Vitamin B12, also known as cobalamin, is an essential nutrient that plays a fundamental role in maintaining the nervous system and producing healthy blood cells. Because the human body cannot manufacture B12, it must be obtained entirely through diet or supplementation. A low level can impair several critical bodily functions and often develops gradually over a long period.
Understanding Vitamin B12 Status
Vitamin B12 is a cofactor required for major metabolic pathways, including DNA synthesis and fatty acid metabolism. It is necessary for the proper creation and maturation of red blood cells, which is why a deficiency often leads to anemia. B12 also helps maintain the myelin sheath, a protective covering that insulates nerve fibers throughout the body. An adequate supply is necessary for normal neurological function.
Clinical guidelines often define low B12 status as a serum level below 200 picograms per milliliter (pg/mL). B12 is stored in substantial amounts, primarily in the liver, meaning it can take two to five years for stores to become depleted after a change in diet or the onset of a malabsorption issue. This long storage time can delay the appearance of deficiency symptoms, making early diagnosis challenging.
Physical Signs of Deficiency
Symptoms of low B12 generally fall into two categories: those related to the blood and those related to the nervous system. Hematological signs often manifest as generalized fatigue, lethargy, and weakness, which are classic indicators of anemia. A specific condition, megaloblastic anemia, occurs when the body produces abnormally large, immature red blood cells that cannot function correctly. This impaired oxygen transport can also lead to paleness or a slight yellowing of the skin and eyes.
Neurological signs can be concerning, as they sometimes become irreversible if treatment is delayed. Individuals frequently report paresthesia, a sensation of numbness or tingling in the hands and feet. The deficiency can affect coordination, leading to difficulty walking, balance problems, and muscle weakness. Cognitive changes such as memory loss, confusion, and difficulty concentrating may also develop.
Factors That Cause Low B12
The most common reason for a low B12 level is not insufficient intake, but rather a problem with absorption within the digestive system. To be absorbed, B12 must first be released from dietary proteins by stomach acid. It then binds to Intrinsic Factor, a protein produced by cells in the stomach lining. This B12-Intrinsic Factor complex travels to the small intestine where it is finally absorbed into the bloodstream.
One primary cause of malabsorption is Pernicious Anemia, an autoimmune condition where the body attacks the stomach cells that produce Intrinsic Factor. Without this protein, B12 cannot be absorbed. Another frequent cause is reduced stomach acid, which can result from natural aging, atrophic gastritis, or the long-term use of acid-suppressing medications like proton pump inhibitors (PPIs). These medications prevent the initial release of B12 from food proteins, leading to deficiency.
Gastrointestinal diseases like Crohn’s disease or celiac disease can also interfere with absorption, particularly if they affect the terminal ileum, the section of the small intestine where the B12 complex is absorbed. Individuals following a strict vegan or vegetarian diet are also at risk because B12 is found almost exclusively in animal products. For this group, a consistent intake of fortified foods or supplements is necessary to prevent depletion.
Restoring Healthy Levels
Treatment for a low B12 level is determined by the underlying cause of the deficiency. For individuals whose deficiency is purely diet-related, high-dose oral supplements are often effective because they allow B12 to be absorbed passively without requiring Intrinsic Factor. Supplementation usually involves doses of 1,000 to 2,000 micrograms (mcg) per day, which can be as effective as injections for raising blood levels.
However, in cases of severe deficiency or when malabsorption is the root cause, such as Pernicious Anemia, intramuscular injections are required. These injections bypass the digestive system, delivering B12 directly into the muscle for systemic absorption. After an initial loading period of frequent injections, maintenance shots are administered periodically to ensure stable B12 levels. Follow-up blood testing confirms that the chosen intervention is successfully restoring healthy B12 status.