For decades, older adults have been advised to take fish oil supplements to strengthen bones, protect the heart and support healthy brain ageing. But now researchers believe the supplement - one of the most widely taken forms of omega-3 - may not be as powerful as once thought, with little evidence the capsules can help protect against Alzheimer's. 'We all wish there was a silver bullet for preventing Alzheimer's, but our findings show that fish oil supplements do not appear to protect brain health,' Dr Hussein Yassine, the study's lead author said. 'While omega-3s play an important role in forming brain cell connections needed for cognition, our results do not support fish oil supplements as a preventative measure against Alzheimer's.' Diets rich in omega-3 fatty acids, such as the Mediterranean diet, have long been linked to health benefits for the heart, brain and other organs. This is partly due to the fact that the body cannot make these essential fatty acids, meaning we have to get it from our diet. Some studies have found that those with higher blood levels of omega-3 fatty acids have better cognitive function. But others have found that giving people who already have dementia omega-3 supplements does not boost their mental capacity. A separate study found that older adults taking omega-3 supplements experienced faster declines in memory and thinking skills over timeThe latest study - published in the journal eBioMedicine - followed 365 adults, aged 55 to 80-years-old who rarely ate fish and were considered at risk for Alzheimer's. Around half of participants carried the risk APOE4 gene - the strongest genetic risk factor for the disease.Participants were randomly selected to receive either daily fish oil supplements - containing 2,000mg of docosahexaenoic acid (DHA), a key omega-3 involved in brain function - or a placebo. Researchers, from the USC Center for Personalised Brain Health, found that DHA levels in fluid around the brain increased by around 17 per cent after six months, confirming the essential nutrient reached its intended target.Two years later, participants underwent a number of memory and cognition tests and brain scans to see if their brain had shrunk since the beginning of the study. Results showed there was no difference in cognitive performance between those who took a daily supplements and those given a dummy pill. Brain scans also showed the supplements failed to protect against brain shrinkage, particularly of the hippocampus - the brain area responsible for memory. The researchers concluded that while the supplements can reach the brain, omega-3s may work better as a part of a Mediterranean-style diet, rather than a standalone supplement.