Vitamin B12 RDA and Maximum Daily Dosage
The recommended dietary allowance (RDA) for vitamin B12 is 2.4 µg/day for adults, with no established upper tolerable intake level due to insufficient data demonstrating toxicity at high doses. 1
Recommended Dietary Allowance
The RDA is 2.4 µg/day for all adults (both men and women), which represents the daily intake sufficient to meet the nutrient requirements of 97-98% of healthy individuals 1
The European Food Safety Authority (EFSA) recommends a slightly higher intake of 4 µg/day, based on intakes of 4.3-8.6 µg/day associated with normal functional markers (methylmalonic acid, homocysteine, serum cobalamin, and holotranscobalamin) 2
The German Nutrition Society recommends 3 µg/day 2
Maximum Daily Dosage
There is no established upper tolerable intake level (UL) for vitamin B12 because insufficient data exists to demonstrate adverse health effects at high doses 1
This absence of a defined maximum reflects vitamin B12's excellent safety profile, even at doses far exceeding the RDA 1
Special Population Considerations
Elderly Adults (≥50 years)
Adults over 50 should consume vitamin B12 primarily in crystalline (synthetic) form rather than food-bound form, as 10-30% of this population has protein-bound vitamin B12 malabsorption due to atrophic gastritis 3, 4
The elderly have higher requirements due to reduced absorption from age-related hypochlorhydria and widespread antacid use 5
Oral doses of 500-1000 µg/day of crystalline vitamin B12 are needed to reverse biochemical deficiency in older adults with malabsorption 6, 7
Patients with Ileal Disease or Resection
Patients with >20 cm of distal ileum resected should receive prophylactic vitamin B12 supplementation with 1000 µg intramuscularly every month indefinitely 1
Resection of <20 cm does not typically cause deficiency 1
Medication-Related Considerations
- Patients on metformin (especially >4 months), sulfasalazine, methotrexate, colchicine, H2 receptor antagonists, phenobarbital, or pregabalin require monitoring and potential supplementation due to impaired B12 absorption or utilization 2
Clinical Pitfalls
Standard serum B12 testing can miss functional deficiency in up to 50% of cases - the Framingham Study demonstrated that half of elderly subjects with normal serum B12 had metabolic deficiency when methylmalonic acid (MMA) was measured 7
Neurological symptoms (gait ataxia, peripheral neuropathy, cognitive difficulties) often precede hematological changes, with approximately one-third of deficiency cases showing no macrocytic anemia 5, 7
Active B12 (holotranscobalamin) and MMA are more sensitive markers than total serum B12 for detecting functional deficiency 2, 7