Older and wiser, the saying goes.
But taking fish oil tablets, also known as omega-3 supplements, does not make the older wiser, a new study has found.
For years, observational studies have suggested that people who consume more omega-3, from food such as fish or with supplements, perform better on memory tests and other measures of brain function.
But new US research has found these supplements do not stave off mental decline and memories loss in older people.
"Contrary to popular belief, we didn't see any benefit of omega-3 supplements for stopping cognitive decline," said lead researcher Emily Chew.
In a study of more than 3000 people with an average age of 72, Chew's team gave a proportion of the group fish oil tablets, while the rest were placed on a placebo. Participants' cognitive function was tested every two years.
After five years, the researchers found no difference in the cognitive scores of people given fish oil compared with those who were not.
The findings, published in the medical journal JAMA, contradict some epidemiological evidence that people who take omega-3 and antioxidants have a lower risk of developing dementia.
The researchers of the latest study speculate that their study may not have been long enough to measure any effects or they may have conducted their program in people who were too old.
Dr Chew said the process of cognitive decline may occur over decades, meaning a short-term supplementation given too late in the disease may not be effective.
"It is possible that eating foods rather than taking any specific single supplement may have an effect," said Dr Chew, deputy clinical director at the National Eye Institute, part of the National Institutes of Health, which funded the study.
In 2012 the evidenced-based medicine group Cochrane Collaboration published a review of the research on fish oil supplements preventing dementia.
The review found the claim lacked any direct evidence, but further study was needed to determine whether the supplements could work in the long term.
In 2014 consumer group Choice found the Australian sales of omega-3 supplements exceeded $200 million annually and were growing at more than 10 per cent per year.
William Brooks, a clinician and researcher at Neuroscience Research Australia (Neura), said while epidemiology studies had suggested the supplement was working, randomised control trials were the gold standard to establish the effect of any drug, supplement or intervention.
"We've seen it in Alzheimer's Disease that epidemiology evidence suggested hormone replacement therapy may prevent it in women, but then RCT's found women on the therapy actually had a higher incidence of Alzheimers' Disease," he said.
Epidemiology studies also found a similar association between Alzheimer's Disease and anti-inflammatory drugs. But randomised control trials found they had no effect.
"I've seen a couple TV programs say Omega-3 is great for kids and learning but the evidence isn't there," said Dr Brooks.
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