasized the importance of fat distribution, stating, “Inside of our belly, we have a lot of different very important organs that you need to be fully working. The liver, for example. If we don’t have any more fat tissue that can accumulate that extra fat, it’s going to go into the liver. And so that’s when we start having issues with what used to be called fatty liver disease.”
Individuals classified as having a normal BMI but who are actually obese may face barriers in accessing necessary medications, as insurance often requires a specific BMI threshold for coverage. This issue is particularly prevalent among Asian Americans, for whom researchers recommend lower BMI thresholds to facilitate earlier access to treatment options.
The Lancet Commission distinguished between pre-clinical obesity, characterized by excess fat with preserved tissue function, and clinical obesity, a chronic condition that alters organ function due to excessive fat. The recent JAMA study merged these definitions, which may complicate understanding and treatment approaches.
Despite its limitations, BMI remains a widely used metric in healthcare due to its simplicity and cost-effectiveness. Dr. King remarked, “It’s mainly a convenience factor. We can get a weight and height really quickly. And so that’s what we all use. But the reality is that it’s probably not a good metric.”
Dr. Escobar added that while BMI can provide a general overview, implementing waist-to-hip measurements would enhance accuracy in assessments. “If we want to be really good about this, we should be getting waist-to-hip on everybody. It’s fairly easy,” she stated.
The JAMA researchers concluded that further investigation is necessary before adopting the new measurement widely. Local medical professionals indicated that transitioning to the updated index would require education for both practitioners and the public. Dr. King noted, “I think in a perfect world, we would never use BMI, knowing what we know now, but I think we’re kind of stuck with it. It’s probably going to be a paradigm shift that will take several years to come off of using it.”
