We have all either had cod liver oil ourselves or have at least known near and dear ones who did. It is a common belief that fish oil supplements, which are high in omega-3 fatty acids, prevent or slow down mental decline. But guess what, they do not, says a study spanning 4,000 old people.
"Contrary to popular belief, we didn't see any benefit of omega-3 supplements for stopping cognitive decline," says Emily Chew, author of the study and deputy clinical director at the National Eye Institute, which is part of the NIH.
The five-year clinical trial, published in the Journal of the American Medical Association, is "one of the largest and longest of its kind," according to a statement from the US National Institutes of Health, which funds the research.
Omega-3 fatty acids are found in fish oils, and people who regularly eat fish such as salmon, tuna and halibut have been shown to have better eye, heart and brain health than people who do not eat these kinds of fish.
Read: How much do you know about the fish you are eating?
But consuming the oils in pill form is not the same. A previous study in 2011 showed that omega-3 supplements, sold over the counter, did not improve the brain health of older patients with pre-existing heart disease.
The current study involved people with a common form of vision loss, called age-related macular degeneration. Patients are 72 years old on average, and 58% of those enrolled are female.
They are randomly assigned to take either a placebo or pills containing omega-3 fatty acids, specifically docosahexaenoic acid (DHA) and eicosapentaenoic acid.
Those enrolled are also given memory and cognitive function tests at the beginning of the study, then again two and four years later.
"The cognition scores of each subgroup decreased to a similar extent over time, indicating that no combination of nutritional supplements made a difference," says the study.
With Alzheimer's disease set to balloon as the global population ages in the coming decades, researchers are keen to find ways to prevent the most common form of dementia.
Some 46.8 million people worldwide currently suffer from dementia, a number that is forecast to reach 131.5 million in 2050, according to Alzheimer's Disease International.