Summary
Rheumatoid arthritis is a chronic autoimmune (inflammatory) disease affecting 0.5-1% of people worldwide. The classic rheumatoid arthritis symptoms of pain and reduced mobility severely impact quality of life, current treatment methods focus on management of these symptoms. Omega-3 fatty acids such as DHA are known for their anti-inflammatory properties, studies have found positive effects of omega-3 fatty acids in the context of rheumatoid arthritis.
 
Introduction
Rheumatoid arthritis (RA) is a chronic condition that can drastically impact the quality of life. The disease is characterized by chronic inflammation that affects the joints, leading to unreversible degradation of cartilage and bone. Specifically, the body’s own immune system of RA patients targets the synovium, a type of membrane present at joints and tendons that produces synovial fluid. This fluid is essential for smooth movement, as its presence reduces friction between joints and allows for site-specific immune responses. Typical symptoms reported by patients are tender and swollen joints, accompanied by pain, stiffness and limited mobility.

Though much is still unclear about the disease mechanism behind RA, immune system components such as pro-inflammatory factors and specialized pro-resolving mediators (SPM) are established modulators of RA pathology. Specialized pro-resolving mediators are derived from omega-3 fatty acids and have been found to be particularly important in resolving RA-associated inflammation. Environmental factors are estimated to account for up to 50% of RA risk, highlighting the importance of lifestyle modifications such as diet in the context of RA risk reduction. A positive effect of omega-3 rich fish oil supplementation in reducing joint pain and swelling in rheumatoid arthritis patients has been scientifically established several decades ago. Recent studies have aided in clarifying the mechanism behind this symptom alleviation; DHA in particular has been implicated in positively affecting harmful factors in RA.
 
Research findings
A collaboration between scientists at Harvard and the University of Calgary investigated the effects of fish oil rich in omega-3 fatty acids DHA & EPA on circulating levels of SPMs in persons with and without RA (Marchand et al., 2023). Fifty-two individuals with and 52 individuals without RA were evenly divided into 4 groups of 26 people, matched by age and sex. Half of the participants with and half of the participants without RA self-reported intake of fish oil, the remainder of participants did not consume fish oil supplements. In the two groups that consumed fish oil supplements, circulating DHA & EPA levels and certain SPM precursors were higher compared to the groups not taking fish oil supplements. On this basis, the authors conclude that individuals with RA are able to synthesize SPMs  upon omega-3 supplementation, which may benefit in resolving RA-associated inflammation. The authors noted that variation in fish oil composition and omega-3 fatty acid concentration as limitations of this study, warranting further studies with a more homogeneous treatment.

Aiming to resolve the specific role of DHA in alleviating rheumatoid arthritis symptoms, Dawczynski and collaborators examined the effects of supplementation with algae oil-derived DHA in RA patients (Dawczynski et al., 2018). Thirty-eight RA patients were divided evenly into two groups in a double-blind setup, given two 10-week supplementation regiments with a 10-week washout period where no supplementation was given in-between. One group received 2.1g/day algae oil-derived DHA for 10 weeks, followed by a washout period with no supplementation of 10 weeks and supplementation with placebo (sunflower oil) for 10 weeks. The other group received a reversal of this regiment, starting with the placebo and ending with the DHA supplementation. The authors reported that in both groups, the DHA supplementation caused an increase of DHA and EPA in the red blood cell membrane, proving that the supplementation was taken up by the body. As a result, the DHA period showed a reduction in the sum of tender and swollen joints and morning stiffness duration compared to the placebo period.
 
Conclusion
Studies researching the benefits of omega-3 fatty acids have found a positive role of DHA, either by itself or combined with EPA, in the treatment of rheumatoid arthritis. Levels of circulating SPMs, EPA & DHA increased upon omega-3 fatty acid supplementation, associated with a positive effect against RA-associated inflammation. Crucially, supplementation with algae oil-derived DHA on its own was found to increase circulating EPA and DHA levels, alleviating RA symptoms that negatively impact quality of life. In the light of these findings, individuals with rheumatoid arthritis may consider to ensure their bodies have adequate levels of DHA, either through dietary changes or supplementation.
 
References
 Dawczynski, C., Dittrich, M., Neumann, T., Goetze, K., Welzel, A., Oelzner, P., Völker, S., Schaible, A. M., Troisi, F., Thomas, L., Pace, S., Koeberle, A., Werz, O., Schlattmann, P., Lorkowski, S., & Jahreis, G. (2018). Docosahexaenoic acid in the treatment of rheumatoid arthritis: A double-blind, placebo-controlled, randomized cross-over study with microalgae vs. sunflower oil. Clinical Nutrition, 37(2), 494–504. https://doi.org/10.1016/j.clnu.2017.02.021

Marchand, N. E., Choi, M. Y., Oakes, E. G., Cook, N. R., Stevens, E., Gomelskaya, N., Kotler, G., Manson, J. E., Lasky-Su, J., Mora, S., Lee, I.-M., Tatituri, R., & Costenbader, K. H. (2023). Over-the-counter fish oil supplementation and pro-resolving and pro-inflammatory lipid mediators in rheumatoid arthritis. Prostaglandins, Leukotrienes and Essential Fatty Acids, 190, 102542. https://doi.org/10.1016/j.plefa.2023.102542