How true is the claim? Quite. In 2006 researchers at Tufts University, in Boston, found that people with the highest concentration of DHA (a fatty acid found in many fish, including wild Alaskan salmon, herring, and mackerel) in their blood had an almost 50 percent reduction in their risk of developing dementia and Alzheimer's disease. While science is still awaiting a definitive study to prove that fish or fish-oil supplements can keep the brain sharp, the American Heart Association recommends two servings of fish―such as salmon―a week.

Why it gets repeated: No one is quite sure how Grandma knew to push the cod-liver oil, but she seems to have been right.

Bottom line: Add low-mercury fish and seafood, such as shrimp, canned light tuna (not albacore, or white, tuna), wild salmon, and catfish, to your diet. Or talk to your doctor about taking supplements.