The epidemiological research indicated that about half of the increase in rheumatoid arthritis cases in one Minnesota county may be linked to rising obesity rates there over three decades.
"The findings outline yet another disease, or disease group, associated with the current obesity epidemic," said study co-author Dr. Sherine Gabriel. "We are likely to see an increasing incidence of rheumatoid arthritis as a result of the increasing prevalence of obesity if we don't address this health crisis."
Moreover, the research suggested that obesity precedes the onset of rheumatoid arthritis, said Gabriel, a professor of medicine and epidemiology at the Mayo Clinic in Rochester.
Women more at risk
The impact of obesity on rheumatoid arthritis risk appeared greater for women in the study, which may be due to the fact that women get the disease three times more often than men. Men often develop the condition later in life, according to the Arthritis Foundation.
The study, conducted at the Mayo Clinic from 1985 to 2007, appeared online recently in the journal Arthritis Care & Research.
Rheumatoid arthritis affects about 1.3 million Americans, or 1% of the US population, according to Arthritis Foundation figures. The autoimmune disorder attacks joint tissues and sometimes organs, causing swelling, inflammation, fever and fatigue. The condition can develop at any time, but it usually develops between the ages of 30 and 60.
The illness is influenced by both genetics and environmental factors, according to background information in the study.
The new research was focused on Olmsted County, Minn., where records of all medical providers have been collected on every resident in one database for decades, Gabriel said.
Adults who developed rheumatoid arthritis were matched with other people based on age, sex and year of diagnosis. Of the 813 people with the disease and another 813 without it, 68% were women and about 30 % were obese. Their average age was 56.
Obestiy rates rising
Researchers reported that during the study the incidence of the disease increased by about nine people per 100 000, and 52% of the change was attributable to obesity.
Obesity rates in the United States have risen steadily, from about 10% of the population in 1980 to almost 36% of adults in 2007, according the US Centers for Disease Control and Prevention. About 17% of children are obese, the agency's data shows.
Commenting on the new findings, one expert expressed concerns about increasing rates of rheumatoid arthritis fueled by rising obesity rates, but agreed with the study's conclusion that more research was needed because the Minnesota group was not racially diverse. Olmsted County is 90 percent white, according to the study.
"The study was pretty well done," said Dr Olivia Ghaw, a rheumatologist at Mt. Sinai Medical Center in New York City. But "the population was limited to one county, so I'm not sure the results can be extrapolated to the entire country."
Increase in arthritis 'worrying'
Ghaw said that because rheumatoid arthritis is an inflammatory disease and obesity has been linked to inflammation, a connection between rising rates of both seemed likely.
"The increase in rheumatoid arthritis is troubling," she said. "Obesity confers a greater risk of inflammatory disease" because certain chemicals in fat cells promote inflammation in the body.
Ghaw also cautioned that treating obese patients for rheumatoid arthritis might be more difficult because they may not respond as well to the medications due to "a chronic inflammatory state."
On a positive note, she said the research showed that some patients may be able to prevent the disease by keeping their weight down.
Although the study found an association between obesity rates and rheumatoid arthritis, it did not prove a cause-and-effect relationship.
Living with arthritis
Why we struggle to lose weight
Treatment for rheumatoid arthritis