Scientists have found further evidence that taking cod liver oil could slow down damage caused by arthritis.
Cod contains healthy oils
A small study of tissue taken from knee replacement patients found lower levels of chemicals linked to the disease in those who took pills before surgery.
Although the impact on pain levels was not tested, experts at Cardiff University say it is possible the oil could delay the onset of the disease.
It contains "omega-3" oils, which are linked to other health benefits.
The UK currently spends £5.5 billion annually dealing with the effects of arthritis on the NHS and social services.
The Cardiff researchers are seeking ways to lessen the impact of this devastating illness.
One, they suggest, might be to return to the age-old remedy taken - with great reluctance - by a whole generation of growing children.
Professor Bruce Caterson, who led the study, wanted to see if cod liver oil affected the disease in live patients.
Rather than recruiting large numbers of patients to take cod liver oil and report back later whether they felt better or worse, he sought chemical "markers" within joint tissue that might reveal how active their arthritis was.
Arthritis is an inflammatory disease, in which cartilage - the tough material within joints that protects the bone surface - is slowly degraded, causing increasing damage.
When it reaches a certain point, knee and hip joints may have to be replaced to reduce the pain levels and disability.
The disease is complex and not fully understood, but scientists have linked higher levels of certain body chemicals within knee tissues to this inflammatory process.
Professor Caterson's team measured levels of these chemicals in knee tissue taken during knee replacement operations from 25 patients.
Of these, 14 were given cod liver oil in the months approaching the operation, while the remainder got a dummy "placebo" pill.
The researchers found 86% of the cod liver oil group had significantly reduced levels of these potentially harmful chemicals - compared with a quarter of the placebo group.
Chemicals linked to joint pain itself were also reduced in the cod liver oil group.
'Not a cure'
The patients were not asked if symptoms had improved while taking cod liver oil - and there is no evidence yet "benefits" would be sustained if the patient carried on taking it beyond a few months.
Professor Caterson stressed this was neither a "cure", nor likely to reverse joint damage already sustained.
He said: "The breakthrough is hugely significant because it demonstrates the efficacy of a dietary intake of cod liver oil in patients with osteoarthritis taken prior to joint replacement surgery.
"It suggests cod liver oil has a dual mode of action, potentially slowing down the cartilage degeneration inherent in osteoarthritis and reducing factors that cause pain and inflammation."
He now hopes to organise a larger study to test whether cod liver oil has a noticeable effect on pain levels and disease progression.
Fergus Logan, chief executive of the Arthritis Research Campaign - which funded the study - said: "These findings provide further proof taking cod liver oil or eating lots of oily fish slows down cartilage destruction in osteoarthritis".
He said this was "great news for those people who have the condition or worry about developing it, and who want to do something positive to help themselves".