Increase in RA Incidence in Women Associated With Obesity

Larry Hand

April 30, 2012

April 30, 2012 — More than half of the increase in rheumatoid arthritis (RA) in a population of Minnesota women can be attributed to obesity, according to an article published online April 18 in Arthritis Care & Research.

Cynthia S. Crowson, MS, from the Division of Rheumatology, Mayo Clinic, Rochester, Minnesota, and colleagues analyzed the records of 813 patients with RA and 813 controls from 1985 to 2007. They found that the incidence of RA increased at a rate of 9.2 per 100,000 women during the period and that obesity accounted for 52% — 4.8 per 100,000 — of the increase between 1995 and 2007. The strong association with obesity remained after adjustment for tobacco-smoking status.

"Given the recent rapid increase in the prevalence of obesity, this risk factor has a significant impact on the incidence of RA and likely accounts for a large proportion of the recent increase in incidence of RA among women," the researchers write. "[U]nless the obesity epidemic is controlled, the incidence and prevalence of RA will continue to rise."

The study population included male and female residents of Olmsted County, Minnesota, with a mean age of almost 56 years; almost 68% were female. Controls were matched to patients with RA for age, sex, and calendar year with or without RA. Extensive medical records allowed for collection of data on height, weight, and smoking status.

"Following a 4 decade period of decline, the incidence of RA has been on the rise since 1995," the researchers write. Because genetic factors do not change so quickly, the incidence is likely attributable to environmental risk factors, they conclude. "[O]besity deserves special consideration due to its recent, dramatic increase in prevalence," the researchers write, although previous research has shown conflicting results for the association.

Using conditional logistic regression models, the researchers found that a history of obesity was a significant risk factor for RA in men and women (odds ratio [OR], 1.24; 95% confidence interval [CI], 1.00 - 1.52; P = .046). Obesity remained a significant factor after adjustment for smoking status (OR, 1.24; 95% CI, 1.01 - 1.53; P = .041). They discounted smoking as a risk factor for the increase because the prevalence of smoking has not increased in recent years.

After using sensitivity analyses to study the variability of obesity's influence on RA among women, they concluded that "obesity could explain 52% of the increase in incidence of RA” among women observed in 1995-2007. For men, the researchers concluded that the proportion of incidence increase attributable to obesity was much less (2.3 per 100,000).

Although the mechanism for obesity's association with RA is unknown, the researchers write that chronic inflammation from obesity could be leading to rheumatic problems. Other possibilities include the link between obesity and vitamin D deficiency or the higher levels of estrogen in obese individuals. Other research, they write, has pointed toward a possible genetic predisposition to both obesity and autoimmune disease.

"We know that fat tissues and cells produce substances that are active in inflammation and immunity," Eric Matteson, MD, chair of the Division of Rheumatology at the Mayo Clinic and a study coauthor, said in a statement. "We know too that obesity is related to many other health problems such as heart disease and diabetes, and now perhaps to autoimmunity. It adds another reason to reduce and prevent obesity in the general population."

Although their study population was 90% white and the findings therefore may not be generalizable to more racially diverse populations, the researchers write that obesity prevalence trends in their study "were remarkably similar to those in the US population reported by the National Health and Nutrition Examination Survey," a National Center for Health Statistics program for assessing the health of children and adults.

The study was funded by the National Institute of Arthritis and Musculoskeletal and Skin Diseases. The authors report receipt of National Institutes of Health research funding and have disclosed no relevant financial relationships.

Arthritis Care Res. Published online April 18, 2012. Abstract

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