Medicosnext
Vitamin B12 tests are commonly requested tests received by laboratories. Vitamin B12 remains the first line test and only reflects the status of vitamin B12 in the body. Testing vitamin B12 does not, however, discriminate the underlying causes of vitamin B12 deficiency. Although Total Vitamin B12 test is predominantly done worldwide, there is emerging evidence of Active Vitamin B12 test being more superior than Total Vitamin B12 in reflecting vitamin B12 deficiency. This article describes in brief, sources, metabolism and deficiency of vitamin B12, emphasizing the importance of Active Vitamin B12 test over Total Vitamin B12 test and other biomarkers.
Vitamin B12
Known as cyanocobalamin, vitamin B12 is a water-soluble vitamin. It is very complex in structure and has tetrapyrrole ring at its center that serves as site for attachment of cobalt molecule. There are various forms of cobalamin, of which methyl cobalamin is mainly present in the serum and adenosyl cobalamin is found in the cytoplasm; the other forms are cyano and hydroxy cobalamin.
How much vitamin B12 is required daily?
The recommended daily allowance (RDA) for vitamin B12
RDA
Adult 2.4µg
Pregnant 2.6µg
Lactation 2.8µg
Children (1-3) years 0.9µg
Children (4-8) years 1.2µg
Children (9-13) years 1.8µg
Children (14-18) years 2.4µg
What food to eat?
Vitamin B12 is not present in plant-based diet. In animals, certain bacteria in the gastrointestinal tract synthesize vitamin B12. The synthesized vitamin B12 is absorbed and stored in animal tissues. Hence, the major sources of vitamin B12 are food of animal origin. Animal meat, such as goat/lamb (liver and kidney), mutton, pork, fish (tuna, Rainbow trout, sardines), chicken, eggs, and dairy products like milk, cheese and yogurt are good sources of B12.[1]
For non-vegetarians, egg, chicken, fish, and dairy could be a less expensive food options as a source of vitamin B12, whereas for vegans, dairy products, and if available, fortified cereals and yeasts. The latter many not be readily available in the Nepali market, and hence, imposes further constraint upon strictly vegetarian consumers.
Metabolism
Vitamin B12 bound to proteins in the food undergoes series of metabolic processes to be freely available for its action inside the body. After ingestion, food is chewed in the mouth, which breaks down the complex structure to simpler form. Further, acid secreted by the stomach digests proteins, which releases tightly bound vitamin B12. Once free, in the stomach, B12 binds to the haptocorrin (HC), also known as R protein, produced by salivary gland. The R-B12 complex travels down to the duodenum, where the R protein gets digested and B-12 is again released to be bound to Intrinsic factor (IF) secreted by gastric parietal cells of the stomach. The B12- IF complex then moves down to the ileum and binds to the cubam receptor complex in the mucosal epithelial cells and is ultimately taken up. Inside the mucosal cells, IF is degraded and B12 is released in the circulation.
B12 in the circulation binds to mainly two proteins—haptocorrin (TCI) and transcobalamin (TCII). When bound to haptocorrin, it is called holohaptocorrin, and when bound to transcobalamin it is called holotranscobalamin. Of the two forms, majority (80%) is in holohaptocorrin form. The transcobalamin bound B12 fraction (20%) is available for uptake by the cell and hence is the bioactive form.Transcobalamin circulates in the blood and delivers B12 to various tissues.
The excess vitamin B12 is stored in the liver, whereas the remainder is excreted from the kidneys. Interestingly, the liver can store roughly 1 mg of vitamin B12, which is sufficient to meet the daily requirement for nearly five and half years. Therefore, manifestation of vitamin B12 deficiency takes years before it becomes apparent.
Deficiency of vitamin B12
The deficiency of vitamin B12 and its implications in health is well established. The most common cause of vitamin B12 deficiency is intake of strictly vegan diet and malnutrition. Vitamin B12 is not present in plant-based diet, so consumers who rely only on veggies may not get adequate amount of vitamin B12 needed for the body. Vitamin B12 deficiency is also found predominantly amongst the elderly because of its weak absorption from the gastrointestinal tract, also known as food cobalamin malabsorption syndrome, and pregnancy. Defect in various steps of metabolism of vitamin B12, i.e. during digestion, absorption, and in circulation also results in its deficiency. Lack of intrinsic factor is a well established cause of vitamin B12 deficiency and may result so due to gastritis, gastric bypass surgeries, or antibody directed to gastric parietal cell, as in case of pernicious anemia. Likewise, any obstruction in process of absorption through ileum, such as inflammatory conditions of bowel, illeal surgeries, tapeworm infestation, and bacterial overgrowth causes vitamin B12 deficiency. Use of proton pump inhibitors causes reduced secretion of acid in the stomach and thus impairs release of food-bound vitamin B12. Several drugs have been known to cause reduction in levels of vitamin B12, such as metformin, phenytoin, nitric oxide, colchicine, antibiotics, and oral contraceptives. Less commonly, genetic conditions resulting into deficiency of transcobalamin II results in vitamin B12 deficiency.[1]
Vitamin B12 in health and disease
Vitamin B12 is required as cofactor by enzyme methionine synthase, which catalyzes the reaction where methyl tetrahydrofolate donates methyl group to homocysteine and in turn generates tetrahydrofolate (TH4) and methionine. With vitamin B12 deficiency, the conversion cannot take place, and hence, methyl tetrahydrofolate and homocysteine (Hcy) accumulate. The reduced generation of tetrahydrofolate then impedes production of thymidine monophosphate, a pyrimidine and an essential component of DNA. This subsequently affects the DNA replication in red blood cells, resulting in macrocytic RBC, called as megaloblastic anemia.
