CHICAGO (AP) -- Overweight workers cost their bosses more in injury claims than their lean colleagues, suggests a study that found the heaviest employees had twice the rate of workers' compensation claims as their fit co-workers.

Obesity experts said they hope the study will convince employers to invest in programs to help fight obesity. One employment attorney warned companies that treating fat workers differently could lead to discrimination complaints.

Duke University researchers also found that the fattest workers had 13 times more lost workdays due to work-related injuries, and their medical claims for those injuries were seven times higher than their fit co-workers.

Overweight workers were more likely to have claims involving injuries to the back, wrist, arm, neck, shoulder, hip, knee and foot than other employees.

The findings were based on eight years of data from 11,728 people employed by Duke and its health system. Researchers found that workers with higher body mass indexes, or BMIs, had higher rates of workers' compensation claims.

The most obese workers - those with BMIs of 40 or higher - had the highest rates of claims and lost workdays. BMI is a measure of height and weight. A 6-foot, 300-pound person, for example, has a BMI of just over 40.

Study co-author Dr. Truls Ostbye said the findings should encourage employers to sponsor fitness programs.

"There are many promising programs," Ostbye said. "We'd like to see more research about what is truly effective."

James Hill, who heads the Center for Human Nutrition at the University of Colorado, said managers will pay attention to the findings because injuries mean more immediate financial losses than the future health-care costs of diabetes and heart disease.

"When you see that claims rates double, I think that's going to get people's attention," Hill said.

But there isn't enough good information about employer-sponsored programs that work, said John Cawley, an expert in the economics of obesity at Cornell University. Employers don't know whether paying for nutrition counseling, obesity surgery or anti-obesity drugs through health insurance makes economic sense, he said.

"It's now apparent to everybody that obesity is a big problem," Cawley said. "But the research isn't there to know where to get biggest bang for the buck."

Cawley noted that BMI does not distinguish muscle from fat and can equate a buff body builder to a couch potato. Although BMI, a measure of height and weight, is used in most obesity research, Cawley's research has found that blacks are particularly likely to be misclassified as obese by BMI.

New York employment attorney Richard Corenthal cautioned employers not to overreact with discriminatory policies.

"Employers need to be careful not to view this study as a green light to treat obese or overweight workers differently," Corenthal said.

The study, appearing in Monday's Archives of Internal Medicine, got funding from the National Institute for Occupational Safety and Health.