An Australian study showing that high-dose fish oil had no benefit over two years compared to a low ‘placebo’ dose in patients with knee osteoarthritis may have unintentionally revealed a benefit from monounsaturated oleic acid.
Adelaide rheumatologist Dr Catherine Hill and colleagues randomised 202 patients with painful OA to high-dose fish oil containing 4.5 g omega-fatty acids daily or what they considered a placebo, containing just 0.45 g omega-3 fatty acids with the balance made up of sunola oil, which is low in saturated, n-6 and n-3 fatty acids.
“The low-dose fish oil group had greater improvement in WOMAC pain and function scores at 2 years compared with the high-dose group, whereas between-group differences at 1 year did not reach statistical significance,” they said.
“There was no difference between the two groups in cartilage volume loss at 2 years. For other secondary endpoints, there was no difference between the two groups at 2 years.”
Writing in the Annals of the Rheumatic Diseases, Dr Hill’s team said that, with hindsight, the most appropriate control would be no oil at all, but it would have made the study unblinded.
“However, given the study we performed, it is a reasonable conclusion that it is still unknown whether low-dose fish oil and/or sunola oil are beneficial for knee OA,” they said.
An editorial commented that oleic acid might be anti-inflammatory despite not containing the polyphenols thought to drive the anti-inflammatory effects of some other oils.
High-dose oleic acid, though, had been shown to suppress insulin resistance and inflammation in animal models and counteract the pro-inflammatory effect of arachidonic acid.
It also influenced adaptive immunity and reduced the activity of cytotoxic T lymphocytes.
A letter in response to the paper, from New York rheumatologist Dr Joel Kremer, said it would be regrettable if the findings were interpreted as proving that fish oil has no benefit in OA.
“It is quite possible that the patients…were in fact given two active interventions,” he said.
“There is a rather rich history of oleic acid-induced changes in immune parameters. Olive oil has been demonstrated to result in significant clinical improvements in the MLR/lpr mouse model of autoimmune disease, and to be associated with significant clinical improvements in rheumatoid arthritis.”