The enzyme methylmalonylCoA mutase involves vitamin B12 for conversion of methylmalonyl CoA into succinyl CoA. Lack of vitamin B12 results in accumulation of methylmalonyl CoA, which affects the myelination of the nervous system. The resulting demyelination of central and peripheral nerves lead to a condition called sub-acute combined degeneration of cord. Most recently, the role of vitamin B12 has been emphasized in risk reduction of cardiovascular and cerebrovascular disease. In certain genotypes, intake of vitamin B12 with folic acid and vitamin B6 has shown to reduce risk of colon cancer.[2] Therefore, vitamin B12 deficiency is associated with increased risk of myocardial infarction and stroke.
Subclinical Cobalamin Deficiency (SCCD) is the state of low-normal vitamin B12 in the body without any clinical sign and symptoms. This mild metabolic abnormality is characterized by mild to no elevation of MMA and Hcy. The serum total vitamin B12 ranges from 150-249 pmol/L.[3]
The presenting symptoms and signs of vitamin B12 deficiency include tiredness, shortness of breathing, pallor, jaundice, and glossitis. With neurologic involvement, the patient usually presents with paresthesia (pin and needle sensation), numbness, and problem in balance and coordination. Neurological examination may reveal weakness and impaired position and vibration sense.[4] B 12 deficiency also manifests as psychiatric problems like irritability, memory impairment, depression, and psychosis.
Reference range Total Vitamin B12[3]
Normal Level >250 pmol/L
Low Level 150 -249 pmol/L
Acute deficiency <149pmol/L
Active B 12 test Vs Total B12 test
The commonly performed tests to understand the status of vitamin B12 in the body are Total Vitamin B12, Active Vitamin B12, Homocysteine (Hcy), and Methylmalonic Acid (MMA). The total vitamin B12 circulates in the serum in two forms, i.e. holohaptocorrin and holotranscobalamin. Biochemical assays that measure total vitamin B12 constitute both the fractions, holohaptocorrin and holotranscobalamin. The main drawback of Total Vitamin B12 test is that it measures the major holohaptocorrin fraction, which is unavailable for cellular uptake and thus does not reflect true cellular status. In which case, it requires second line tests for confirmation to determine the actual deficiency at cellular level.[3]
Active Vitamin B12 test, on the other hand, measures only holotranscobalamin, which is the metabolically active form of B12 taken up by the tissues. Recent studies have reported superiority of Active Vitamin B12 in detecting B12 deficiency than Total Vitamin B12, MMA, and Hcy. Active Vitamin B12 better reflects the cellular status of vitamin B12 deficiency.[3] Total vitamin B12 level is affected by pregnancy; therefore, Active Vitamin B12 test provides better indication of deficiency than Total B12 test in pregnancy. In addition, Active Vitamin B12 is more reliable in confirming inadequacy of vitamin B12 in case of low normal level of total vitamin B12.[5] The reference range of Active Vitamin B12 is 20-125 pmol/L.[3]
Methylmalonic acid (MMA) and Homocysteine (Hcy)
Also known as functional biomarkers, MMA and Hcy are elevated in the case of vitamin B12 deficiency, and are considered as second line tests. MMA and Hcy can be assessed in patients before treatment and helps confirm the deficiency of vitamin B12, as well as when vitamin B12 level itself is in low normal range. Post-treatment, MMA and Hcy levels are used to determine the response to therapy. The MMA and Hcy levels are expected to return to normal with sufficient vitamin B12 supplementation. Of the two tests, MMA is more specific for vitamin B12 deficiency, as Hcy is also elevated in folate deficiency, homocystinuria, and renal failure.[4]. The use of MMA test in confirming the diagnosis of vitamin B12 deficiency is limited by its limited availability, high cost, and unstandardized assay and reference interval.
Conclusion
Although Total Vitamin B12 testing is widespread, because Active Vitamin B12 test measures the biologically active fraction, it provides a better indicator of vitamin B12 deficiency.
Very informative