Vitamin B12: diet, testing, supplementation and risks

The European Food Safety Authority (EFSA) sets an adequate intake of 4 µg per day for adults, 4.5 µg during pregnancy, and 5 µg during breastfeeding. The US NIH recommends 2.4 µg for adults; the difference comes from the methods used.

B12 is naturally found in fish, seafood, meat, eggs, milk, and dairy products. Unfortified plant foods aren't a reliable source. Some plant-based drinks, cereals, or nutritional yeasts are fortified, which only the label can confirm. For details on foods and choices within a plant-based diet, see the article B12: animal foods, fortified products and plant-based choices.

Dietary B12 is bound to proteins. Stomach acid must release it, then it binds to intrinsic factor, a protein produced by the stomach, before being absorbed at the end of the small intestine. Each step can be where things go wrong.

The 2024 British NICE guideline lists the main risk factors:

• Diet: a vegan diet or one low in animal products without fortified foods or a supplement, food allergies, difficulty buying or preparing meals, eating disorders.
• Diseases: autoimmune atrophic gastritis, celiac disease, other autoimmune diseases, family history.
• Medications: metformin, proton pump inhibitors, H2 blockers, colchicine, certain anticonvulsants (phenobarbital, primidone, topiramate), and pregabalin.
• Surgery and other factors: bariatric surgery, removal of the stomach or the end of the small intestine, abdominal radiotherapy, recreational nitrous oxide use.

NICE recommends offering testing to people who have at least one compatible sign and at least one risk factor. Suggestive signs are varied and not very specific: anemia or oversized red blood cells, unexplained fatigue, concentration or memory problems, tingling or numbness, balance problems, a sore tongue, visual disturbances. The absence of anemia doesn't rule out a deficiency.

The first test is total B12 or active B12 (holotranscobalamin). NICE considers total B12 below 180 ng/L (133 pmol/L) as a confirmed deficiency, and a value between 180 and 350 ng/L as indeterminate. The lab's own thresholds take precedence when they differ.

Several factors can distort the reading. A recent supplement or injection can raise the level without correcting the deficiency: the blood test should therefore come before any supplementation, and any products already taken should be reported. The combined contraceptive pill can lower total B12 without a real deficiency. With an indeterminate result, a second-line marker, such as methylmalonic acid, may be requested; it's interpreted alongside kidney function.

A supplement helps secure intake when a reliable source is missing, for example on a vegan diet. Treating a confirmed deficiency is a different process: correcting the shortfall, investigating its cause, choosing the route, and organizing follow-up.

The route depends on the cause. NICE recommends lifelong intramuscular injections for autoimmune gastritis or total removal of the stomach or the end of the small intestine. For other forms of malabsorption, the oral route remains possible at at least 1 mg per day, a dose far above needs, since only a small fraction then passes through by diffusion. A 2018 Cochrane review (3 trials, 153 participants) found low-quality evidence suggesting that high-dose oral B12 normalizes blood levels as well as injections do.

Symptoms often improve within two weeks, but can take up to three months, or even longer, to fully resolve.

Cyanocobalamin, methylcobalamin, or hydroxocobalamin: the data don't show that one supplement form is superior for everyone. No tolerable upper intake level has been set for B12, for lack of established adverse effects at dietary and usual supplement doses.

That said, this doesn't justify the "energy" claims associated with high doses: in someone without a deficiency, a supplement provides no demonstrated benefit.

1. 01List your B12 sources: animal foods, fortified products, supplements.
2. 02Identify your absorption risk factors: medications, surgery, digestive or autoimmune diseases.
3. 03If you have a compatible symptom and a risk factor, ask for testing before taking any supplement.
4. 04Have the result interpreted alongside your symptoms, blood count, and treatments.
5. 05Distinguish preventing a shortfall from treating a confirmed deficiency, which calls for medical follow-up.