Vitamin B12 Deficiency

Vitamin B12 deficiency is a treatable condition that develops when dietary intake is insufficient or when the body cannot absorb the vitamin properly. B12, also known as cobalamin, serves as a cofactor for enzymes involved in DNA synthesis, fatty acid production, and myelin formation, the protective sheath that insulates nerve fibers. Without adequate B12, red blood cell production falters and nerve function can deteriorate.

Analysis of national survey data from 2007 to 2018 found that approximately 3.6% of adults aged 19 and older are B12-deficient. Prevalence increases with age: roughly 6% of individuals under 60 are affected, rising to nearly 20% of those over 60. The condition can develop gradually over months or years because the body maintains substantial B12 reserves, which can mask a declining supply for an extended period. Depending on the underlying cause, treatment may involve short-term supplementation until levels normalize or lifelong B12 therapy.

B12 deficiency arises from two broad pathways: inadequate dietary intake or impaired absorption. Because B12 is found almost exclusively in animal-derived foods (meat, fish, dairy, and eggs), individuals who avoid these foods or do not consume B12-fortified alternatives are at elevated risk.

The most prevalent cause of deficiency in clinical practice is food-bound cobalamin malabsorption (FBCM), a condition in which the body cannot efficiently release B12 from the proteins in food. FBCM can result from chronic gastritis, long-term use of acid-suppressing medications, or age-related reductions in stomach acid production. Pernicious anemia is an autoimmune condition in which the body produces antibodies against intrinsic factor, a protein required for B12 absorption in the small intestine, effectively blocking uptake from food. Other contributing factors include medications like metformin, surgical procedures that alter the stomach or small intestine, and digestive conditions such as celiac disease or Crohn's disease that impair nutrient absorption.

Testing for B12 deficiency should be considered when symptoms such as persistent fatigue, numbness or tingling in the hands or feet, memory difficulties, or unexplained anemia are present. Screening is also warranted for individuals with risk factors including prior gastric or small intestine surgery, inflammatory bowel disease, metformin use exceeding four months, proton pump inhibitor or H2 blocker use exceeding twelve months, strict vegetarian or vegan dietary patterns, and age over 75 years.

Testing may also be appropriate for people taking other medications that interfere with B12 absorption, those with digestive disorders, or anyone following a diet that significantly limits animal products. Early detection matters because some neurological effects of B12 deficiency can become permanent if the condition goes untreated for extended periods.

Serum B12 levels above 300 pg/mL are generally interpreted as normal. Levels between 200 and 300 pg/mL fall into a borderline zone where additional testing may help clarify the picture. Levels below 200 pg/mL are considered deficient. However, B12 results should always be evaluated alongside symptoms and other laboratory findings, as the clinical picture is not always straightforward.

When B12 levels are borderline, healthcare providers may order methylmalonic acid (MMA) and homocysteine levels to confirm deficiency, since both of these metabolites accumulate when B12 is insufficient. A complete blood count may reveal enlarged red blood cells (macrocytic anemia) or other abnormalities consistent with B12 deficiency. It is worth noting that some individuals experience symptoms even at borderline B12 levels, and treatment decisions should weigh both the laboratory results and the clinical presentation.

Maintaining adequate B12 levels centers on dietary awareness and, for some individuals, supplementation. B12 is found primarily in animal products including meat, fish, eggs, and dairy. Individuals following vegetarian or vegan diets should pay particular attention to B12 intake through fortified foods (such as fortified cereals, plant milks, and nutritional yeast) or dedicated supplements, since plant-based foods contain virtually no naturally occurring B12.

For people with absorption issues, dietary changes alone may not be enough, and medical treatment with oral supplements or injections may be necessary to maintain adequate levels. Regular monitoring of B12 status is important for those with gastrointestinal conditions, individuals taking acid-suppressing medications or metformin, and adults over 60. Limiting alcohol consumption and managing stress can also support overall nutrient absorption and general health.