What vitamin B12 is
Vitamin B12, also called cobalamin, is a water-soluble B vitamin that the body needs in tiny amounts. Unlike many vitamins, it is made not by plants or animals but by certain microorganisms. Animals accumulate it by eating food containing those microbes, or through bacteria in their digestive systems, which is how it ends up in meat, fish, eggs and dairy.
This is general information, not medical advice. Whether you need testing or a supplement is a decision for you and a clinician.
What it does
B12 is involved in several basic jobs:
- Making red blood cells. Without enough B12, red blood cells can form abnormally and become large and inefficient, a condition often called megaloblastic anemia.
- Nerve function. B12 is needed to maintain the protective covering of nerve fibers. Low levels over a long period can affect nerves.
- DNA synthesis. It works closely with folate, another B vitamin, in making DNA, which matters for any rapidly dividing cells.
- Energy metabolism. B12 helps the body process certain fats and proteins. It does not directly give you energy in the way food calories do, so claims that extra B12 boosts energy in people who already have enough are not well supported.
The body can store B12, mainly in the liver, so deficiency usually develops slowly, sometimes over years.
Typical food sources
B12 occurs naturally in foods that come from animals:
- Meat, poultry and especially organ meats such as liver
- Fish and shellfish, including clams, sardines, salmon and tuna
- Eggs
- Milk, yogurt and cheese
For people who eat few or no animal foods, the practical sources are fortified foods and supplements. Some breakfast cereals, plant milks and nutritional yeast products are fortified, but fortification differs between brands and countries, so labels are worth checking. Seaweed, fermented foods and mushrooms are sometimes claimed to be reliable B12 sources, but their B12 content varies and some of what is measured may not be usable by humans, so they are generally not relied upon.
Why absorption matters
Eating B12 is only part of the story. In the stomach, B12 is released from food with the help of stomach acid. It then binds to a protein called intrinsic factor, made by the stomach lining, and is absorbed in the small intestine. A problem at any step can lead to low levels even when the diet looks adequate. Understanding how the digestive tract handles nutrients helps explain why.
Who may be at higher risk of low levels
Several groups are commonly mentioned in health guidance:
- People who eat vegan or very low-animal-product diets, who may get little B12 without fortified foods or supplements. Vegetarians who eat eggs or dairy usually get some, but the amount may be limited.
- Older adults. Stomach acid production often declines with age, which can make it harder to release B12 from food. Fortified foods and supplements contain B12 in a form that does not depend on stomach acid in the same way, which is why older adults are often advised to consider them.
- People with pernicious anemia, an autoimmune condition in which the body cannot make intrinsic factor.
- People with digestive conditions or surgery, such as celiac disease, Crohn’s disease, or gastric bypass or other stomach and intestinal surgery.
- People taking certain medications over long periods. Metformin (a diabetes medicine) and acid-reducing drugs such as proton pump inhibitors have been associated with lower B12 levels in some people. This does not mean anyone should stop their medicine; it means a clinician may want to check levels from time to time.
- Pregnant and breastfeeding people on restricted diets, since a baby’s early development relies on adequate B12.
- People with heavy long-term alcohol use, which can affect both diet and absorption.
Possible signs of low levels
Symptoms can be vague and develop gradually. They may include unusual tiredness, weakness, pale or slightly yellow skin, a sore tongue, tingling or numbness in the hands and feet, difficulty with balance, memory problems or mood changes. Many other conditions cause the same symptoms, so none of these signs can tell you that B12 is the cause. Also, the neurological effects can sometimes be present even without anemia.
Because nerve problems can become permanent if low levels go untreated for a long time, doctors tend to take suspected deficiency seriously.
Testing and treatment
A clinician can check B12 in a blood test, and may add other tests, such as methylmalonic acid or homocysteine, if the result is borderline. Test interpretation can be tricky, and a normal result does not always rule out a problem. Never rely on self-diagnosis from a list of symptoms, and avoid starting large doses before testing if a problem is suspected, since this can complicate evaluation.
If treatment is needed, it can include oral supplements at various doses or, when absorption is a problem, injections. A clinician decides which, and for how long.
Things to keep in mind about supplements
Because B12 is water soluble and the body absorbs only a limited fraction of large oral doses, excess is typically excreted in urine, and a tolerable upper limit has not been set in many countries. That does not mean more is better, nor that high doses are risk free for everyone. People with certain conditions should check with a clinician. Folic acid can also correct some of the anemia caused by low B12 while the nerve effects continue, which is another reason to get tested before treating yourself.
Supplements also differ in form. Some people prefer cyanocobalamin, others methylcobalamin; research has not shown that one is clearly better for everyone.
A practical takeaway
For most people who eat a varied diet that includes animal foods, B12 intake is likely to be adequate. If you follow a plant-based diet, are older, have a digestive condition, take certain medicines or have symptoms that worry you, it is reasonable to raise the question of B12 with a doctor or dietitian. Needs differ, and personalized guidance is more reliable than general rules.