More than half of U.S. adults with diabetes also have arthritis, raising a serious obstacle for diabetic patients urged to exercise, according to a government study.

The survey of nearly 800,000 people is the first extensive look at the overlap between the two conditions, said Dr. John Klippel, president of the Arthritis Foundation.

And its findings highlight a significant challenge: Most diabetics are told exercise is important to their health, but experts say many of them don't do it.

People with diabetes who exercise have better control of their blood sugar and a much lower risk of heart disease complications. But the new research suggests many diabetics see themselves as unable to exercise because of arthritis, said Julia Simard, a Harvard School of Public Health researcher who has studied rheumatoid arthritis and diabetes.

"If you have this perception of ill health, it may affect your willingness to be active," said Simard, who was not involved in the new research.

More than 46 million Americans have some form of arthritis, and nearly 21 million have diabetes. Other research indicates exercise is important in managing both conditions.

The study found that 52 percent of diabetics said they also had arthritis. The conditions and the overlap were most common in Americans 65 and older.

The analysis was based on telephone surveys in the years 2005 and 2007 by the Centers for Disease Control and Prevention. Researchers relied on what people said about their health, and did not verify diagnoses.

The researchers also asked about exercise and physical activity. People with both diabetes and arthritis were 30 to 40 percent more likely to be physically inactive than those who had diabetes alone.

"If we're ever going to successfully control a disease like diabetes, we're going to have to pay a lot of attention to arthritis," Klippel said.

Exercise that put less stress on joints, such as walking, biking and aquatics, are recommended for people with arthritis, said Dr. Chad Helmick, a CDC epidemiologist who co-authored the study.