Can anyone advise or send me to research papers or pertinent information on what b12 levels are stored in the liver of people with pernicious anaemia?
An average adults body is supposed to require 2.4mcg of b12 daily, where does the excess go from a 1000mcg b12 injection?
Hoping you can help. Thank you.
the liver can store quite significant amount of B12 - and it has been estimated that the amount is sufficient for 3 years, which is why a dietary deficiency takes a long time to develop.The mechanism for releasing stores of B12 is to release it in bile for absorption in the terminal ileum. If you have PA then the mechanism for absorbing B12 in the terminal illeum doesn't work and the B12 will just pass on through the gut.
B12 transfer from blood to liver is done by B12 binding to a specific protein heptacorin. This mechanism isn't affected by PA so that the B12 from your injections can replenish the stores of B12 in the liver but your body won't be able to use those stores to in your liver to regulate amounts in the blood because it doesn't get absorbed back into your blood stream, it just passes through your gut.
There is a limit to the amount that can be absorbed in the ileum in a single go - somewhere around 10mcg so very little of a 1000mcg dose will be absorbed in the ileum. This is because the B12 needs to be bound to particular proteins to be absorbed there and the body has a limited capacity to make those proteins. Most of the dose will pass through the ileum and carry on down the gut. This doesn't mean that all of the B12 is lost because there is a secondary mechanism called passive absorption that happens in the rest of the gut but it is much less efficient than the specialised absorption process in the ileum, but some does get absorbed - estimate is that this averages about 1% of the amount passing through so having a large dose going through means it is possible for enough to be absorbed and get into the blood stream.
However, the efficiency of passive absorption shows a great deal of variation between individuals meaning that it could be much less than 1% that is being absorbed.
Whilst all of a 1000mcg dose from an injection goes into your blood the vast majority of that dose doesn't get bound to the protein that is needed to enable it to pass into your cells - transcobalamin - because your body has a finite ability to produce transcobalamin. This means that it is either bound to another protein like heptacorin or it isn't bound to a protein at all and will be removed from blood through various filtration systems - the main one being the kidneys. The heptacorin bound will be stored in the liver but any attempt to release it will just result in the small amounts of B12 released passing down the gut.
So, most of the 1000mcg dose is removed by kidneys and leaves your body in urine.
That's a very simple explanation and real life seems to be much more complicated in terms of what actually happens with transfer from blood to cells once B12 levels have been raised by injections and that's something that is being researched. It does mean that significant numbers of patients who have injections report that they need to maintain their B12 levels above the normal range to feel okay.
That’s brilliant, thank you for that information. If the body is only able to use such a small amount at any one time from your b12 injection, are there not smaller doses available that could be taken more often?
1000mcg worked out as being the most effective dose to ensure the most efficient balance between storing B12 in the blood (which is what is happening if you are using injections) and losing it through removal by the kidneys - not sure what the sums are around rate of removal of B12 from injected B12 and it just varies so much its quite probable that there averages wouldn't work. The early studies of using injected B12 showed some people had high levels over a year - think it in one case it was 4 years later.
How does it feel to have too much B12?
seeing neurologist, advice on b12 injections welcomed…
