After decades of decline, rheumatoid arthritis is on the rise among white women in the US, new research shows.

While the study wasn’t designed to investigate why this is happening, the speed of the change points to environmental, rather than genetic, factors, Dr. Sherine E. Gabriel of the Mayo Clinic College of Medicine in Rochester, Minnesota, one of the study’s authors, told Reuters Health. “It’s pretty unlikely that the genetic makeup of a population changed that quickly.”

In rheumatoid arthritis, or RA, a person’s immune system attacks the joints, leading to inflammation, pain, stiffness and in some cases erosion of the bone and joint deformity.

In their study, Gabriel and her team used data from the Rochester Epidemiology Project, which has been tracking medical records from health care providers in Olmstead County, Minnesota since the early 1900s, and has been funded by the National Institutes of Health for more than 40 years. The project includes information on virtually everyone who has sought medical care in the county for the past several decades.

The researchers had previously analyzed data for the period 1955 to 1994, which showed a steady decline in cases of RA for both men and women. In the current analysis, of data from 1995 to 2007, they identified 466 people with a confirmed diagnosis of the disease.

During that time, the researchers found, RA risk held steady for men, with about 28 men diagnosed annually for every 100,000 men in the general population.

But for women, there was a slight increase in incidence of 2.5 percent per year between 1995 and 2007, with an average annual incidence of 53 diagnoses per 100,000 women for the entire time period.

The actual percentage of women with RA in the county rose from about three-quarters of a percent in 1995 to nearly 1 percent in 2005; for men, fewer than half a percent had RA at both time points.

There are many possible environmental factors that could be playing a role in the decline and subsequent increase of RA among women, Gabriel and her team note. For example, while smoking - one of the few confirmed risk factors for the disease -declined sharply since the 1950s, it hasn’t declined as quickly among women as it has among men. “Women were sort of slower to pick up that bad habit, and they seem to be a little slower to drop it,” the researcher said.

Other factors could include the changing composition of oral contraceptives. The pill has been shown to protect women against RA. However, because the oral contraceptives of today contain far less estrogen than they used to, they may be less protective, according to Gabriel and her team.

Vitamin D deficiency, which has been linked to RA, is another possibility, the researchers say. Deficiency in this nutrient has been on the rise in recent years, particularly among women.

Because the population of Olmstead County is 90 percent white, the findings likely can’t be extrapolated to other ethnic groups. Nevertheless, the researchers say, the pattern observed in the current study is very likely to reflect that in the general US white population.

While the findings are mostly of interest from a scientific perspective, they offer a couple of messages for women, too, according to Gabriel: don’t smoke, and get checked for vitamin D deficiency. The findings provide “just one more reason to quit that bad habit,” she said.

SOURCE: Arthritis & Rheumatism, online February 26, 2010.