It was Christmas week 1990 in Oakland. Steven Bloom was wandering through The Lot, that timeless gathering of hippies that springs up in the parking lot before every Grateful Dead concert, when a Deadhead handed him a yellow flyer.
Because of the general safety of the vitamin, upper limits for normal intake do not exist. If your blood values for the vitamin fall below a range of 170 to 250 picograms per milliliter, you may develop a B12 deficiency.
A vitamin B12 test measures the amount of vitamin B12 in the blood. The normal values listed here-called a reference range-are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses.