Inflammation of the synovial membrane in rheumatoid arthritis is mediated by specialized cells necessary for immune response. The most prominent features are the accumulation of mononuclear phagocytes, lymphocytes and leukocytes in the proliferating tissue.

Pro-inflammatory and proliferative signals are transmitted to the bone marrow and to the synovial membrane. The result is a monoclonal stimulation of specific cell lines, and synovial proliferation in the inflamed joint. Angiogenesis, synovial hypertrophy, and increased perfusion facilitate the accumulation of inflammatory cells. Components of the autoimmune reaction are described in the international system of classification, the CD-System (cluster of differentiation).

Pro-inflammatory signals are mediated by metabolites of arachidonic acid. Prostaglandins, leukotrienes, lipoxines and hydroxy fatty acids, derived from this PUFA, stimulate the formation and the activity of adhesion molecules (integrines), cytokines (gamma-interferon, interleukin-1, interleukin-6, tumor-necrosis factor), chemokines (interleukine-8, macrophage-chemotactic peptide, RANTES and colony -stimulating factors ((CSF, granulocytes/ monocytes-CSF, Multi-CSF (= IL-3)).

Dietary means to mitigate inflammation comprise reduction of arachidonic acid, and increased intake of eicosapentaenoic acid and antioxidants. In the literature 12 randomized, placebo-controlled double-blind studies, fulfilling GCP-criteria, demonstrate a moderate but consistent improvement of clinical findings and laboratory parameters in patients with RA. A dose-response relationship was established up to an daily dose of 2.6 gram fish oil, equivalent to about 1.6 gram EPA.

In these experiments EPA was the omega-3 fatty acid responsible for improvement, with distinct effects on inhibition of cytokines formation (IL-1 to IL-6, IL-8, TFN-alpha, GM-CSF), decreased induction of proinflammatory adhesion molecules (selectines, intercellular adhesions molecule-1 (ICAM-1)), and degrading enzymes (e.g. phospholipase A2, cyclooxygenase-2, inducible NO-synthetase).

Only one study reports the relevance of the background diet. From this study it became apparent that reduction of dietary arachidonic acid improves the incorporation and the clinical benefit of EPA.