Vitamin B12, also known as cobalamin, is an essential water-soluble nutrient the human body cannot produce. Low B12 levels signify an insufficient supply of this compound, which is necessary for several fundamental biological processes. Deficiencies are relatively common, particularly in certain populations, and can lead to serious health consequences if not identified and treated.
The Essential Functions of Vitamin B12
Vitamin B12 is indispensable for the proper functioning of the nervous system. It plays a significant part in the development and maintenance of the central nervous system. It is required for the synthesis of myelin, the fatty sheath that insulates nerve fibers, allowing for rapid and efficient nerve signal transmission. Disruption of this process is the root cause of many neurological symptoms associated with deficiency.
The vitamin is also a cofactor for enzymes involved in the metabolism of fatty acids and amino acids. Crucially, B12 works alongside folate to facilitate DNA synthesis, a process required for the creation of all new cells. This role is particularly important in the production of red blood cells (hematopoiesis). A lack of B12 disrupts the maturation process of these cells, leading to the characteristic large, fragile blood cells seen in megaloblastic anemia.
Why Levels Drop
Low B12 levels occur either because of inadequate dietary intake or, more commonly, due to a problem with absorption in the digestive tract. Since cobalamin is naturally present only in animal products like meat, fish, and dairy, individuals following strict vegetarian or vegan diets are at a heightened risk of deficiency. They must rely on fortified foods or supplements to meet their daily requirements.
Impaired absorption is the most frequent cause of deficiency in the general population. In the stomach, hydrochloric acid releases B12 from food proteins. The vitamin must then bind to Intrinsic Factor (IF), which is secreted by the stomach’s parietal cells. The B12-IF complex travels to the small intestine, where it is finally absorbed.
Conditions that interfere with this process significantly increase the risk of low B12. Pernicious anemia, an autoimmune disorder, prevents the production of Intrinsic Factor, meaning the vitamin cannot be absorbed. Gastrointestinal surgeries, such as gastric bypass, also remove or bypass the sections responsible for IF production and absorption.
Age-related changes, such as reduced stomach acid production (achlorhydria), impair B12 release from food, making older adults particularly susceptible. Certain medications, including proton pump inhibitors and metformin, can also interfere with B12 absorption.
Signs of Deficiency
The clinical signs of B12 deficiency are varied and often develop gradually. Hematological symptoms are often the earliest manifestations, including fatigue, general weakness, and lethargy, which result from megaloblastic anemia. A pale or slightly jaundiced skin tone may also be noted due to the ineffective production and breakdown of red blood cells.
Neurological symptoms are concerning because B12 deficiency directly affects nerve health. Individuals may experience paresthesia, described as a tingling or “pins and needles” sensation, especially in the hands and feet. This nerve damage can progress to difficulty with balance and walking, a condition known as ataxia.
Cognitive changes, such as confusion, memory problems, and difficulty concentrating, can also be present. Other physical signs include glossitis, which presents as a sore, smooth, and red tongue, along with mood disturbances like depression. Neurological damage caused by B12 deficiency, if left untreated, may become irreversible.
Testing and Restoring Levels
Diagnosis of low B12 typically begins with a standard blood test to measure serum B12 levels. A value below 200 picograms per milliliter (pg/mL) is generally considered deficient, but symptoms can sometimes appear even at low-normal levels. For a more definitive diagnosis, especially when serum levels are borderline, healthcare providers may order secondary tests.
These secondary tests measure levels of methylmalonic acid (MMA) and homocysteine in the blood. Since B12 is required to process these substances, both MMA and homocysteine levels become elevated when a functional B12 deficiency is present. This indicates that the body is not utilizing B12 effectively, confirming the diagnosis.
Treatment depends on the underlying cause and the severity of the deficiency. For mild cases caused solely by inadequate dietary intake, high-dose oral B12 supplements are often effective. However, when the deficiency is caused by an absorption issue like pernicious anemia or gastric surgery, B12 injections are the preferred method of delivery.
Intramuscular injections bypass the digestive system entirely, ensuring the vitamin is absorbed directly into the body. Patients with severe neurological symptoms may initially receive frequent injections to rapidly replenish stores, followed by a maintenance schedule, often monthly. Regular follow-up testing is necessary to ensure B12 levels are maintained and prevent the recurrence of symptoms.