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).

If you're an older adult with early memory problems, taking B vitamins might help slow brain shrinkage—but only if you already have enough omega-3s in your blood. Your starting nutrient levels can change whether the vitamins work or not.

For older adults with mild memory problems and low levels of omega-3 fatty acids, taking B vitamins doesn't seem to slow down brain shrinkage.

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.

Taking B vitamins might help slow brain shrinkage in older people with mild memory problems—but only if they also have enough omega-3s in their body. If they don’t, the B vitamins might not help as much.

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.