What amount of consumed vitamin B12 (cobalamin) equals the vitamin B12 level in blood in an average adult?

Vitamin B12 Absorption and Blood Levels

The relationship between consumed and blood B12 is not linear—approximately 50% of dietary vitamin B12 is absorbed by healthy adults, but absorption dramatically decreases as dose increases due to saturation of the intrinsic factor-mediated system at about 1.5-2.0 μg per meal. 1, 2, 3

Absorption Efficiency by Dose

The intrinsic factor-mediated absorption pathway becomes saturated at relatively low doses, fundamentally changing the absorption dynamics:

• At physiologic meal doses (1.5-2.0 μg): The intrinsic factor pathway absorbs approximately 50% of ingested B12 2, 3
• Beyond saturation (>2 μg per meal): Bioavailability significantly decreases, with only ~1% absorbed via passive diffusion 1
• High-dose supplementation (500-1000 μg): Relies almost entirely on the 1% passive diffusion mechanism, meaning 5-10 μg actually enters the bloodstream 4

Food Source Bioavailability

The absorption rate varies substantially by food source, even when intrinsic factor is present:

• Fish meat: 42% bioavailability 3
• Chicken meat: 61-66% bioavailability 3
• Sheep/red meat: 56-89% bioavailability 3
• Eggs: <9% bioavailability (poorly absorbed) 3
• Fortified cereals: Crystalline form with superior absorption in older adults 4, 2

Recommended Intake to Maintain Blood Levels

Guidelines vary based on the target population and whether crystalline or food-bound B12 is consumed:

• Healthy adults: 2.4 μg/day meets requirements for 97-98% of the population, assuming 50% absorption 5, 2
• European recommendations: 4 μg/day based on intakes (4.3-8.6 μg/day) associated with normal functional markers (MMA, homocysteine) 6, 5
• Elderly adults (>50 years): Should consume crystalline B12 rather than food-bound forms due to reduced gastric acid production 4, 2
• Therapeutic dosing for deficiency: 500 μg/day oral crystalline B12 reverses biochemical deficiency in older adults 4

Critical Factors Affecting the Consumption-to-Blood Relationship

Age-related changes: Atrophic gastritis affects up to 20% of older adults, dramatically impairing food-bound B12 absorption while preserving crystalline B12 absorption 7, 4, 2

Medication interference: Metformin (especially >1 g/day for >4 months), PPIs, and H2 blockers significantly reduce B12 absorption, increasing deficiency prevalence from 17.8% in the general population to significantly higher rates in treated patients 7, 8

Meal timing and dose: Because the intrinsic factor system saturates at 1.5-2.0 μg per meal, consuming B12 across multiple meals throughout the day maximizes absorption compared to a single large dose 1, 3

Practical Clinical Algorithm

For healthy adults under 50: Consuming 5-15 μg/day from animal foods (the average diet) provides adequate absorption through the intrinsic factor pathway, with approximately 2.5-7.5 μg absorbed 1

For adults over 50 or those with absorption issues: Use crystalline B12 (fortified foods or supplements) at 500-1000 μg/day, which delivers 5-10 μg to the bloodstream via passive diffusion, bypassing the impaired intrinsic factor pathway 4, 2

For confirmed deficiency: Intramuscular administration of 100-1000 μg results in 50-98% urinary excretion within 48 hours, with peak plasma levels at 1 hour, making it highly effective but with minimal tissue storage opportunity 1

Common Pitfalls

Assuming linear absorption: The saturation of intrinsic factor at low doses means doubling dietary intake does not double blood levels—absorption efficiency drops precipitously above 2 μg per meal 1, 3

Relying on serum B12 alone: Standard serum B12 testing misses functional deficiency in up to 50% of cases; methylmalonic acid (MMA) testing identifies an additional 5-10% of patients with true cellular deficiency despite "normal" serum levels 7, 5

Ignoring food matrix effects: Eggs provide minimal bioavailable B12 (<9%) despite containing the vitamin, while fortified cereals provide highly bioavailable crystalline B12 3