What does vitamin B6 and B12 absorption depend upon?

Question

I'm looking at my question about homocysteine metabolism and am doing a followup inquiry into vitamins B6(Pyroxidine) and B12(Cyanocobalamin). I've found this interesting bit about vitamin B12 and am looking at its implications:

Approximately 56% of a 1 mcg oral dose of vitamin B12 is absorbed, but absorption decreases drastically when the capacity of intrinsic factor is exceeded (at 1–2 mcg of vitamin B12)

Quote from vitamin B12 fact sheet

I'm trying to understand the implications of an intrinsic factor involved in B12 absorption. Lets say that a person gets vitamin B12 from both food and a single supplement pill. Will taking that pill "use up" the absorption factor and prevent food's vitamin B12 from being absorbed over time?

This is the first time I'm hearing of additional "factors" that may limit vitamin absorption in the human body. Is this common across vitamins, or is it B12 specific?

Thank you for your input!

Answer 1

Vitamin B12 (cobalamin) is a water-soluble vitamin required for homocysteine methyltransferase. In order to be absorbed, it must bind to intrinsic factor (IF), which is produced by the parietal cells of the stomach. The absorption of the vitamin B12-IF complex occurs in the terminal ileum (of the small intestine). The quote just means that if there isn't enough IF, then you can't absorb vitamin B12.

There are many things that limit absorption of vitamins, and even vitamin B12. Because vitamin B12 is found in animal products, vegetarians and vegans may need to supplement it. Pernicious anemia is an autoimmune disease that destroys IF or the parietal cells, Gastric bypass surgery can lead to vitamin B12 deficiency. Anything that affects vitamin B12 absorption in the terminal ileum, such as in Crohn's, celiac disease, or enteritis, can lead to deficiency.

To answer your question, no. Yes, it will use some IF (provided you have IF), but that would allow your body to absorb vitamin B12. For people who have diseases mentioned above, they can take vitamin B12 via injections, subligually (under the tongue), or intranasal gels, according to the fact sheet you cited.

Because all the B vitamins and vitamin C are water-soluble, they are easily absorbed through the intestinal wall. Excess amounts are usually excreted in urine. I believe vitamin B12 is the only vitamin that needs to be bound to a factor, due to its larger size. It can be stored in the liver for several years. Fat-soluble vitamins (A, D, E, K) are absorbed with lipids.