Several studies have shown that eating fish high in omega-3 fatty acids protects against various diseases, including cognitive decline. The research has spawned a big industry for fish oil supplements that its makers tout as being beneficial to brain health.

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But a new study casts doubts on the claims. Research from the National Eye Institute and the National Institutes of Health concluded that omega-3 fish oil supplements "did not slow cognitive decline in older persons."

The study, published in JAMA, the Journal of the American Medical Association, does not counteract the previous research that eating foods rich in omega-3s, such as mackerel, salmon and tuna, protects against an array of health problems, including cardiovascular disease and cognitive decline.