Vitamin B12 plays a central role in three of your body’s most essential processes: making red blood cells, maintaining your nervous system, and producing energy from food. Adults need 2.4 mcg per day, and because your body can’t make B12 on its own, every microgram has to come from food or supplements. Here’s what it actually does once it’s inside you.
How B12 Keeps Your Nerves Working
Your nerves are wrapped in a protective coating called myelin, which works like insulation on an electrical wire. Myelin allows nerve signals to travel quickly and accurately through your brain, spinal cord, and limbs. B12 is essential for building and repairing this coating. Without enough of it, myelin breaks down, and the damage tends to hit the longest nerve fibers first, particularly those running through the spinal cord that control your sense of balance, vibration, and body position.
This is why tingling or numbness in the hands and feet is one of the hallmark signs of B12 deficiency. As the damage progresses, it can affect coordination, cause difficulty walking, and lead to cognitive changes like memory problems and difficulty concentrating. B12 also contributes to the production of neurotransmitters, the chemical messengers your brain relies on for mood, motivation, and mental clarity. Importantly, nerve symptoms can show up months before any blood-related signs of deficiency appear, which means they’re easy to miss or attribute to something else.
Building Red Blood Cells
B12 is required for your bone marrow to produce healthy red blood cells. It helps with a key step in DNA replication: converting one building block of DNA into another. When B12 is missing, that conversion stalls. Developing red blood cells can’t divide properly, but they keep growing. The result is fewer, abnormally large cells called megaloblasts that don’t carry oxygen efficiently.
This condition, megaloblastic anemia, leads to fatigue, weakness, lightheadedness, rapid heartbeat, pale skin, and shortness of breath. Because DNA replication matters in every rapidly dividing cell, not just blood cells, B12 deficiency can also cause a sore tongue, digestive problems, and easy bruising. The good news is that once B12 levels are restored, blood cell production typically returns to normal relatively quickly.
Converting Food Into Energy
B12 acts as a helper molecule for an enzyme that feeds fats and proteins into your body’s main energy-production cycle (the citric acid cycle). Specifically, it converts a compound from the breakdown of certain fats and amino acids into a form your cells can actually use for fuel. That same compound is also needed to make hemoglobin, the protein in red blood cells that carries oxygen to your tissues.
This is why fatigue is such a common complaint with low B12. It’s not that B12 itself is an energy source, but without it, your cells struggle to extract energy from the food you eat. If your B12 levels are already normal, taking extra won’t give you a noticeable energy boost. The benefit comes from correcting a deficiency.
Protecting Your DNA
Every time a cell divides, it copies its entire DNA. B12 helps ensure those copies are accurate. It supports the production of thymidine, one of the four building blocks of DNA. When B12 is low, the wrong building block gets inserted during copying, which can cause single- or double-strand breaks in the DNA, chromosome damage, and genetic instability over time. B12 also influences DNA methylation, a process your cells use to switch genes on and off. Disruptions to methylation are linked to a range of health problems beyond just blood cell production.
Where You Get B12
B12 is found almost exclusively in animal-based foods. Clams are the richest source by a wide margin, with 84 mcg in a 3-ounce serving. Trout (5.4 mcg) and salmon (4.9 mcg) are also strong sources, followed by canned tuna (2.5 mcg), beef (1.5 mcg), and Greek yogurt (1.3 mcg per 6 ounces). A single large egg provides 0.6 mcg, and a 3-ounce chicken breast has just 0.3 mcg.
If you eat little or no animal food, fortified products are your main option. Many breakfast cereals are fortified to 100% of the daily value per serving (6 mcg), and fortified soy milk typically provides around 1.7 mcg. Checking labels matters because fortification levels vary between brands. People following a vegan diet will almost certainly need a supplement to meet the 2.4 mcg daily recommendation. Pregnant women need slightly more, at 2.6 mcg per day.
How Your Body Absorbs It
Absorbing B12 is more complex than most vitamins. Your stomach lining produces a protein called intrinsic factor, which binds to B12 in the stomach and escorts it to the small intestine, where it enters your bloodstream. Without enough intrinsic factor, B12 passes through your gut without being absorbed, no matter how much you consume.
Several conditions disrupt this process. Pernicious anemia is an autoimmune condition where the body destroys the cells that make intrinsic factor. Stomach surgery or gastric bypass can remove the tissue that produces it. And as you age, your stomach produces less acid, making it harder to release B12 from food in the first place. This is why adults over 50 are often advised to get their B12 from supplements or fortified foods, which don’t require stomach acid for absorption.
Who’s Most Likely to Be Low
Beyond vegans and older adults, people taking metformin for type 2 diabetes face a well-documented risk. Research shows metformin causes dose-dependent drops in B12 levels over time. One randomized trial of 469 people with diabetes found that B12 levels remained stable in those taking a placebo but dropped significantly in the metformin group after 18 months. The risk increases after four or more years of use, especially when metformin is combined with acid-reducing medications like proton pump inhibitors. Annual B12 testing is recommended for long-term metformin users.
People with digestive conditions that affect the stomach or small intestine, including celiac disease, Crohn’s disease, and chronic gastritis, are also at higher risk. Heavy alcohol use can interfere with absorption as well.
Recognizing a Deficiency
B12 deficiency develops slowly, often over years, because your liver stores several years’ worth. Early symptoms are vague: fatigue, low motivation, mild weakness, and difficulty concentrating. As levels drop further, you may notice tingling or numbness in your hands and feet, a sore or swollen tongue, balance problems, dizziness, and mood changes including irritability or depression. Some people develop rapid heartbeat, shortness of breath, or pale skin as anemia sets in.
A blood test can measure your B12 level directly. Normal serum B12 falls between 160 and 950 pg/mL. Values below 160 pg/mL suggest deficiency, though levels in the low-normal range can still cause symptoms in some people. When results are borderline, testing for methylmalonic acid in the blood provides a more sensitive measure, since that compound builds up specifically when B12 is too low to do its job.