Vitamin B12 Needs Calculator

Vitamin B12 (cobalamin) is an essential water-soluble vitamin that plays a crucial role in nerve function, red blood cell formation, and DNA synthesis. Unlike most vitamins, B12 is primarily found in animal-based foods, making supplementation important for certain dietary groups.

Recommended Dietary Allowance (RDA) by age:

• Infants 0-6 months: 0.4 mcg (Adequate Intake)
• Infants 7-12 months: 0.5 mcg (Adequate Intake)
• Children 1-3 years: 0.9 mcg
• Children 4-8 years: 1.2 mcg
• Children 9-13 years: 1.8 mcg
• Teens 14-18 years: 2.4 mcg
• Adults 19+ years: 2.4 mcg
• Pregnant women: 2.6 mcg
• Breastfeeding women: 2.8 mcg

There is no upper limit. The Institute of Medicine found no harm from high intakes, partly because so little of a large dose is absorbed.

Why the dose on a supplement label looks absurd

B12 is absorbed in two ways. The main route needs intrinsic factor, a protein made in the stomach, and it saturates at roughly 1.5 to 2 mcg a meal. Beyond that, about 1% of a dose slips through on its own. So a 2,000 mcg weekly tablet delivers about 20 mcg, and a 10 mcg daily one a little under 2. Both are standard advice and neither is an overdose.

Diet type matters greatly:

• Omnivores typically get enough from meat, fish, eggs, and dairy. A 3 oz serving of salmon is 4.8 mcg, twice the adult need.
• Vegetarians can get there from dairy and eggs (two cups of milk is 2.4 mcg), but many run low, and a fortified food is an easy backstop.
• Vegans need fortified foods or a supplement, because plant foods carry virtually no usable B12. The Vegan Society’s three options: fortified foods giving 3 mcg a day spread over two or three meals, a 10 mcg supplement daily, or 2,000 mcg once a week.

Age-related absorption decline:

Somewhere between 10 and 30% of adults over 50 absorb the B12 bound to food poorly, mostly because of low stomach acid. The crystalline B12 in supplements and fortified foods does not need stomach acid, which is why the Institute of Medicine advises everyone over 50 to get most of their B12 that way.

Medicines: metformin and long-term acid blockers (proton pump inhibitors such as omeprazole, and H2 blockers) both lower B12 absorption. The American Diabetes Association advises checking B12 now and then in anyone on metformin for the long term, especially with numb or tingling feet.

Signs of B12 deficiency:

Fatigue, weakness, numbness or tingling in hands and feet, difficulty walking, mood changes, and cognitive difficulties. Severe deficiency can cause irreversible nerve damage if left untreated. Pernicious anaemia, where the stomach stops making intrinsic factor, is treated with injections or with 1,000 to 2,000 mcg a day by mouth, which works for most people because of that 1% passive route.

Food sources, per serving (NIH):

Beef liver 70.7 mcg (3 oz), clams 17 mcg (3 oz), rainbow trout 5.4 mcg (3 oz), sockeye salmon 4.8 mcg (3 oz), canned light tuna 2.5 mcg (3 oz), fortified nutritional yeast 2.4 mcg (1 tbsp, check the label), milk 1.2 mcg (1 cup), plain yogurt 1.0 mcg (6 oz), Swiss cheese 0.9 mcg (1 oz), a large egg 0.5 mcg. Older tables list clams at 84 mcg; the current USDA figure is far lower.