Why the incidence of RA in women is increasing, but not in men?

Thanks to Shauna from RA Chicks for asking this question and giving me the topic to my next blog!  It seems that RA is more prominent in women, than in men and the incidence in women seems to be increasing and in men it seems to be stable.  According to a Mayo Clinic Study, after 40 years of decline (during the years 1955 to 1994), the incidence (frequency of occurrence) and prevalence (total number of cases in a given population) of rheumatoid arthritis among women is rising. From 1995 to 2005, the incidence of rheumatoid arthritis among women was 54 per 100,000 compared to 36 per 100,000 for the previous 10 years.  For men, the incidence was stable at 29 per 100,000. Researchers concluded that an environmental factor could explain the reverse in trend for women.

The Mayo Clinic also did research as to why it seems to be increasing in women. The incidence and prevalence of RA vary widely across populations, geographic areas, and time periods, probably as a result of an interplay between genes and environment.  One factor that has been considered, and the strongest, is cigarette smoking.
While smoking overall has declined in recent years, the decline has been much slower among women. The percentage of men who smoked fell from 78% in the earlier decade to 59%  whereas rates of smoking among women remained almost unchanged.

Another factor is Vitamin D deficiency.  Which seems to also be rising among women in recent decades, may further contribute, along with other environmental factors such as infections, obesity, and socioeconomic factors.

Another potentially important contributor has been the change in the formulations used in oral contraceptives. Oral contraceptives have been shown to be clearly protective against RA in women, but contemporary pills contain significantly lower doses of estrogen than the older formulations. In this study the researcher stated, "Because these lower doses of synthetic estrogen's confer less protection against the development of RA than higher doses, it is almost certain that the protective effect of birth control has diminished over time."

Estrogen, the hormone, has played a part in RA.  More women develop RA during the months after childbirth, or during the pre-menopause years. A study was done about this in 2003, you can read about it HERE.  It stated that, irregular menstrual cycles correlated with an increased risk of RA. And women who breastfed longer than twelve months were less likely to develop Rheumatoid Arthritis.

Also in the study it stated that certain infections could have played a role in the increase of RA.  An infection called Porphyromonas gingivalis,  a gram-negative organism associated with periodontists. It stated that this bacteria could mediate changes in the body resulting in expression of RA-specific autoantibodies. 

UPDATE:  Here is an email from my Rheumatologist responding to this very question:
Hi Mallory,
Thanks for your email.
The short answer to your question is that we don't know.
Women are more frequently affected not only by RA but also by lupus erythematodes (SLE) and other rheumatic diseases. Differences in sex hormone levels are the most commonly cited reason. In line with this, RA often gets better during pregnancy (which is characterized by drastically changed hormone levels). In contrast, SLE tends to get worse during pregnancy (these things are complicated).
Women have two X chromosome while man have an X and a Y chromosome. It could also be that genes located on the X or Y chromosome increase (or decrease) the risk for getting the disease. And then there are other possible factors such as differences in environmental exposures, eating habits, etc... So the long answer is that there are hypotheses but no conclusive explanation.
Best,
Joerg Ermann

Sadly, it seems that more research needs to be done as to why it is increasing in women, and not in men.  Nothing seems to be definite.  In the study they basically said to not smoke, and to get checked for Vitamin D deficiency, which is only great if you don't already have RA.  My advice to those who don't have RA yet, don't smoke, and make sure you have enough Vitamin D and Calcium in your diet.  Drinking milk, eating yogurt and sunlight are three examples of Vitamin D.  They also sell Vitamin D (with calcium) supplements and your local vitamin stores.  However, even doing that doesn't seem to necessarily help.  I've had JRA since I was 3, never smoked a day in my life and have always been a big milk drinker/yogurt eater and I take 2 Vitamin D pills a day.  Scientists are doing what they can, but are more concerned about finding a cure for RA, which in my book is excellent.  In meantime, us women need to stay strong and stick together, because we are strong, wonderful women who don't need an illness to let us down!