To keep your brain working its best, you need enough omega-3s (found in fish and nuts) and low levels of a substance called homocysteine, which means you're getting enough B vitamins from your diet.

Taking B vitamins doesn't seem to slow brain shrinkage in older people with mild memory problems—if they already have low levels of omega-3 fatty acids in their blood. It's like the B vitamins just don't work well unless you have enough omega-3s.

For older adults with mild memory problems, having high levels of omega-3s in the blood might help slow brain shrinkage—but only if they’re also taking B vitamins. Alone, omega-3s don’t seem to help.

If you're an older adult with mild memory problems, your blood levels of omega-3 fatty acids might tell doctors whether taking B vitamins will help slow down brain shrinkage — but only if your levels are either very high (above 590) or very low (below 390).

For older adults with mild memory problems and high levels of omega-3s in their blood, taking daily B vitamin pills for two years can slow brain shrinkage by 40% compared to taking a placebo — but only if they already have lots of omega-3s in their system.

For older adults with mild memory problems, taking a daily combo of B vitamins might slow brain shrinkage by 40% over two years—but only if they already have high levels of omega-3s in their blood. If their omega-3s are low, the vitamins don’t seem to help.