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WASHINGTON - Obese workers may cost their employers more, filing more workers' compensation claims for injuries on the job, and then more frequently going on to become disabled, according to two reports published today.

The researchers, writing in the Archives of Internal Medicine, said employers should consider offering advice not only on safe work practices, but on healthy eating and exercise.

A person is considered technically obese if he or she has a body mass index of 30 or above. BMI is calculated as weight in kilograms divided by height in metres squared.

A person 165 cm tall, is obese at 82 kilograms and morbidly obese, with a BMI of 40, at 109 kilograms. Obesity brings a higher risk of cancer, heart disease and diabetes.

Dr. Truls Ostbye and colleagues at Duke University Medical Centre in North Carolina studied 11,728 health care and university employees who completed at least one health risk assessment questionnaire between 1997 and 2004.

Over an average of three years of follow-up, workers with higher BMIs tended to file more workers' compensation claims. People with BMIs of 40 or more had twice the rate of claims as people at recommended weights -- especially claims related to back, wrist or arm, neck or shoulder, knee, foot or hip injuries.

"The number of lost workdays was almost 13 times higher, medical claims costs were seven times higher and indemnity claims costs were 11 times higher among the heaviest employees compared with those of recommended weight," Ostbye's team wrote.

For the second study, Dr. Soham Al Snih of the University of Texas Medical Branch in Galveston and colleagues studied 12,725 adults 65 years or older.

None were disabled at the beginning of the study, which started in 1982 and ended in 1993.

The researchers questioned the volunteers every year about health conditions, demographics and psychosocial characteristics. Blood pressure, height and weight and physical function were also measured.

Over the 11 years, 3,570 of the volunteers became disabled, and 2,019 died.

"Subjects with BMIs of lower than 18.5 (underweight) or 30 or higher (obese) at baseline were significantly more likely to experience disability during the follow-up period," Al Snih's team wrote.

But being simply overweight did not cause health trouble. People with BMIs of 25 to 30 -- 68 to 82 kilograms for someone 165 cm -- lived longer than people who weighed less.

"Disability-free life expectancy is greatest among subjects with a BMI of 25 to less than 30," the researchers wrote.

Other studies have also found underweight and normal weight older adults may have a lower immediate risk of death.