Smoking is a risk factor for rheumatoid arthritis (RA), a new analysis of 16 studies confirms.

The effect is especially strong in men and heavy smokers, the researchers found. And men who tested positive for rheumatoid factor (RF), a self-attacking antibody found in about 80 percent of RA patients, were at even higher risk if they smoked.

Research over the past two decades has linked smoking to RA, especially in men, Dr. S. Kumagai of Kobe University Graduate School of Medicine in Kobe, Japan and his colleagues write. But findings on smoking and RA in women have been “inconsistent.”

The researchers conducted the first systematic analysis of research on RA risk and smoking, looking at 16 studies in all.

Men who were current smokers were at nearly double the risk of RA, Kumagai and colleagues found, and the effect was roughly the same in ever- and past smokers. When the researchers looked at RF-positive RA, they found male smokers were at nearly four-fold risk of the disease, while risk was tripled in ever-smokers and about 2.5 times greater for past smokers.

Smoking also increased RA risk in women, but to a lesser degree. Female current, ever- and ex-smokers had a 1.2 to 1.3 times greater likelihood of developing RA, whether or not they were RF-positive.

The men who had logged at least 20 pack years - meaning they had smoked at least 20 cigarettes a day for 20 years - were 2.3 times more likely to develop RA, while for women risk was increased 1.75-fold.

Smoking has been linked to RF production, the researchers note. The relationship among RF, RA, and smoking may be different for women, they add, due to hormonal factors.

“Any type of smoking constitutes a significant risk factor for the development of RA,” Kumagai and colleagues write. “Because RA is associated with a poor quality of life and life prognosis, we recommend cessation of smoking for current smokers, especially heavy smokers to prevent or reduce the risk of developing RA.

SOURCE: Annals of the Rheumatic Diseases, January 2010.