First-line treatments for osteoarthritis (OA) are targeted at the inflammatory reaction that occurs after breakdown of articular cartilage through regular use of nonsteroidal anti-inflammatory drugs (NSAIDs), corticosteroid injections, or surgical intervention. Associated activity restrictions and chronic pain have spurred a search for alternative treatments, commonly daily supplements such as glucosamine, chondroitin, and fish oil, to name a select few of the innumerable products reported to benefit patients with OA.
Background
Fish oil is 1 of the 2 most popular supplements among patients with OA. However, its effectiveness and precise benefit are still debated,1,2 and there is confusion about the definition of the product, the nature of investigations into its effectiveness, and the standardization of research unique to OA. Most fish oil research relates to patients with rheumatoid arthritis (RA). The anti-inflammatory benefits seen in patients with RA are generally applied to characterize fish oils as anti-inflammatory agents with a logical benefit in reducing OA symptoms. However, there is a dearth of independent and focused clinical results justifying that assumption. Further, lack of federal regulation of the supplement industry hinders conducting generalizable studies regarding medical benefit in a regulated and verified dose and form.3
The benefits of fish oil in RA treatment are well supported and accepted. In patients with RA, daily fish oil supplementation has been shown to reduce use of other medications and improve pain scores reported by both physicians and patients.4-10 The clinical efficacy of fish oil use in RA has been determined to be “reasonably strong,” with multiple studies confirming suppression of inflammatory cytokines in vitro and in vivo.11,12 The mechanism by which the inflammatory processes are augmented by fish oil supplementation suggests potential benefit to patients with OA, though review articles as recent as 2011 have concluded that research in that capacity is not sufficient to warrant recommendation.13,14
Most studies of OA-specific use of fish oils have been conducted in in vitro models. Treatment of bovine chondrocytes with omega-3 fatty acids causes reductions in inflammatory markers induced by interleukin 1, one of several proinflammatory cytokines that induce inflammation in OA at the gene and plasma levels, and these reductions have been reproduced.15-17 Although a preventive benefit was found in a study of pig medial collateral ligament fibroblasts, findings of later studies have been inconsistent.18 It also appears that fish oils may alter lipid composition in membranes, favoring incorporation of anti-inflammatory precursor n-3 fatty acids over proinflammatory precursor n-6 fatty acids in these model systems.19,20
Animal in vivo models have also been used to describe the effects of fish oil supplementation on OA. Assessment of dogs with OA before and after supplementation with the omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) revealed improvement in clinical signs observed by owners, improvement in weight-bearing measured by veterinary clinicians, and decreased use of NSAIDs.21-24
Fish oil studies using osteoarthritic cartilage samples harvested during surgical procedures have demonstrated results consistent with other model systems described thus far. They have demonstrated a dose-dependent decrease in induced inflammatory destruction of tissue associated with fish oil supplementation. In addition, finding a lack of cellular toxicity, they have validated the safety of supplements.25,26 Proposed but unproven mechanisms for the anti-inflammatory actions of EPA and DHA include competition with n-6 fatty acids; presence of resolvins (anti-inflammatory molecules derived from EPA and DHA); presence of n-3 products that compete with proinflammatory molecules for receptors; reduction in gene expression of cytokines, cyclo-oxygenase 2, and degrading proteinases; interference in the signaling pathways of inflammation; and reduction in lymphocyte proliferation.26,27
Reduction in the n-6/n-3 ratio has been correlated with reduced inflammatory conditions such as OA, stemming from the epidemiologic evidence that higher n-3 intake in Eastern diets and lower intake of n-6 result in a lower incidence of these diseases.18,28,29 Studies have found sufficient evidence to suggest that this ratio has a role in OA, though not sufficient to recommend supplement use over diet modification.19 One study demonstrated an ability to favorably alter bone marrow lipid composition with n-3 fatty acid supplementation.10
The evidence leads to a conclusion of anti-inflammatory benefits from fish oils in these abstracted models. The multitude of basic science studies conducted on the anti-inflammatory properties of omega-3 fatty acids, only briefly reviewed here, supports the potential benefits colloquially ascribed to fish oil in the treatment of OA yet also implies the need for human clinical trials to address these properties clinically.
We reviewed the literature to address claims that fish oil supplementation can prevent or decrease severity of OA. We hypothesized there would be insufficient clinical studies to justify recommending supplementation to patients. Of note, the degree of heterogeneity in the evidence precluded performing a meta-analysis with any statistical validity.