Being overweight increases the chances of premature death, according to a USC researcher.

Although people accept that obesity can shorten a person’s life span, scientists have debated whether the weight category between normal and obese is, in fact, a health risk.

Being overweight definitely does not help people live longer, said Jonathan Samet, MD, MS, director of the USC Institute for Global Health. His statement is backed by a four-continent effort involving 239 studies and data from 10.6 million people. The collaborative and comprehensive study — one of the largest to date — was published in The Lancet.

The results have important clinical and public health implications, Samet said.

“Physicians should identify being overweight as posing a risk to health,” said Samet, one of the study’s authors and holder of the Flora L. Thornton Chair in Preventive Medicine at the Keck School of Medicine of USC. “Increasing the risk of dying is a powerful indicator of health. The new results on overweight should be strong motivation for people to return to a healthy weight.”

The World Health Organization states that more than 1.9 billion people — about 39 percent of adults — are overweight, defined as having a body mass index between 25 and 30. For example, a 5-foot-4 person who weighs 140 pounds has a healthy BMI of 24. Add five pounds and this individual would be considered overweight.

The study analyzed participants who had never smoked, did not have chronic disease and were still alive five years after the research began. The study involved 385,879 deaths in 239 studies.

Overweight and obesity were strongly connected to coronary heart disease, stroke and respiratory disease death, and were moderately linked to cancer mortality. The findings were reflected in Europe, North America and East Asia.

The new finding on overweight is counter to the results of a prior paper in The Journal of the American Medical Association, which found that being overweight actually adds to one’s life span, and “grade 1 obesity” (BMI 30-35) did not increase mortality. The JAMA article was based on a review of 97 studies with 2.9 million participants.

The Lancet study did more than just summarize published results; the data from all 239 studies was analyzed in a standard way.

“Our study was able to reproduce (the JAMA review’s) findings when conducting crude analyses with inadequate control of reverse causality, but not when we conducted appropriately strict analyses,” the Lancet paper stated.

The WHO states that about 95 million children — mostly in less developed regions — are underweight.

Being underweight, according to the study, was associated with substantially higher respiratory disease mortality and somewhat higher death rates from coronary heart disease, stroke and cancer. These findings held true in Asia, Australia, New Zealand, Europe and North America.

One of the study’s limitations, like many other research papers in this arena, is the use of BMI as a surrogate for visceral fat, which is thought to be key in bringing about the negative consequences linked to obesity.

“The health risk associated with obesity is thought to be driven by abdominal fat,” Samet said. “BMI is an imperfect measure of fat in someone’s belly. There are more gold standard ways to measure fat, but that can’t be done for 10.6 million people.”

Using an accepted but imperfect measure, the study was able to find a connection between premature death and being overweight. The proportion of premature deaths that could be avoided with a healthy weight (between BMI 18.5 and 25) is about 1 in 5 in North America, 1 in 6 in Australia and New Zealand, 1 in 7 in Europe and 1 in 20 in East Asia.

In short, like smoking, the health problems associated with underweight, overweight and obesity are substantial but potentially preventable.

Sun Ha Jee, PhD, MPH, an epidemiology professor at Yonsei University in Seoul, contributed to the research. He shared data from 1.3 million people in the Korean Cancer Prevention Study.

— Zen Vuong