People whose body mass index (BMI) is below 18.5, formally considered underweight, carry the same risk for early death as obese people at the opposite end of the spectrum, a new study finds.

As of late, experts have started to denounce BMI as a reliable, if crude, indicator of overall health. The measurement reflects the ratio of someone’s height to weight, failing to account for how that weight is distributed or what kind of weight it is. For example, the 2013 New York City Marathon winner, Kenyan runner Geoffrey Mutai, has a BMI of 18.6 — just above the threshold for underweight — while Arnold Schwarzenegger in his bodybuilding prime had a BMI of 30, putting him squarely in the “obese” category.

“If we want to continue to use BMI in health care and public health initiatives, we must realize that a robust and healthy individual is someone who has a reasonable amount of body fat and also sufficient bone and muscle," said Dr. Joel Ray, study leader and physician-researcher at St. Michael's Hospital in Toronto, in a statement. If body fat is our main concern, he added, then BMI may need a replacement.

His latest study seems to highlight that need. Ray and his colleagues conducted a meta-analysis of 51 studies examining the links between BMI and death from any cause, including data on newborns and stillbirths. He found that underweight adults face a risk of early death 1.8 times greater than people of a healthy weight. Overweight people face a risk 1.2 times greater, and obese people 1.3 times greater.

The risks differ markedly between the obese and underweight. In addition to its recent classification as a disease, obesity puts people at risk for diabetes, cardiovascular disease, high blood pressure, and stroke. Underweight people, meanwhile, face lung disease, cardiovascular disease, falls and injury, alcoholism and cirrhotic liver disease, and suicide as the result of mental illness.

Because the risk factors are different, in underweight cases BMI may actually be useful, Ray told Medical Daily. “As a measure of obesity-related disease and death, abdominal circumference should be used,” he said, although when it comes to disease and death related to being underweight, “BMI works fine.”

It should come as no surprise that America’s growing obesity problem crowds out the underweight population. According to the Centers for Disease Control and Prevention, the prevalence of adults aged 20 to 74  who were underweight decreased markedly over the last several decades, from 4.0 percent in the 1960s to 1.7 percent between 2007 and 2010. Meanwhile, the obesity rate has risen to 35.7 percent. It is among the leading causes of preventable death, and each year it burdens the health care system to the tune of $147 billion.

Finding the correct measurement tool to establish where a healthy weight resides should be a top priority, Ray argues. Solving the obesity problem is important. We also have an “obligation to ensure that we avoid creating an epidemic of underweight adults and fetuses who are otherwise at the correct weight,” he said. “We are, therefore, obliged to use the right measurement tool."

 

Source: Cao S, Moineddin R, Urquia M, Razak F, Ray J. J-shapedness: an often missed, often miscalculated relation: the example of weight and mortality. Journal of Epidemiology & Community Health. 2014.