Arthritis affects about one in five adults in the U.S. and limits the activities of more than one-third of them, the U.S. Centers for Disease Control and Prevention said Thursday.

The agency's Morbidity and Mortality Weekly Report looked at the prevalence of doctor-diagnosed arthritis in 2004 to 2005, using data from a national survey of 30,000 people.

"The findings in this report indicate that 21.6 per cent (46.4 million) of U.S. adults reported doctor-diagnosed arthritis, and 8.3 per cent (17.4 million) reported arthritis-attributable activity limitation during 2003-2005," the report said.

"Women, older adults, persons with little education or those who are obese, overweight or physically inactive are more likely affected."

Rates were higher among those who were obese — 31.6 per cent — or overweight — 21.7 per cent — compared with 16.3 per cent among those who were normal weight or underweight.

Prevalence among women was 25.4 per cent compared with 17.6 per cent for men.

Excess weight a risk factor

Respondents were defined as having doctor-diagnosed arthritis if they answered "yes" to the question: "Have you ever been told by a doctor or other health professional that you have some form of arthritis, rheumatoid arthritis, gout, lupus or fibromyalgia?"

Given the cross-sectional nature of the study, researchers can't say if there is a cause-effect relationship between obesity or physical activity and arthritis.

The report's authors noted that other studies have identified excess weight as a risk factor for osteoarthritis, the most common type of arthritis. Physical activity has helped prevent or delay the onset of disability among adults with osteoarthritis.

More than 100,000 women, men and children will be diagnosed with arthritis this year in Canada, according to the Arthritis Society.

That number is expected to rise by one million people within the next decade, making arthritis the leading cause of disability in Canada.