Breast-feeding may reduce mother's RA risk.(Obstetrics)

Mothers who breast-feed for at least a year have a reduced risk of developing rheumatoid arthritis a decade or so later, according to a major longitudinal study.

Results from the Nurses' Health Study, a prospective cohort study of 121,700 women, suggest that a total lifetime breast-feeding duration of 12-23 months could reduce the risk of eventual RA by 20%--after adjusting for possible confounding variables--compared with parous women who do not breast-feed.

The risk reduction rises to a significant 50% if women breast-feed for more than 24 months, indicating a dose-response effect. (See table.)

"The biologic mechanism for this is unclear," reported Elizabeth W. Karlson, M.D., of Brigham and Women's Hospital, Boston, and colleagues (Arthritis Rheum. 2004;50:3458-67). Indeed, rather than a "strong trend" toward an inverse relationship between breast-feeding and the risk of RA, which they found, the investigators had suspected that the helper T cells type 1 (Th1)-promoting effect of prolactin, the primary hormone that is elevated during breast-feeding, would have led to more RA from breast-feeding.

The investigators also reported that a marked irregularity in menstrual cycles between the ages of 20 and 35 years appeared to be a risk factor for RA, "which, to our knowledge, has not previously been studied."

Earlier age at menarche significantly increased the eventual risk of RA, but no other reproductive hormonal factor was associated with RA risk, including the use or nonuse of oral contraceptives and the number of pregnancies.

"Although [the] participants are not a random sample of women living in the U.S., it is unlikely that the basic biologic relationships among these women will differ from those among women in general," the investigators ...