Pernicious Anaemia Society
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Why Do We Load?

I am trying to find information about the reasons that we Load B12 when starting out.

I know it is the accepted wisdom and that is is written in the treatment guidelines - but my question is why do we do it and do any of you know of any documents online that actually give valid biological reasons for the practice.

Really hope one of you can help :-)

13 Replies

my thoughts are, that the loading is to replenish those organs, predominantly the liver that's b12 depleted. the liver can store 5 yrs worth of b12. that's why it takes so long to totally deplete the body of b12 & ppl don't always make a time frame correlation between dietary changes/habits/malabsorbtion etc..



Although the body generally stores a lot of B12 in the liver this isn't actually useable if you have an absorption problem because reabsorption depends on the same mechanism as normal absorption, so filling up the liver isn't really going to make a lot of difference ... but I certainly think there is an element of 'leaky bath' going on.


The idea behind loading doses is, I think, to help the body build up levels quickly to prevent any more damage and give the system a bit of a boost in terms of repairing damagage. The fact that a lot of the B12 will be lost is also a factor - so a bit like trying to fill up a leaky bath - you have to start of with a lot of buckets first and then once it is full you only need to add the odd bucket now and then to keep it topped up - the maintenance doses.


Hi Gambit, thank you so much for your input.

My questions still is though. Why load? Why put 1mg in every second day for a prolonged period when you could put in say 5mg or 20mg every second day for less of a period.

You are a forum legend Gambit, and I dont mean to sound argumentative - X


Would really have to admit that I don't know why 1mg was the dose chosen - except that it probably seemed to be the dosage that got the best response over people that were being given it at the time.

I am aware that some people actually have different responses ... and in some cases lower doses can be more effective than higher because of reactions (possibly autoimmune to high serum B12). There may be some research out there that has been done but it's probably buried in the mists of time ... well before the internet :)


Sorry - just realised that there is probably a good reason for not using higher doses which is the fact that the rate of loss of B12 is dependent on how much B12 there is in your system - the higher the amount of b12 the more quickly it is lost, so actually higher doses may well be inefficient in getting B12 levels built up than 1mg. Of course the calculations are all based on averages and we all know that the average B12 person is a rare beast indeed but ...

so to go back to the leaky bath - the maths might be that if there you double the size of the bucket you are using then you quadruple the rate at which water is lost .... so doubling the size of the bucket is potentially a lot less effective than doubling the requency with which you put buckets in.


Love that, and you've given me a totally new piece of information in the B12 loss rate increasing the more we have in our system. Thank you.

And thank you for the thoughts and all the effort you have put in!!! x

Between you and the other answers I feel as though we have given this one a really good bash :-)


In my pre-retirement job I did a bit of pharmacokinetics (the study of the behaviour of drugs in the body - absorption, distribution, metabolism and elimination). My guess is that the body can only utilise a certain amount of B12, with any excess being eliminated. So, in most cases 1mg every other day is the optimum for IM administration.

But it is just a guess. I'll try to find out for sure when I've got the time and the energy.


Sounds about right and if you have the resources to put the science behind that it would be awesome to read.

Thank you so much


Well, I had a look around today and found that it's quite difficult to read the whole paper from a scientific journal if you don't have professional access. However, I think I've found the reason...

The human body has some wierd foibles. One of which is the way the kidneys work. Almost all of the smaller chemicals in the blood get filtered out by the kidney - and the stuff that the body wants to keep gets reabsorbed in the kidney tubule. It's as if you threw everything in your house into the rubbish bin, then took out all the stuff you want to keep. Hardly intelligent design.

But this reabsoption has a maximum throughput. If there's too much B12 in the blood then it all gets filtered but only some of it will get reabsorbed, the rest just goes out in the urine. I'm going to guess that the loading doses are designed to stay below the tubular reabsorption saturation point.

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My partner had B12 of 220 and neurological problems and was given two weeks of 1mg cobalamin every second day. I have been injecting him with it once weekly for two months and he now has a level of 1400. Presumably that means there has been an accumulation - i.e. it has not all been eliminated. Just my input into the discussion.


Thank you. That certainly proves that the loading system works. Now your partner is up to 1400 how are his symptoms?


His symptoms are good but there is long term damage. There has been some improvement there but I am considering trying him out on daily methyl B12, since I have read of two reversals of long term peripheral neuritis using that method. I have both my parents (dementia and severe peripheral neuritis) on injections of cobalamin every second day. I don't know where their levels are at, but had intended to persist for about six months.


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