One of the most popular nutritional supplements these days is fish oil. It contains a high proportion of omega 3 fatty acids (EFA), notable eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). This supplement is purported to have a broad range of beneficial effects in many disease conditions due to its effect on chemicals in the body involved in the inflammatory response (for more details see this article on eicosanoids). Some of these effects, such as a reduction in the rate of heart attacks in people with established heart disease and heart attack risk factors, are well-supported by research data. Other claims are less clearly valid. Several literature summaries are available from the Agency for Healthcare Research and Quality, and Medline.
In the veterinary arena, EFA supplements are widely used for allergic skin disease, with modest supportive clinical trial evidence (see also 1, 2, 3, 4). In humans, there is limited evidence to support an effect on some clinical variables in patients with rheumatoid arthritis, and some have suggested osteoarthritis treatment as a veterinary application for these supplements. There is reasonable biologic plausibility to support investigating this use of EFA supplements, and two articles in a recent issue of the Journal of the American Veterinary Medical Association (JAVMA) report studies evaluating the use of fish oil supplements for dogs with confirmed osteoarthritis. I shall briefly review each of them.
The first study  was a multicenter, randomized, double-blind, placebo-controlled prospective study lasting 24 weeks. 167 dogs were randomized into two groups, one receiving a diet supplemented with EPA and DHA, the other receiving a pretty closely-matched control diet. There were no significant differences in any relevant variables between the groups at baseline. 23% of the dogs failed to complete the study (9% in the treatment group and 14% in the control group), and there were no significant differences between the groups with respect to these dropouts.
Assessment measures were subjective, with an owner survey and a clinical assessment by participating veterinarians. Dogs were evaluated at 6, 12, and 24 weeks after beginning the diets. Bloodwork showed significant increases in the blood levels of EHA and DHA in the dogs fed the test diet, so these substances were clearly absorbed. The owner survey evaluated 13 measures of comfort and function over three time periods (0-6 weeks, 6-12 weeks, and 12-24 weeks on the diets). Of these, 2 measures were significantly different between the groups at the first evaluation, and 1 measure was different at the last evaluation. There were no significant differences between the groups in clinical evaluation by veterinarians.
The study was apparently well-designed and well-conducted. The measures of outcome were subjective, which is less than ideal. It seems fairly clear that the results do not support the use of EFA supplementation for osteoarthritis. After all, out of 39 possible points at which the groups were compared in terms of owner evaluation only 3 showed changes not attributable to chance, and none of the evaluations by veterinarians showed significant difference. Unfortunately, the discussion section of the article is less an objective survey of the trial or the overall preponderance of the evidence than it is an attempt to put the most positive possible spin on the results.
The authors suggest their subjective measurement instrument may not have been sufficiently sensitive to detect a change and try to attribute the failure of the veterinarians to detect a difference to the hospital environment and limitations on clinical evaluation. They then conclude, “Our results suggest an ameliorative effect of omega-3 fatty acid supplementation in arthritic dogs,” and “ingestion of the test food….appeared to improve the arthritic condition in pet dogs with osteoarthritis.” Such a conclusion so clearly at odds with the data presented in the report seems to be more an example of confirmation bias and cognitive dissonance than a reliable presentation of the evidence.
The second study  was conducted by several of the same authors. It too was a well-designed randomized, double-blinded, placebo-controlled, prospective study of the effects of dietary EFA supplementation on dogs with osteoarthritis. 44 dogs were enrolled in the study, and 14% failed to complete it (9% in the treatment group and 5% in the control group). Again there were no significant differences detected at baseline and no differences with respect to dogs that failed to finish the study. Both subjective and objective measures were evaluated at baseline and after 45 and then 90 days of the diets. Owner and clinician subjective evaluations were compared as were the results of force plate gait analysis.
In contrast to the previous study, there were no significant differences in owner evaluations of the subjects’ comfort and function. The authors attributed this to the low number of subjects rather than the more parsimonious explanation that there was no differences of sufficient magnitude to be noted. The clinician evaluations at 90 days showed significant differences from baseline for the test group in 3 of 5 measures. However, there were no significant differences between the test and control group for any measure.
The authors stated, correctly in my opinion, that “subjective assessment of limb function lacks repeatability as an outcome measure and is inferior to objective data obtained from force platform gait analysis.” Such an analysis was performed on all subjects. The results showed no change from baseline to 90 days for any of six variables measured (peak vertical force, vertical impulse, braking and propulsive peak forces, and braking and propulsive impulses). The test group did show a statistically significant difference in the mean percentage change in one measure, peak vertical force.
Again, these results provide lackluster support for the contention that EFA supplementation may be beneficial for dogs with osteoarthritis. Some subjective clinical measures showed a difference, but this is not consistent with the results of the other, larger trial, and the authors themselves minimized the significance of these results in both papers. One objective measure did show a statistically significant, and likely clinically significant change. However, the combined results of the two trials offer tepid support for the hypothesis under examination, and an interpretation of no meaningful effect seems much better supported by the results.
Unfortunately, the authors again spin these results in the most positive way possible:
“Together with the findings of our other study, findings of the study reported here supported the hypothesis that ingestion of fish oil omega-3 fatty acids improves clinical signs in dogs with osteoarthritis….Dietary supplementation with fish oil omega-3 fatty acids resulted in an improvement in weight bearing in dogs with osteoarthritis.”
The authors do acknowledge some of the limitations of their study and suggest that further research is necessary for definitive recommendations. I would agree that these results might justify further study, though I see no reason to expect dramatic findings of benefit. However, the reality is that in the world of veterinary medicine, with limited resources and clinical trial evidence, the more likely outcome of these reports is going to be an increase in the prescription for EFA supplements intended to treat osteoarthritis. The positive statements in the abstracts and discussion sections are likely to be the “take-home” message many readers get from the reports, despite the reality that the data is considerably less positive. I always appreciate well-designed and conducted research, but these reports emphasize the difficulty in conducting such trials without having an investment in their outcome that affects the interpretation of the results. This is a large part of why careful and critical evaluation of the primary literature for oneself is such a key component to sound, evidence-based practice.
1. Roush JK, Dodd CE, Fritsch DA, Allen TA, Jewell DE, et al. Multicenter veterinary practice assessment of the effects of omega-3 fatty acids on osteoarthritis in dogs. J Am Vet Med Assoc. 2010 Jan 1;236(1):59-66.
2. Roush JK, Cross AR, Renberg WC, Dodd CE, Sixby KA,, et al. Evaluation of the effects of dietary supplementation with fish oil omega-3 fatty acids on weight bearing in dogs with osteoarthritis. J Am Vet Med Assoc. 2010 Jan 1;236(1):63-73.