Vitamin B12 deficiency: What to know

Where do our bodies get vitamin B12?

You get vitamin B12 in your diet primarily by eating animal products and fortified foods.

Examples of foods that contain vitamin B12 include:

• Meat
• Fish
• Shellfish
• Eggs
• Dairy
• Fortified grain products

You can also get vitamin B12 from a multivitamin supplement or prescription medication.

Bacteria in the body also make vitamin B12, but our bodies can't absorb it. This is because the body absorbs vitamin B12 in the ileum, part of the small intestine. The bacteria that make vitamin B12 are active after the ileum, so the B12 can’t be absorbed and used by the body. This means we must get vitamin B12 from sources outside the body.

How much vitamin B12 should I get each day?

For healthy individuals, the recommended vitamin B12 intake is 2.4 to 2.8 micrograms (mcg) per day.

The National Academies of Sciences, Engineering, and Medicine recommend:

• People ages 14 and older get 2.4 mcg of B12 per day
• Pregnant individuals get 2.6 mcg of B12 per day
• Breastfeeding individuals get 2.8 mcg of B12 per day

There hasn't been a tolerable upper intake level set for B12. While extremely high doses of vitamin B12 can lead to toxicity, excess amounts are excreted in the urine.

How do you find out if you have a vitamin B12 deficiency?

Not sure if you are getting enough B12? Here are two ways to tell.

Look for symptoms of vitamin B12 deficiency

Symptoms of low levels of vitamin B12 include:

• Anemia
• Fatigue
• Neuropathy
• Neurological decline
• Glossitis, or swelling of the tongue
• Low counts of white and red blood cells
• Dementia
• Weight loss
• Infertility

Your body can store vitamin B12. As a result, it can take years to develop deficiency symptoms.

Lab tests

Your doctor may also use a blood test to see if you have low vitamin B12 levels.

However, inflammation can lead to inaccurate B12 results. For example, many cancer patients have inflammation in their bodies. In this case, your doctor may order other tests — such as methylmalonic acid, homocysteine and C-reactive protein — to better understand your vitamin B12 levels.

What causes low levels of vitamin B12?

Many factors can cause low vitamin B12. Examples include:

• Diet. Not consuming enough vitamin B12 in your diet can lead to a deficiency. People who eat a vegetarian or vegan diet may be at a higher risk for B12 deficiencies.
• Age. As you age, your body makes less stomach acid and a type of protein called intrinsic factor. Stomach acid and intrinsic factor are needed for digestion and vitamin B12 absorption.
• Certain surgeries. Procedures such as total gastrectomy, gastric bypass and Whipple procedure bypass or remove parts of the body involved in digestion and the absorption of vitamin B12.
• Chemotherapy. This cancer treatment can impact digestion and absorption of vitamin B12.
• Anemia. When you have anemia, your body makes less intrinsic factor. As a result, your body can’t release vitamin B12 from foods with protein.
• Certain medications, including acid reducer medication — which interferes with B12 absorption — and oral contraceptives.
• Chronic diseases such as Crohn's, celiac disease and gastritis can impair vitamin B12 absorption.

Is vitamin B12 deficiency linked to any disease?

Yes. Vitamin B12 deficiency is linked to:

• Hematologic diseases, such as anemia
• Cardiovascular decline
• Neurological and psychiatric diseases

Is vitamin B12 linked to cancer?

Right now, the answer is unclear.

There is inconclusive evidence linking very high or very low levels of vitamin B12 to cancer risk. It is unclear if B12 has a causal relationship with cancer risk, meaning it directly affects risk. B12 levels could be a result of the cancer diagnosis and not the cause.

Low B12 is a risk factor for stomach cancer, but this is not because of B12 specifically. Instead, the underlying stomach condition may cause low B12 levels or be caused by low B12 levels.

I have low vitamin B12 levels. Now what?

If you have low vitamin B12 levels, you can make lifestyle changes to improve your levels. It is always best to get your B12 from food sources. This can be done as part of a healthy diet.

There are also vitamin B12 multivitamins or supplements. These may be recommended if you can’t get enough vitamin B12 in your diet due to a medical condition or absorption issue, like the ones listed above. Always talk with your doctor before starting a supplement. This is especially vital for cancer patients, as some supplements may interfere with treatment. Your doctor will consider your labs, symptoms and health history to make sure a B12 supplement is right for you; they’ll also determine what supplement dose you need.

Your health care team can also help you manage your B12 levels and any chronic conditions that may lead to a B12 deficiency. They may recommend B12 prescription medications or injections.

For example, after a total gastrectomy, we recommend that patients get B12 supplementation — often in the form of B12 injections — for the rest of their lives. With no stomach, your body doesn’t have a way to remove B12 from food so it can be absorbed.

When in doubt, ask your care team for advice on what’s right for you.

Grace Whitmer is an inpatient senior clinical dietitian at UT MD Anderson.

Request an appointment at UT MD Anderson online or call 1-877-632-6789.