B-12

Vitamin B12 (cobalamin) absorption is a complex, multi-step process:

  1. Release from Food Proteins: In the stomach, hydrochloric acid and digestive enzymes release B12 from the proteins in food. 

  2. Binding to Haptocorrin: The free B12 then binds to haptocorrin, a protein secreted in saliva and the stomach.

  3. Transfer to Intrinsic Factor: In the small intestine, pancreatic enzymes degrade haptocorrin, allowing B12 to bind with intrinsic factor (IF), a glycoprotein produced by the stomach's parietal cells. 

  4. Absorption in the Ileum: The B12-IF complex travels to the distal ileum, where it binds to specific receptors on the intestinal lining and is absorbed into the bloodstream.

This intricate process can be disrupted by various factors, including aging, gastrointestinal disorders, certain medications, and autoimmune conditions like pernicious anemia, leading to B12 deficiency.

Elevated serum B12 levels without supplementation can be indicative of underlying health issues:

  • Liver Diseases: Conditions such as hepatitis, cirrhosis, and liver cancer can cause liver cells to release stored B12 into the bloodstream or impair the liver's ability to clear excess B12. 

  • Blood Disorders: Myeloproliferative disorders like chronic myeloid leukemia and polycythemia vera can increase levels of B12-binding proteins, leading to elevated serum B12. 

  • Kidney Disease: Chronic kidney disease can reduce the kidneys' ability to excrete B12, resulting in higher blood levels.

  • Inflammatory Conditions: Chronic inflammation from autoimmune diseases can damage cells, releasing B12 into the bloodstream. 

  • Macro-B12 Complexes: In rare cases, B12 binds to immunoglobulins, forming complexes that are detected in blood tests but are not bioavailable, leading to falsely elevated B12 readings. 

Why B12 would be high if it is stored in the liver and excreted in urine:

  • Vitamin B12 is stored in the liver, often in large amounts that can last several years.

  • It’s a water-soluble vitamin, and any excess from food or supplements is typically excreted in urine.

So, under normal circumstances, the body regulates B12 quite well. But when B12 is high without supplementation, it usually signals that something abnormal is happening. 

Here's why: 

1. Liver Release

  • Liver damage or disease (e.g., hepatitis, cirrhosis, fatty liver, or liver cancer) can cause stored B12 to leak into the bloodstream.

  • Think of it like a "storage tank" breaking open — you haven’t added more B12, but more is circulating in the blood.

2. Overproduction of Binding Proteins

  • In some blood cancers or disorders (e.g., chronic myeloid leukemia, polycythemia vera), the body may produce more haptocorrin, a protein that binds B12.

  • This increases total B12 levels, but not necessarily the amount your cells can use.

3. Kidney Impairment

  • Chronic kidney disease may reduce B12 clearance. It’s not that you’re absorbing more, but rather that you’re excreting less.

4. Chronic Inflammation or Autoimmunity

  • Inflammation from autoimmune diseases (like Hashimoto’s, rheumatoid arthritis, or lupus) can disrupt cell membranes or increase certain proteins that affect B12 levels.

5. Macro-B12

  • A rare phenomenon where B12 binds to antibodies (immunoglobulins), forming "macro-B12" complexes that don’t get cleared properly, causing artificially high levels on blood tests.

  • It’s not harmful, but can mislead clinicians unless they test for it specifically.

If your B12 is elevated without supplementation, it's not likely due to too much absorption or intake. Instead, it's often:

  • A sign of something affecting storage, transport, or excretion,

  • Or a laboratory artifact (like macro-B12).

In these cases, doctors typically look at:

  • Liver and kidney function tests

  • Complete blood count (CBC)

  • Markers of inflammation (CRP, ESR)

  • And sometimes tests for macro-B12


RDA requirement for adults over age 19 is 2.4 mcg daily.


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