Body Mass Index (BMI) is the most widely used measure of obesity, but two new studies shows that it may be misleading, leading to the observation that some people who have higher BMI levels are actually healthier than those in the normal range. This has become known as the obesity paradox.
BMI distinguishes between people who are overweight and obese from those with normal healthy weight through a height-weight calculation. Coined by mathematician Adolphe Quetelet in the 1800’s, it was used as a study tool in measuring the growth rate of a normal man, and had nothing to do with fat.
Modern problems with using BMI as a measurement of obesity are based on the fact that it does not consider bone structure, muscle mass, fat distribution, age or sex. As individuals age their muscle mass decreases and body fat increases, leading to normal BMI measurements in elderly people who are actually obese. Conversely, a lean person who has high muscle mass may have their body fat overestimated.
Results from the first study, from Canada, were published online in the journal Annals of Internal Medicine on March 8. It included over 49,000 women who were followed for 7 years, and 5,000 men who were followed for 5 years. During the time of the study about 5,000 of the women and 1,000 of the men died.
The Canadian study showed that, when combined with body fat percentage a low BMI did not necessarily indicate a healthy person. Both men and women with low BMI but high body fat were found to be at higher mortality risk.
Research from the University of California Santa Barbara and University of California Los Angeles, which appeared in the International Journal of Obesity, also showed that millions of Americans who have been labeled overweight or obese based solely on BMI are actually healthy. In addition, those with healthy BMI measurements but a high percentage of body fat may have an increased risk of death. This is thought to be the root of the obesity paradox.
Co-author of the California study results, Jeffrey Hunger, says BMI is a deeply flawed measure of health. “Not only does BMI mislabel 54 million heavier individuals as unhealthy, it actually overlooks a large group of individuals considered to have a ‘healthy’ BMI who are actually unhealthy when you look at underlying clinical indicators.”
Many U.S. insurance companies use BMI as a factor for determining health insurance costs. However, study lead author Janet Tomiyama, from UCLA, notes that healthy people who have a BMI of more than 24.99 are no more likely than people with lower BMIs to incur higher medical expenses.
The conclusions drawn by both studies are that high body fat percentages and low BMI are independently associated with increases mortality, which helps explain the conflicting relationship between mortality and BMI.