People with low levels of vitamin B12 in their blood may have more damage in their brain, like the insulation around nerve fibers breaking down and the brain shrinking over time.

People with vitamin B12 levels between 400 and 600 units in their blood are 34% more likely to die from any cause than people with lower levels—even though those levels are still considered normal by doctors.

People with very high levels of vitamin B12 in their blood—above 600 pmol/L—might be 50% more likely to die from any cause compared to those with lower levels, which could mean there’s a dangerous upper limit to how much B12 is safe.