Vitamin B12 is a small nutrient with a big job. It keeps your nerves and blood cells healthy and helps make DNA, the genetic material in every cell. Because your body can store large amounts, deficiency often develops slowly and silently, sometimes over years, before symptoms appear. In Part 2 of Vitamins and Minerals Explained, we look at vitamin B12: how much you need, where to find it and who is most at risk of running low.
What does vitamin B12 do?
Vitamin B12, also called cobalamin, is a water-soluble vitamin with several essential roles. According to the US National Institutes of Health (NIH):
- Healthy blood: it is needed to make normal red blood cells. Without enough, the body produces large, immature red blood cells that do not work properly, a condition called megaloblastic anaemia, which causes tiredness and weakness.
- Healthy nerves: it helps maintain the protective coating around nerves and supports normal brain and nerve function.
- DNA synthesis: it is needed to make DNA in all cells.
B12 also works closely with folate (Part 9), another B vitamin, in several of these processes.
How much vitamin B12 do you need?
B12 is needed in tiny amounts, measured in micrograms (mcg). The NIH recommends:
| Age group | Recommended daily amount |
|---|---|
| Birth to 6 months | 0.4 mcg |
| 7–12 months | 0.5 mcg |
| 1–3 years | 0.9 mcg |
| 4–8 years | 1.2 mcg |
| 9–13 years | 1.8 mcg |
| Teens 14–18 and adults | 2.4 mcg |
| Pregnancy | 2.6 mcg |
| Breastfeeding | 2.8 mcg |
Your body stores a remarkable amount of B12, mainly in the liver: the NIH notes stores can equal about 1,000 to 2,000 times your typical daily intake. That is why it can take years for deficiency symptoms to appear after intake drops.
Best food sources of vitamin B12
Vitamin B12 is found naturally almost only in animal foods: meat, fish, poultry, eggs and dairy. Plant foods do not naturally contain B12 unless fortified. The table below shows amounts per serving from the NIH (the US Daily Value is 2.4 mcg).
| Food | Serving | Vitamin B12 | % DV |
|---|---|---|---|
| Beef liver, pan fried | 3 oz (85 g) | 70.7 mcg | 2,944% |
| Clams, cooked | 3 oz | 17 mcg | 708% |
| Oysters, cooked | 3 oz | 14.9 mcg | 621% |
| Nutritional yeast, fortified | about ¼ cup | 8.3–24 mcg | 346–1,000% |
| Atlantic salmon, cooked | 3 oz | 2.6 mcg | 108% |
| Tuna, light, canned in water | 3 oz | 2.5 mcg | 104% |
| Ground beef, 85% lean | 3 oz | 2.4 mcg | 100% |
| Milk, 2% | 1 cup | 1.3 mcg | 54% |
| Yoghurt, plain, fat free | 6 oz | 1.0 mcg | 43% |
| Breakfast cereal, fortified with 25% DV | 1 serving | 0.6 mcg | 25% |
Fortified foods, such as some breakfast cereals, plant milks and nutritional yeast, are important sources for vegetarians and vegans. Check the label, because the amount added varies widely between brands.
How B12 is absorbed
Absorbing B12 from food is a two-step process, which explains why so many people have trouble with it:
- Stomach acid releases B12 from the protein in food.
- B12 then combines with intrinsic factor, a protein made in the stomach, which allows it to be absorbed in the small intestine.
Problems with either step, such as low stomach acid or a lack of intrinsic factor, can lead to deficiency even if you eat plenty of B12. Supplements and fortified foods contain "free" B12 that does not need stomach acid to be released, though they still need intrinsic factor for normal absorption.
Who is at risk of vitamin B12 deficiency?
- Older adults: many have reduced stomach acid, which makes it harder to absorb B12 from food. The NIH notes deficiency affects between 3% and 43% of older adults, depending on the study, and recommends that adults over 50 get most of their B12 from fortified foods or supplements.
- People with pernicious anaemia: an autoimmune condition in which the body cannot make intrinsic factor, so B12 cannot be absorbed normally.
- People with atrophic gastritis, which reduces stomach acid and intrinsic factor.
- People who have had stomach or intestinal surgery, including weight-loss surgery.
- People with digestive disorders such as coeliac disease or Crohn's disease.
- Vegetarians and vegans: plant foods contain no B12 unless fortified. Breastfed babies of vegan mothers can also be at risk if the mother's B12 is low.
- People taking certain medicines, particularly metformin for diabetes and long-term acid-reducing drugs (see below).
