Background/Purpose: Breastfeeding (BF) has been associated with both a decreased and an increased risk of developing rheumatoid arthritis (RA). Regarding oral contraceptive (OC) use and the risk of RA a few studies have shown a negative association but the majority of reports have been unable to demonstrate an association. To the best of our knowledge no previous study has investigated the impact of BF and OC use on the two subgroups of RA, characterized by presence/absence of antibodies to citrullinated peptides (ACPA). Our aim was to study the association between BF history and OC use and the risk of ACPA-positive/-negative RA among women.

Methods: Data from the Epidemiological Investigation of RA (EIRA), a population-based case-control study, comprising women aged 18-75+ living in Sweden, between 1996 and 2013 was analyzed (2637 cases and 4244 controls). For BF, data was available between 2006 and 2013 (884 cases and 1947 controls, parous women). An extensive questionnaire was answered by the participants, including questions regarding duration of BF for each delivered child, OC use and potential confounders (education, smoking, BMI, alcohol consumption, postmenopausal hormone therapy, and reproductive factors). Total BF history among parous women was calculated as the sum of the duration of BF for each delivered child and categorized as 0-6, 7-12 and ≥13 months. Current use of OCs was defined as those who were currently using OCs during the index-year and started at least one year before symptoms debut. Past users were defined as those who used OCs in the past and had stopped at least the year before the index-year. Ever users were defined as current and past users while never users correspond to women who had never used OCs before the index-year. We calculated odds ratios (ORs) with 95% confidence intervals (CI) of developing ACPA-positive/-negative RA, by means of unconditional logistic regression, adjusting for age, residential area and pack-years.

Results: A longer history of BF was associated with a reduced risk of developing RA (OR=0.76, 95% CI 0.62-0.93 and OR=0.91, 95% CI 0.74-1.12 for breastfeeding ≥13 months and 7-12 months respectively, compared with up to 6 months). The trend was significant for ACPA+ RA (p-value 0.0088), but not for ACPA- RA (p-value 0.2833). These estimates were attenuated when we adjusted for pack-years. Ever users of OC had a decreased risk of developing RA compared with never users (OR=0.88, 95% CI 0.79-0.98), mainly due to a decreased risk among past OC users (OR=0.88, 95% CI 0.80-0.98). The association was significant for ACPA+ but not for ACPA- RA. A longer duration of ever OC use (≥8 years) was significantly associated with a decreased risk of RA overall (OR=0.83, 95% CI 0.73-0.94) and ACPA+ RA (OR=0.82, 95% CI 0.71-0.95), while a borderline association was found for ACPA- RA (OR=0.83, 95% CI 0.69-1.01).

Conclusion: Both BF and OC use were associated with a reduced risk of developing RA, especially for ACPA+ RA. Our study contributes to the knowledge of environmental risk factors and its different impact on the subgroups of RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/breastfeeding-oral-contraceptive-and-the-risk-of-rheumatoid-arthritis-results-from-the-swedish-epidemiological-investigation-of-ra-study/