An ingredient in fish oil doesn't slow cognitive decline in people with Alzheimer's disease, according to a new clinical trial.

Alzheimer's disease patients who took docosahexaenoic acid (DHA) supplements had the same rate of cognitive decline as people who didn't take DHA, said study researcher Dr. Joseph Quinn, of Oregon Health & Science University.

"I am not recommending fish oil or DHA supplements for people who have dementia or Alzheimer's in the absence of evidence for it," Quinn told MyHealthNewsDaily.

However, it's still possible that DHA, one of the omega-3 fatty acids in fish oil, could ward off Alzheimer's disease, because the study did not test the effectiveness of the supplement in healthy people, he said.

Quinn and his colleagues conducted the study on 295 people with mild to moderate Alzheimer's disease. About 70 percent of the people took two grams of DHA a day, and the other 30 percent took a placebo.

After 18 months, the patients' cognitive decline was measured. The patients who took the supplement had the same rate of cognitive decline as those who took the placebo, according to the study.

Although DHA did not seem to have an effect, taking this supplement isn't the same thing as eating a diet high in fish, Quinn pointed out. It's possible that other components of fish oil, or other non-fish elements of the average fish-eaters diet, might be more effective than DHA alone, he said.

Previous studies have linked fish oil and Alzheimer's disease, with varying results.

An animal study published earlier this year in the journal Neuroscience Letters found fish oil enhanced the effects of green tea in reducing the amyloid plaques in the brain that are linked to Alzheimer's disease. And another mouse study done in 2005 found DHA helped reduce buildup of the plaques.

However, these studies only looked at how fish oil can affect risk of developing dementia, not the decline that happens once it has set in, Quinn said.

The new findings support another study published this year in the American Journal of Clinical Nutrition, which found taking fish oil supplements over a two-year period did not improve cognitive function in people 70 to 80 years old.

Next, Quinn said he will investigate using fish oil to prevent cognitive decline in elderly people who have mild cognitive impairment, which is a risk factor for later developing dementia.

The study will be published tomorrow (Nov. 3) in the Journal of the American Medical Association.

This article was provided by MyHealthNewsDaily, a sister site to LiveScience.