Symptoms of vitamin B12 deficiency
Because B12 affects both blood and nerves, deficiency can cause a wide range of symptoms:
Blood-related symptoms
- Tiredness and weakness
- Pale skin
- Heart palpitations or shortness of breath
- Loss of appetite and weight loss
Nerve-related symptoms
- Numbness or tingling in the hands and feet
- Problems with balance and walking
- Memory problems, confusion or difficulty thinking
- Depression and mood changes
- A sore mouth or tongue
In babies
Infants with B12 deficiency may fail to grow and thrive, have delayed development and develop megaloblastic anaemia.
Importantly, the NIH warns that nerve damage can happen even without anaemia, and some nerve damage may be permanent if deficiency is not treated promptly. That is why symptoms like persistent tingling or numbness should be checked by a doctor.
How B12 deficiency is diagnosed
Doctors usually start with a blood test for vitamin B12 levels, often alongside a complete blood count. Because B12 levels alone can be misleading, they may also check other markers such as methylmalonic acid or homocysteine. For a detailed guide to the test, see Lab Tests Explained: the Vitamin B12 test.
Vitamin B12 supplements and treatment
B12 is available in multivitamins, B-complex supplements and single-nutrient supplements. Common forms include:
- Cyanocobalamin: the most common and well-studied form.
- Methylcobalamin, adenosylcobalamin and hydroxocobalamin: other forms often marketed as more "natural" or "active".
According to the NIH, research has not shown that any one form is better than the others for most people.
B12 is also available as tablets that dissolve under the tongue, as a prescription nasal gel and as injections. Injections are often used to treat severe deficiency or pernicious anaemia, although very high oral doses can also work for many people because a small percentage is absorbed without intrinsic factor.
Is too much vitamin B12 harmful?
The NIH states that vitamin B12 has not been shown to cause harm even at high doses, and no official upper limit has been set. Supplements of 500 to 1,000 mcg are considered safe, though the body absorbs only a small percentage of such large doses. Unusually high blood levels without supplements can sometimes signal other health problems, so they should be discussed with a doctor.
Medicine interactions
- Metformin: long-term use of this diabetes medicine may reduce B12 absorption. People taking metformin may benefit from periodic B12 checks.
- Acid-reducing drugs: proton pump inhibitors such as omeprazole and lansoprazole, and H2 blockers, reduce stomach acid and can make it harder to absorb B12 from food over time.
Does B12 boost energy or prevent disease?
B12 is often marketed as an energy booster. According to the NIH, extra B12 does not improve energy or athletic performance in people who already get enough. Research has also found that B vitamins can lower homocysteine, a substance linked with heart disease, but taking them does not reduce the risk of heart disease or stroke. Most studies have not found that B12 supplements slow cognitive decline in people with normal levels.
Practical tips
- If you eat animal foods, a varied diet including fish, eggs, dairy or meat usually provides enough B12.
- If you are vegetarian or vegan, rely on fortified foods or a reliable B12 supplement every day or week.
- If you are over 50, consider fortified foods or a supplement, as recommended by the NIH.
- If you take metformin or long-term acid-reducing medicines, ask your doctor about B12 testing.
- Do not ignore persistent numbness, tingling, balance problems or unusual tiredness.
Frequently asked questions
Can vegans get B12 from seaweed or mushrooms?
Some plant foods contain B12-like compounds or small, inconsistent amounts, but they are not considered reliable sources. Fortified foods or supplements are recommended.
How quickly do B12 injections work?
Many people with deficiency notice improvements in energy within days to weeks, though nerve symptoms may take months to improve and may not fully resolve.
Should I take B12 and folate together?
They work together, and taking high doses of folic acid can hide the blood signs of B12 deficiency while nerve damage progresses. If you are considering high-dose folic acid, ask your doctor to check B12 too.
In Part 3, we look at iron, the world's most common nutritional deficiency.
Sources
- NIH Office of Dietary Supplements – Vitamin B12 (consumer fact sheet)
- NIH Office of Dietary Supplements – Vitamin B12 (health professional fact sheet)
Vitamins and Minerals Explained: the full series
- Part 1: Vitamin D
- Part 2: Vitamin B12 (you are here)
- Part 3: Iron
- Part 4: Magnesium
- Part 5: Vitamin C
- Part 6: Calcium
- Part 7: Zinc
- Part 8: Omega-3 Fatty Acids
- Part 9: Folate and Folic Acid
- Part 10: Vitamin A
- Part 11: Vitamin E
- Part 12: Vitamin K
- Part 13: Potassium
- Part 14: Iodine
- Part 15: Biotin
- Part 16: Do You Need Supplements?
This article is for general information only and is not medical advice. Recommended amounts shown are US figures from the NIH Office of Dietary Supplements; guidance in other countries may differ slightly. Talk to a doctor or pharmacist before starting supplements, especially if you are pregnant, take medicines or have a health condition.
