There's no reason to recommend fish oil supplements, centre, for patients with Alzheimer's disease, U.S. researchers concluded. (Eric Risberg/Associated Press)

Taking omega-3 supplements doesn't help people with mild to moderate Alzheimer's disease, U.S. researchers have found.

Previous studies found that eating fish, a main source of omega-3 fatty acids, was associated with a reduced risk of dementia, but the research did not control who consumed it.

Now researchers have looked at nearly 300 people aged 76 on average who were randomly assigned to take pills of an omega-3 fatty acid called DHA, or a placebo, every day for 18 months. Results were similar in both groups.

"There is no basis for recommending DHA supplementation for patients with Alzheimer disease," said Dr. Joseph Quinn of Oregon Health and Science University in Portland and his co-authors conclude in Wednesday's issue of the Journal of the American Medical Association.

Study participants were assessed for changes in their cognitive and functional abilities, using standard scales for rating Alzheimer's disease and dementia.

No benefit seen

On the Alzheimer's assessment scale, for example, DHA supplements had no beneficial effect: the average change in the score over 18 months was 8.27 points for the placebo group, compared with 7.98 points for the DHA group.

Taking the supplements after symptoms appear may be too late since it's thought the disease process may start years earlier, Dr. Kristine Yaffe, a dementia researcher at University of California at San Francisco, said in a journal editorial.

Genetic makeup could also make a difference, said Prof. Henry Brodaty, director of the Dementia Collaborative Research Centre at the University of New South Wales in Australia.

Brodaty said there is a genetic risk factor for Alzheimer's, based on a gene called apolipoprotein E that comes in several forms. People who carry the E4 type variant have an increased risk for early-onset Alzheimer's disease, although it's not guaranteed.

When the Oregon researchers removed people carrying E4 from the analysis, they found that DHA did have a positive effect on slowing progression of the disease. However, they noted this finding needs replication to be confirmed.

Laurie Ryan, program director of Alzheimer's studies at the U.S. National Institute on Aging, called the results disappointing. Ryan noted the institute is funding research into other possible treatments such as lifestyle changes and drugs.

The institute paid for most of the research. The rest of the funding came from Martek Biosciences, maker of the DHA pills used in the study.

Two co-authors are Martek employees and Quinn is an unpaid consultant for the company. Quinn and two other study authors are also inventors of a patent for using DHA pills to treat Alzheimer's with a certain genetic variation.

With files from The Associated Press and Australian Broadcasting Corp.