What are normal vitamin B12 (cobalamin) levels?

From the Guidelines

Normal vitamin B12 levels in adults typically range from 200 to 900 picograms per milliliter (pg/mL) or 148 to 664 picomoles per liter (pmol/L), depending on the laboratory reference range used. Values below 200 pg/mL are generally considered deficient, while levels between 200-300 pg/mL may be borderline and warrant further investigation if symptoms of deficiency are present 1. B12 is essential for proper nerve function, DNA synthesis, and red blood cell formation. The body stores B12 in the liver, so deficiency often develops gradually over several years as stores become depleted. Common symptoms of deficiency include fatigue, weakness, tingling in hands and feet, difficulty walking, memory problems, and anemia. Those at higher risk for low B12 include older adults, vegetarians and vegans, people with gastrointestinal disorders like Crohn's disease or celiac disease, and those taking certain medications such as metformin or proton pump inhibitors.

Key Considerations

• The recommended dietary allowance (RDA) for vitamin B12 is 2.4 µg/day, as stated in the evidence-based nutrition principles and recommendations for the treatment and prevention of diabetes and related complications 1.
• Enteral nutrition should provide at least 2.5 mg cyanocobalamin per day in 1500 kcal, according to the ESPEN micronutrient guideline 1.
• Regular testing is recommended for at-risk individuals, as early detection and treatment can prevent permanent neurological damage.
• It is essential to treat vitamin B12 deficiency immediately, especially in cases with possible neurological involvement, to prevent long-term damage. Hydroxocobalamin 1 mg intramuscularly should be administered on alternate days until there is no further improvement, then hydroxocobalamin 1 mg intramuscularly administered every 2 months 1.

From the Research

Normal B12 Levels

• The definition of normal B12 levels is not explicitly stated in the provided studies, but it is mentioned that diagnostic biomarkers for B12 status include decreased levels of circulating total B12 and transcobalamin-bound B12, and abnormally increased levels of homocysteine and methylmalonic acid 2.
• The exact cut-offs to classify clinical and subclinical deficiency remain debated, and the chosen test often depends on its availability rather than on the diagnostic performance and sensitivity to irrelevant factors interfering with vitamin B12 markers 3.
• Serum B12 is still the most commonly used and widely available test, but diagnostics by holotranscobalamin, serum methylmalonic acid, and plasma homocysteine measurements have grown in the last several years in routine practice 3.

B12 Deficiency Treatment

• High oral doses of B12 (1000 mcg and 2000 mcg) were as effective as intramuscular administration in achieving haematological and neurological responses in vitamin B12 deficient patients 4.
• The oral route is comparable to the intramuscular route for rectifying vitamin B12 deficiency, and treatment with a combination of methylcobalamin and adenosylcobalamin or hydroxocobalamin is recommended 5.
• Management of B12 deficiency depends on B12 supplementation, either via high-dose oral routes or via parenteral administration 2.

B12 Status Assessment

• Vitamin B12 status can be assessed through various tests, including serum B12, holotranscobalamin, serum methylmalonic acid, and plasma homocysteine measurements 3.
• The lack of a robust absorption test, coupled with compromised sensitivity and specificity of other tests, hinders determination of the cause for depleted B12 status 3.
• Future directions for diagnostics of B12 status and health interventions are discussed, including the need for improved diagnostic methods and a better understanding of the prevalence, causes, and manifestations of B12 deficiency 2, 3.