Altogether, say the authors, an estimated 75 million adults in the US are misclassified as either healthy or unhealthy when BMI is used as the sole health indicator.
The study provides more evidence to support the idea that a person's body mass index (BMI = weight in kg divided by height in m2) is a flawed measure of health.
In spite of this, BMI continues to be used as a yardstick for determining health status. Many employers use it to calculate workers' health care costs, note the researchers behind the new study, who report their findings in the International Journal of Obesity.
And soon, if a rule proposed by the Equal Employment Opportunity Commission (EEOC) is adopted, US employers will be allowed to charge employees up to 30% of health care costs if they fail to meet certain health criteria such as not having a BMI in the normal range (between 18.5 and 24.99).
The study, led by A. Janet Tomiyama, an assistant professor of psychology at the University of California-Los Angeles (UCLA), analyzed the link between BMI and cardiometabolic health using data from the most recent National Health and Nutrition Examination Survey (NHANES).
BMI misclassifies nearly 75 million Americans as healthy or unhealthy
The analysis shows nearly half of Americans whose BMI puts them in the overweight category (34.4 million people) are actually healthy according to cardiometabolic measures, as are 19.8 million whose BMI classes them as obese.
The researchers also found that 15% of Americans (2 million people) whose BMI calculation is 35 or higher - thus classing them as very obese - are also healthy.
Prof. Tomiyama says:
"Many people see obesity as a death sentence. But the data show there are tens of millions of people who are overweight and obese and are perfectly healthy."
She says that these people would be unlikely to incur higher medical expenses and it would be unfair to charge them higher premiums based only on their BMI.
The study also finds over 30% of Americans whose BMI puts them in the normal range (nearly 21 million people) are actually unhealthy according to cardiometabolic measures.
Altogether, say the authors, nearly 75 million adults in the US are misclassified as either healthy or unhealthy when BMI is used as the sole health indicator.
'Final nail in the coffin for BMI'
Prof. Tomiyama says she is surprised by the large numbers they found in the new study and remarks how unfair it is that many healthy people may be penalized, while many unhealthy people will not get charged more for their health insurance, based on their BMI. She notes:
"Employers, policy makers and insurance companies should focus on actual health markers."
In previous work, her lab - which specializes in diet, stress and health - has also found no clear link between weight loss and improvements in blood pressure, diabetes, cholesterol, glucose and other health measures.
Prof. Tomiyama now intends to study people with high BMI who are very healthy.
Co-author Jeffrey Hunger, a doctoral candidate at the University of California-Santa Barbara, urges people to obsess less about their weight and focus on following a healthy diet and taking regular exercise.
He says their study confirms how flawed BMI is as an indicator of health and concludes that "this should be the final nail in the coffin for BMI."
In 2013, Medical News Today reported on a Science paper that discusses why BMI is an inaccurate measure of body fat content and how there is an urgent need for accurate, practical and affordable tools to measure biomarkers that can better predict the risks of disease and death.