People with chronically low body weight face the same chances of dying as those who are obese, a review of major studies finds.

Joel Ray, a physician-researcher at St. Michael’s Hospital in Toronto, reviewed 51 studies on the link between body mass index (BMI) and death, choosing only studies lasting for five years or more. The researchers considered such factors relating to BMI as smoking, consumption of alcoholic, lung disease, among others in establishing a “J-shaped” curve describing the relationship.

Among findings, Ray says adults with low body weights — a BMI less than 18.5 — face a 1.8 times higher risk of death than people with healthier weights within a BMI range of 18.5 to 24.9. Yet the same risk of death builds as BMI rises on the other side of that J-curve. For people who are obese with BMIs ranging between 30 and 34.9, the risk of death is 1.2 times higher than average.

Even worse, people deemed severely obese face an even higher death risk at 1.3 times the average.

Still, public health officials may spend more time worrying about the greater prevalence and cost of obesity in the United States, which affects more than one-third of adults with direct medical costs of $147 billion per year. The health problems of people who are chronically underweight in a country of predominantly overweight people are masked further as the trend runs in the opposite direction. As obesity continues to rise in the U.S., the prevalence of people who are underweight has decreased significantly during the past half-century, according to the Centers for Disease Control and Prevention.

Today, 2.2 percent of American women are underweight, along with 0.9 percent of men, down from 5.7 percent and 2.3 percent, respectively. Among common causes of low body weight are malnourishment and poor self-care, heavy alcohol and drug use, smoking, mental health problems, and poverty.

Ray also criticized the use of BMI as a stand-alone measurement of health, however. "BMI reflects not only body fat, but also muscle mass,” he said in a statement. “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.”

Ray suggests clinicians replace BMI with another measure, such as waist circumference.

Source: Ray, Joel, Cao, Sissi, Moineddin, Rahim, Urquia, Marcelo L., Razak, Fahad. J-Shapedness: An Often Missed, Often Miscalculated Relation: The Example Of Weight And Mortality. Journal of Epidemiology and Public Health. 2014.

Fryar, Cheryl D., Ogden, Cynthia L. Prevalence Of Underweight Among Adults Aged 20 Years And Over: United States, 2007-2008. Centers for Disease Control and Prevention.