TUESDAY, Jan. 22 (HealthDay News) -- Obese drivers are up to 80 percent more likely to die in a car crash than normal-weight drivers, a new study finds.

Car designers may need to take heavier drivers into consideration to keep them safer, the researchers said.

"This study highlights yet another negative consequence of obesity," said study co-author Thomas Rice, a research epidemiologist with the University of California, Berkeley's Safe Transportation Research & Education Center.

"Our findings suggest two things: first, that there is something about obese vehicle occupants that causes poorer outcomes. That thing is probably a higher prevalence of comorbidities -- other health conditions -- related to obesity that inhibit survival and recovery from severe injury," he said.

Second, earlier research has shown that the proper interaction between seat belts and the human body is inhibited in the obese, Rice said.

"Specifically, the lap belt is prevented from engaging the pelvis due to excess body fat. It is this engagement between lap belt and pelvis that impedes the forward motion of occupants during frontal collisions," he said.

Rice stressed the importance of proper seat belt use, especially among the obese. "It is critical that the lap belt be positioned as low as possible on the lap and as close to one's pelvis as possible," he said.

The report was published in the Jan. 21 online edition of the Emergency Medicine Journal.

For the study, Rice and Dr. Motao Zhu, from the department of epidemiology and Injury Control Research Center at the University of West Virginia, used fatality data from the U.S. National Highway Traffic Safety Administration for 1996 to 2008.

During that time, details of more than 57,000 car accidents were documented. Rice and Zhu looked specifically for accidents involving two cars resulting in a death. They refined their search further and included only accidents involving cars of similar size and type. In all, their final pool included more than 3,400 pairs of drivers.

Obesity was determined by body mass index (BMI), a measurement that takes both height and weight into account.

As the level of obesity increased, so did the odds of dying in the crash. Compared to normal-weight drivers, those at the lowest level of obesity were 21 percent more likely to die, those at the next level were 51 percent more likely to die and those who were most obese were 80 percent more likely to die, Rice and Zhu found.

Obese women had a greater risk of dying than obese men, the researchers noted.

In addition, underweight men were slightly more likely to die in a crash than normal-weight drivers, the study found.

These risks remained even for drivers wearing seat belts and even when the airbag deployed, the authors noted.

Car design may have to be adapted to reduce the risk to these drivers, Rice and Zhu add, especially in light of the U.S. obesity epidemic.

"It may be the case that passenger vehicles are well designed to protect normal-weight vehicle occupants but are deficient in protecting overweight or obese occupants," they wrote in their study.

Commenting on the findings, Dr. David Katz, director of the Yale University Prevention Research Center, said: "We have a serious and pernicious problem of anti-obesity bias in the United States. Efforts to address that may at times invite us to pretend that size doesn't matter, but, in fact, it does."

The world around us has been built to accommodate prevailing norms of height, weight, he noted. "It just stands to reason that safety systems such as those in cars, designed for people of a certain average size, may serve a population of a larger average size less well," Katz said.

That may be what's behind the new findings, he said, but it also could be that obesity-related illnesses affect recovery from trauma.

"But minimally, [the findings] appear to mean that in car crashes, size does matter, because it affects outcome. How, why and what we can do about it now become the important questions," Katz said.

Although the study found an association between obesity and death rates in car crashes, it did not establish a cause-and-effect relationship.

More information

For more about obesity, visit the U.S. National Library of Medicine.