For older people with mild memory problems, taking B vitamins might help their memory and thinking skills—but only if they already have high levels of a certain omega-3 fat (DHA) in their blood. If their DHA levels are low, the vitamins don’t seem to help at all.

We analyzed two assertions about B vitamins and memory in older adults with mild cognitive impairment, and both point to the same pattern: B vitamins may help memory and thinking skills only when DHA levels are already high. If DHA levels are low, the vitamins don’t appear to make a difference [1]. What we’ve found so far suggests that the effect of B vitamins isn’t the same for everyone—it seems to depend on whether the person has enough DHA in their blood. DHA is an omega-3 fat found in fatty fish and some supplements, and it plays a role in brain health. The evidence doesn’t say B vitamins cause improvement, but it does show a consistent link between high DHA and a possible benefit from the vitamins. Neither study found cases where B vitamins helped memory without high DHA, nor did any show harm. We don’t know why this connection exists, and we don’t know how much DHA is “high enough” or how long someone needs to take B vitamins to see any change. The evidence we’ve reviewed leans toward the idea that DHA status might be a key factor in whether B vitamins help memory in this group. For now, if you’re an older adult with mild memory concerns and considering B vitamins, it might be worth checking your DHA levels first—but we don’t yet know if taking DHA supplements will make B vitamins work better.