Most people get vitamin B12 from animal-based foods like meat, fish, eggs, and dairy. Adults need about 2.4 micrograms (mcg) per day, and a single 3-ounce serving of beef or tuna covers that entirely. But if you follow a plant-based diet, take certain medications, or have an absorption issue, you may need fortified foods, supplements, or injections to maintain healthy levels.
Best Food Sources of B12
Vitamin B12 is naturally found only in animal products. The richest source by far is beef liver: a 3-ounce cooked serving delivers 70.7 mcg, roughly 30 times the daily requirement. Clams come in second at 17 mcg per 3-ounce serving. For more everyday options, Atlantic salmon (2.6 mcg), canned light tuna (2.5 mcg), and ground beef (2.4 mcg) each provide a full day’s worth in a single serving.
Dairy and eggs contribute smaller amounts. A cup of 2% milk has 1.3 mcg, and one large egg provides 0.5 mcg. These foods add up over the course of a day, but relying on eggs and dairy alone means you need multiple servings to hit the target.
Plant-Based and Fortified Options
If you eat little or no animal food, fortified products are essential. Fortified nutritional yeast is the standout: just 2 teaspoons (5 grams) can supply over 300% of the daily value for B12. It has a savory, slightly cheesy flavor that works well sprinkled on popcorn, pasta, or roasted vegetables.
Fortified plant milks (soy, oat, almond), breakfast cereals, and some meat substitutes also contain added B12, though amounts vary by brand. Check the nutrition label for the percentage of daily value per serving. Unfortified plant foods, including seaweed and fermented soy, contain B12 analogs that your body cannot use reliably, so they should not be counted as sources.
How Your Body Absorbs B12
Getting B12 into your food is only half the equation. Your body has a surprisingly complex process for absorbing it. First, stomach acid separates B12 from the proteins it’s bound to in food. Then a protein called intrinsic factor, produced by cells in the stomach lining, binds to the freed B12. This combination travels to the lower part of the small intestine, where it’s absorbed into the bloodstream.
Any disruption along this chain, whether low stomach acid, reduced intrinsic factor, or damage to the small intestine, can lead to deficiency even if your diet looks adequate on paper. This is why some people eat plenty of B12-rich food and still end up low.
Who Is Most Likely to Be Deficient
Several groups face a higher risk. Vegans and strict vegetarians are the most obvious, since B12 occurs naturally only in animal products. But dietary shortfalls take a long time to show up. Your body stores enough B12 to last 2 to 5 years, so problems may not surface until years after a dietary change.
People with pernicious anemia have an autoimmune condition where the immune system attacks the stomach cells that produce intrinsic factor. Without intrinsic factor, B12 simply passes through without being absorbed. Pernicious anemia is more common in women around age 60 and in people with other autoimmune conditions.
Medications can also interfere. Metformin, widely prescribed for type 2 diabetes, is linked to B12 deficiency in up to 1 in 10 people who take it long term. The risk increases with higher doses and longer use. Proton pump inhibitors, the acid-reducing drugs used for reflux, also impair absorption by lowering stomach acid levels. Older adults in general tend to produce less stomach acid, making absorption less efficient even without medication.
Signs You Might Be Low
B12 deficiency affects two major systems: your blood and your nervous system. On the blood side, it causes a type of anemia where red blood cells become abnormally large and can’t carry oxygen efficiently. This leads to fatigue, weakness, and pale skin. Severe anemia can cause an abnormally fast heart rate or even heart failure.
The neurological effects can appear with or without anemia, which makes them easy to miss. They include pins and needles in the hands or feet, memory problems, difficulty with balance and coordination, and vision changes. In advanced cases, nerve damage in the legs can become permanent. B12 deficiency has also been linked to temporary infertility.
Supplements: What to Know
B12 supplements come in two main forms. Cyanocobalamin is the synthetic version used in most supplements. It’s stable, inexpensive, and well studied. Methylcobalamin is the form that occurs naturally in the body. Marketing often positions methylcobalamin as superior, but the evidence is mixed. One study found that the body absorbed about 49% of a cyanocobalamin dose compared to 44% for methylcobalamin. Another study found that the body excreted three times as much cyanocobalamin in urine, suggesting methylcobalamin may be retained better. Overall, the practical difference appears small and may depend on individual factors like age and genetics.
Oral supplements work well for most people with dietary deficiency, because B12 in supplement form is not bound to food proteins and doesn’t require stomach acid to be released. Even people with mild absorption issues can often get enough from high-dose oral supplements, since a small percentage of B12 is absorbed passively (without intrinsic factor) when the dose is large enough.
When Injections Are Needed
For people with pernicious anemia or severe deficiency with neurological symptoms, oral supplements may not be enough. Injections bypass the digestive system entirely, delivering B12 directly into muscle or under the skin. A typical treatment schedule starts with frequent injections (daily or every other day for the first one to two weeks), then tapers to once a month for ongoing maintenance. People with pernicious anemia generally need monthly injections for life, since the underlying absorption problem doesn’t resolve.
Testing for Deficiency
A standard blood test measures the level of B12 circulating in your bloodstream. But this test has a blind spot: it can come back normal even when your cells aren’t actually using B12 properly. A more sensitive marker is methylmalonic acid (MMA). When your body lacks functional B12, MMA levels rise. Normal blood MMA is usually under 0.40 micromoles per liter. If your MMA is elevated but your B12 looks fine on paper, you may still have an early or mild deficiency, particularly if you’re experiencing symptoms like tingling or fatigue.
If you fall into a higher-risk group, whether from diet, medication, age, or an autoimmune condition, periodic testing can catch deficiency before nerve damage sets in. The neurological effects of long-standing B12 deficiency are not always reversible, which makes early detection particularly worthwhile.