STEVE INSKEEP, HOST:

Let's stay with Your Health for a moment, because researches think they have a better understanding of another debilitating disease. Rheumatoid arthritis causes painful joint inflammation. The latest research identifies factors that may put some people at risk of developing it. The female hormone estrogen, along with proteins produced by the body's fat cells, may play an important role. Gretchen Cuda-Kroen reports.

GRETCHEN CUDA-KROEN, BYLINE: Being overweight is unhealthy for many reasons - obese people are at greater risk for diabetes, heart disease, stroke and even cancer. Now doctors have added rheumatoid arthritis to that list.

GRETCHEN CUDA KROEN, BYLINE: A recent study at the Mayo Clinic found that obese individuals were 25 percent more likely to develop rheumatoid arthritis than people of normal weight.

The link, says Eric Matteson who led the study, isn't related to stress on the joints from being heavy.

DR. ERIC MATHESON: The link we think, has to do with the activity of the fat cells themselves.

KROEN: Unlike osteoarthritis, a form of arthritis that is caused by wear and tear on the joints, rheumatoid arthritis is an autoimmune disease, explains Matteson. It occurs when the body's immune system attacks the lining around the joints and the resulting inflammation leads to the destruction of bone and cartilage. Matteson says it's the fat cells that stoke the fire of inflammation.

MATHESON: We have recognized in the past several years that fat cells are important mediators of inflammation. They are immunologically active, they produce proteins that are inflammatory and so we thought that there might be a connection between the amount of fat cells that a person has and the risk for developing an inflammatory disease like rheumatoid arthritis.

KROEN: Along with a host of molecules that increase inflammation, fat cells produce the female hormone estrogen. What's more, there seems to be a gender bias in inflammatory diseases: A woman's risk of developing an inflammatory disease is nearly twice that of a man's and three out of four people who have rheumatoid arthritis are women.

But studies on estrogen replacement and oral contraceptives have yielded conflicting results, leading Matteson and others to conclude that estrogen is likely just a small piece of a complicated puzzle.

MATHESON: So there's definitely a hormonal link although we don't know yet what the exact nature of that link is.

KROEN: Over the last several decades, obesity rates have risen, and so have the rates of arthritis. In addition, drugs used to treat rheumatoid arthritis generally don't work very well in obese patients. But if being obese increases your risk, does losing weight help?

Matteson believes it can help a lot; not only does it help relieve stress on painful and inflamed joints, he says losing weight makes the drugs work better.

Fifty-eight-year-old Gail Bishop agrees. She's had arthritis since she was 16, and although she is a healthy weight now, she was once 65 pounds heavier. She says losing the weight, and watching her diet has had an enormous impact on her arthritis symptoms.

GAIL BISHOP: For example, on one of my knees, the x-ray looks terrible and it would look like I would need a joint replacement. And when I was heavier, that knee gave me quite a bit of trouble. And now that that weight is off, it's really pretty much pain free. So it makes a dramatic difference.

KROEN: Bishop says she may not be able to control her arthritis, but she can control what she puts in her mouth - and as a result, how she feels. Another way do to reduce your risk is: don't smoke. Smoking dramatically increases the chances of developing rheumatoid arthritis.

For NPR News, I'm Gretchen Cuda Kroen.

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