The Basics
Vitamin B12 supports red blood cell formation and nerve function. The NIH Office of Dietary Supplements sets the adult RDA at 2.4 mcg/day [1]. Because natural B12 is found almost exclusively in animal foods (meat, fish, eggs, dairy), intake and absorption — not toxicity — are the main concerns.
Who Is at Risk of Deficiency
NIH identifies several groups more likely to be low in B12 [1]:
- Older adults — reduced stomach acid lowers absorption; an estimated 3%–43% of community-dwelling older adults are deficient depending on the cutoff used.
- People with pernicious anemia — an autoimmune condition that is the most common cause of clinically evident B12 deficiency.
- Those with GI disorders or surgery — celiac disease, Crohn's disease, or procedures that remove parts of the stomach or intestine reduce absorption.
- Vegans and vegetarians — natural food sources are limited to animal foods; fortified foods or supplements are needed.
- Infants of vegan mothers — at risk of severe deficiency if the mother's intake is low.
- Long-term users of metformin or proton pump inhibitors — these medicines can reduce B12 absorption [1].
Why There's No Upper Limit
NIH did not set a UL for vitamin B12 because of its low potential for toxicity — the body doesn't store harmful amounts and excretes what it doesn't use [1]. That's why high-dose B12 supplements are common, though 'no UL' doesn't mean megadoses are necessary (see Upper Intake Levels).
What to Do
If you fall into a risk group, ask a clinician about checking your B12 status. A shortfall can develop slowly and may show up as fatigue, tingling, or memory changes — but these are non-specific, so testing (not self-diagnosis) is the right path. Fortified foods or a modest supplement usually restore adequate intake for dietary causes.