Fish oils are the current wonder supplement, good for all that ails you. I have actually taken them when the evidence appeared to support a benefit in prevention of cardiovascular disease, but the evidence is not looking as strong these days even for that indication, which has been the most strongly supported of the many suggested uses in humans. Other uses include prevention or treatment of dementia, arthritis, inflammatory diseases, and many others.
I have reviewed in detail suggestions that fish oils might have a benefit for arthritis in dogs, and it is weak but not definitive either way. Others have reviewed the use of fish oils in dogs with allergic skin disease, and there is reasonable evidence of some mild benefits for that condition. There have been suggestions that fish oils, as supplements or added to dog food, can affect the development of canine cognitive dysfunction, but there is no robust research evidence to support this.
A recent Cochrane review looking at fish oil supplementation and dementia in humans did not find evidence to support a beneficial effect despite a number of quite large clinical studies.
Emma Sydenham. Alan D Dangour. Wee-Shiong Lim. Omega 3 fatty acid for the prevention of cognitive decline and dementia.
Three randomized clinical trials involving over 4000 people were reviewed. Some subjects were supplemented and followed for as long as 2 ½ years. Despite this, no evidence of benefit was found. Few adverse events were seen either, apart from gastrointestinal upset. The conclusion of the review was:
Direct evidence on the effect of omega-3 PUFA on incident dementia is lacking. The available trials showed no benefit of omega-3 PUFA supplementation on cognitive function in cognitively healthy older people. Omega-3 PUFA supplementation is generally well tolerated with the most commonly reported side-effect being mild gastrointestinal problems.
Certainly not the last word, but yet another fairly strong piece of evidence suggesting that even one of the most intensively studied and widely used supplement has yet to definitively demonstrate the benefits claimed for it. And while data of similar strength is unlikely to be developed for dogs with cognitive dysfunction, the tentative conclusion based on extrapolating from the human research has to be that fish oils could have benefits in this condition, but it is at least as likely that they do not.