What Does Vitamin B12 Do for the Body? Your Guide to Its Essential Functions

October 16, 2025 •

4 min read

According to the NIH, an estimated 6% of people under age 60 and 20% of those over 60 have a vitamin B12 deficiency, highlighting its widespread importance. This essential nutrient plays numerous critical roles that are often overlooked, leading many to wonder exactly what does vitamin B12 do for the body.

Quick Summary

Vitamin B12 is a crucial nutrient for maintaining healthy nerve cells, aiding in the production of DNA, and forming red blood cells that transport oxygen throughout the body. A deficiency can impair these vital functions, leading to fatigue and neurological issues. The vitamin is primarily found in animal products, making supplementation important for vegans and older adults.

Key Points

Nervous System Health: Vitamin B12 is vital for maintaining the myelin sheath, which protects nerve fibers, ensuring proper neurological function.
Red Blood Cell Formation: It is crucial for producing healthy red blood cells and, with a lack of it, can cause megaloblastic anemia, leading to fatigue and weakness.
DNA and Cell Synthesis: B12 is a key component in creating and regulating DNA, a process fundamental for cellular health and division.
Energy Metabolism: It helps convert food into usable energy, supporting your body's overall metabolic processes.
Dietary Considerations: As it is found mainly in animal products, those on vegan or vegetarian diets, as well as older adults, are at higher risk of deficiency and may need supplements.

The Core Functions of Vitamin B12

Vitamin B12, also known as cobalamin, is a water-soluble vitamin that is fundamental to several major bodily processes. While your body cannot produce it on its own, it plays a vital role in ensuring the proper functioning of your cells and organs. Its functions range from keeping your nervous system healthy to aiding in the creation of your body's genetic material.

Supporting Nervous System Health

One of the most critical functions of vitamin B12 is its role in maintaining a healthy nervous system. It is essential for the production of myelin, the protective sheath that insulates nerve fibers and allows for fast and efficient nerve impulse transmission. When B12 levels are low, the myelin sheath can deteriorate, leading to nerve damage and impaired function. A deficiency can manifest as neurological symptoms such as tingling or numbness (paresthesia), muscle weakness, poor balance, and memory problems.

Aiding in Red Blood Cell Formation

Vitamin B12 works in tandem with folate (vitamin B9) to produce healthy red blood cells in the bone marrow. A deficiency in B12 disrupts this process, causing red blood cells to become unusually large and oval-shaped (megaloblastic anemia). These abnormal cells cannot function properly, leading to a reduced oxygen-carrying capacity in the blood. This can cause persistent fatigue, weakness, and shortness of breath, which are common symptoms of anemia.

Role in DNA Synthesis and Cellular Health

Vitamin B12 is a key cofactor in the synthesis of DNA, the genetic material found in every cell of your body. It is involved in metabolic pathways that help create the building blocks of DNA and regulate gene expression. This is particularly important for tissues with a high turnover rate of cells, such as the gastrointestinal tract and bone marrow. Impaired DNA synthesis due to a B12 deficiency can lead to defective cell division and contribute to various health issues.

Facilitating Energy Metabolism

While B12 is often promoted as a source of energy, it primarily helps your body produce energy by assisting in the metabolism of carbohydrates, fats, and proteins. It acts as a coenzyme in the Krebs cycle, a key part of cellular respiration, to convert food into usable energy (ATP). For those with a deficiency, supplementing can restore energy levels, but it does not act as a stimulant for those with normal levels.

Understanding B12 Deficiency: Symptoms and Causes

Vitamin B12 deficiency develops when the body does not get or cannot absorb enough of the vitamin. Your body can store B12 for several years, so symptoms can take a long time to appear. Common risk factors include following a vegan or vegetarian diet without supplementation, being over 60, having gastrointestinal disorders like Crohn's or celiac disease, or taking certain medications like metformin or proton pump inhibitors.

Symptoms of Vitamin B12 Deficiency

Fatigue and Weakness: A lack of healthy red blood cells reduces oxygen delivery throughout the body.
Neurological Issues: Numbness, tingling, balance problems, memory loss, and confusion can occur due to nerve damage.
Psychological Changes: Depression, irritability, and mood swings are associated with low B12 levels.
Gastrointestinal Problems: Constipation, loss of appetite, and a painful, smooth, red tongue (glossitis) are possible.
Skin and Nail Changes: In some cases, pale or jaundiced skin, as well as nail discoloration, can be observed.

Optimal Intake: Food Sources and Supplements

Since vitamin B12 is not naturally present in plant-based foods, sourcing it requires a conscious effort for some dietary patterns. The Recommended Dietary Allowance (RDA) for most adults is 2.4 micrograms (mcg) per day.

How Different Food Sources Provide B12


Food Category	Examples	Bioavailability & Notes
Animal Products	Beef, clams, salmon, milk, eggs	Highest natural source, with varying bioavailability; dairy products are absorbed more efficiently than meat.
Fortified Foods	Breakfast cereals, nutritional yeast, plant-based milks	Excellent source for vegans and vegetarians; contains B12 in its free, highly absorbable form.
Supplements	Tablets, sublingual drops, injections	Effective for treating deficiency or for risk groups; absorption via passive diffusion can make oral supplements viable even with absorption issues.

For those who follow a vegan or predominantly plant-based diet, regular consumption of fortified foods and/or taking a supplement is necessary to prevent deficiency. For others, a balanced diet including animal products usually provides sufficient B12.

Conclusion

Vitamin B12 is far more than just an 'energy vitamin'; its influence permeates nearly every aspect of cellular function. From safeguarding the delicate nerve fibers to ensuring the continuous renewal of red blood cells and the integrity of our DNA, its roles are indispensable. An adequate supply, through a balanced diet rich in animal products or reliable supplementation for those at risk, is essential for preventing long-term neurological and hematological damage. Understanding what this nutrient does for your body is the first step toward safeguarding your health for the long term. For more on B12, see the NIH Office of Dietary Supplements fact sheet.

Frequently Asked Questions

Symptoms of a B12 deficiency can include fatigue, weakness, numbness or tingling in the hands and feet, memory problems, a sore tongue, and mood changes like depression.

You can increase your intake by eating animal products like meat, fish, eggs, and dairy. For vegans, fortified foods like cereals, nutritional yeast, and plant-based milks, along with supplements, are reliable sources.

People most at risk include older adults, individuals on strict vegan diets, those with gastrointestinal conditions (Crohn's, celiac disease), and people taking certain long-term medications like metformin.

Vitamin B12 helps convert food into energy, so a deficiency can cause fatigue. For those with low levels, increasing B12 intake can restore energy. However, it does not provide an energy boost to people who already have sufficient levels.

Both can cause megaloblastic anemia. However, a B12 deficiency, if left untreated, is more likely to cause irreversible nerve and brain damage, a complication typically not seen with folate deficiency.

A doctor can diagnose a deficiency with blood tests that measure the level of vitamin B12. Tests for methylmalonic acid (MMA) and homocysteine may also be used to confirm the diagnosis.

Vitamin B12 is water-soluble, and excess amounts are typically excreted by the body, so toxicity is rare. However, some studies have noted a possible link between very high blood levels and certain health issues, though more research is needed.

The absorption of B12 is a complex process. In the stomach, B12 is released from food proteins by acid and then binds to a protein called 'intrinsic factor' before it can be absorbed in the small intestine. Problems with this process can lead to malabsorption.