What it does in the body
- Methionine synthase cofactor (homocysteine to methionine, regenerates THF from 5-MTHF)
- L-methylmalonyl-CoA mutase cofactor (propionate pathway, essential for myelin maintenance)
- SAM-dependent methylation reactions (DNA, proteins, lipids, neurotransmitters)
- Red blood cell maturation (via folate-dependent DNA synthesis)
- Myelin sheath synthesis and maintenance
How much you need (Daily Value)
| Group | Recommended | Source |
|---|---|---|
| Adult male | 2.4 mcg | NIH/IOM |
| Adult female | 2.4 mcg | NIH/IOM |
| Pregnancy | 2.6 mcg | WHO/IOM |
| Children | 0.9-1.8 mcg (ages 1-13) | WHO |
| Older adults | 2.4 mcg (recommend supplementation or fortified foods as absorption declines) | NIH |
Richest food sources
| Food | Amount | Where |
|---|---|---|
| Beef liver | 70.7 mcg per 100g | global |
| Clams | 98.9 mcg per 100g | global |
| Sardines | 8.9 mcg per 100g | Mediterranean/global |
| Salmon | 3.2 mcg per 100g | global |
| Beef (ground, cooked) | 2.6 mcg per 100g | global |
| Fortified nutritional yeast | 8.3 mcg per 15g serving | global |
| Milk (whole) | 0.45 mcg per 100ml | global |
| Nori seaweed (dried) | trace-77 mcg per 100g (variable, bioavailability debated) | East Asia |
In our food database, per 100 g
Highest among our free foods — open the Food Explorer to compare.
If you don't get enough
Mild: Fatigue, elevated MMA and homocysteine, subtle cognitive slowing
Moderate: Megaloblastic anemia (macrocytic, hypersegmented neutrophils), glossitis, paresthesias
Severe: Subacute combined degeneration of spinal cord (posterior column + lateral corticospinal tract demyelination), pancytopenia, dementia, psychosis (megaloblastic madness)
Time to onset: Liver stores last 3-5 years; clinical deficiency takes 2-5 years to develop on a completely B12-free diet
Too much
Upper limit: No established UL; considered safe even at very high doses
No documented toxicity from oral or parenteral administration. Rare acneiform eruptions reported with high-dose injections.
How well you absorb it
Active absorption via intrinsic factor: ~56% of 1 mcg dose (saturatable, max ~1.5 mcg per meal). Passive diffusion: ~1% of dose at any amount.
Helped by: Intrinsic factor (from gastric parietal cells), Adequate stomach acid (releases B12 from food protein), R-proteins in saliva
Hindered by: Proton pump inhibitors/H2 blockers (reduce acid-dependent B12 release from food), Metformin (reduces ileal absorption via calcium-dependent mechanism), Pernicious anemia (autoimmune destruction of parietal cells), Gastric bypass surgery
Cooking & storage
B12 is relatively heat-stable at neutral pH. Boiling milk for 2-5 minutes preserves most B12. Prolonged cooking at high temperatures or alkaline conditions (baking soda) can cause 10-30% loss. B12 in fermented foods may increase during fermentation.
Nutrisize Health totals every nutrient from what you actually eat, against targets set for your age and sex, and flags what's short — across 4,995 foods, on your device.
