MORRIS, Minn. -- A new study has found that severely obese truck drivers are more likely to crash in the first two years of driving on a job.

The findings, published in the scientific journal Accident Analysis & Prevention, are expected to fuel the ongoing debate over the best way to screen commercial drivers for medical conditions that pose a risk on the road.

With its long hours, sedentary nature, and often stressful conditions, truck driving has never been considered conducive to good health.

Previous studies have shown that truck drivers generally don't get enough sleep, and they're more likely than the general population to smoke, be overweight, and suffer from sleep disorders.

These health problems can lead to safety problems: Commercial drivers are involved in more than 4,000 fatal crashes each year, and more than 13 percent of those are due to fatigue and other physical issues, according to a 2007 study by the U.S. Federal Motor Carrier Safety Administration.

For more than a decade, behavioral economist and former truck driver Stephen Burks of the University of Minnesota, Morris, has been working with trucking company Schneider National to identify factors that contribute to truck driver health and safety.

Burks and his team have studied drivers' cognitive abilities, tolerance for risk and loss, and even their ability to plan.

Two years ago, the team turned its attention to a simple but readily available bit of data: drivers' height and weight.

The researchers asked 744 new drivers, who trained with Schneider National in 2005 and 2006, for their height and weight, and then calculated the body mass index (BMI) for each. A BMI greater than 25 is considered overweight, and greater than 30 is obese.

Then the researchers kept tabs on the rookie drivers over the next 2 years or until they ended their employment with the company, noting every accident they had during that period.

"That's when the data stood up and shouted at us," said Jon Anderson, a biostatistician at the University of Minnesota, Morris, who co-authored the study.

"We found really clear evidence that the highest-BMI drivers are at higher risk of having an accident."

During their first two years on the road, drivers with a BMI higher than 35 ("severely obese") were 43 to 55 percent more likely to crash than were drivers with a normal BMI, the team reported Accident Analysis & Prevention.

Drivers who are overweight or obese, but not severely, did not appear to be at higher risk, and the study does not indicate why.

The relationship held even when the researchers corrected for number of miles on the road, geographic location, age, and other crash risk factors.

The increased risk may be due to sleep apnea, a sleep disorder strongly associated with obesity that can cause potentially dangerous daytime drowsiness, Burks says.

But he emphasizes that other factors -- limited agility, for example, or fatigue thats associated with obesity but not due to sleep apnea -- may be responsible.

Nevertheless, the studys findings fan the flames of the sleep apnea-related controversy, says Kurt Hegmann, a physician who directs the University of Utah Rocky Mountain Center for Occupational and Environmental Health in Salt Lake City and was not involved in the study.

Federal regulators have been debating whether to require all commercial drivers above a certain BMI to be screened for sleep apnea, and what that BMI should be. This study suggests screening should start at a BMI of 35, Hegmann says.

"This is a valuable study; it shows there's enough information to warrant screening," he says. "It sure looks like there's risk here and that we probably should be doing something about it."