How long before an oral B12 tablet affects your Active/Serum B12 level?

[Katanaqui]

Assuming for a moment that you don't have PA, and that you can successfully absorb oral B12 tablets like a healthy person (who may just have a very poor diet):

1) How long does it take an oral B12 tablet to be absorbed, such that it raises an Active B12 blood test result (or failing that, a Serum B12 result)?

2) And what dose of tablet/s would a typical, healthy person need for a noticeable effect?

For example: my Serum B12 is currently 180ng/L (right on the lower bound of the normal range 180--650); I will shortly know my Active B12 too, which of course I also expect to be low. Could I take 1x 50mcg Cyanocobalamin tablet, take a Serum/Active B12 test 24 hours later, and expect my blood result to have noticeably increased to something like 400ng/L?

Thank you in advance to anyone who can help, or point me towards some reliable reading

For context: I have low Serum B12, so we're in the process of figuring out if I have PA; my Intrinsic Factor Ab blood test came back negative, which I know can't rule out a dx of PA. I thought we'd already ruled out malabsorption issues e.g. PPI use, because coincidentally I'd already been taking prescription oral vitamin B tablets for quite some time, and they clearly weren't helping raise my consistently low Serum B12 result.

However, I've belatedly learned that B Compound Strong tablets don't actually contain B12. So before I finally start injections very soon and they completely skew my Active/Serum B12 blood results, I want to ascertain if I can in fact absorb B12 tablets. But after two months of fighting for correct protocol with several doctors who refused at every turn, I'd rather figure it out myself by taking another private blood test than watch them get even more confused and find a reason to delay my loading doses even further.

Please note I'm already aware that someone with PA cannot typically absorb B12 due to lack of IF in the gut (unless very high dose oral gives them enough via passive diffusion), and that oral B12 can take a while to affect someone's symptoms (if at all). I'm specifically interested in the blood result, as a quick indication that I may be able to return to typical-dose tablets in the future as maintenance, instead of being on injections for life or having to take super-high-dose tablets in the hope that some trickles through.

[Nackapan]

My husbands serum b12 blood test was 180.pg/ml

Note ; He had NO symptoms.

He took 2.5-5mcg daily fir 3 months in a tablet form.

His level rose to about 356pg/mL ( without looking it up)

Range 200-900pg/ml

It was not diet related but I think absorbing at a slower rate. He has tyoe1 diabetes.

Anyhow he continues to take this very low b12 tablet daily.

He feels no different!

If that helps

[mickeymouse42]

I'm in a similar position to you. I discovered in January that my Serum B12 level was 201 ng/L in a range of 197 to 711. The GP wouldn't do the PA tests (IFAB) because I was in the normal range and told me to take supplements.

On advice from this forum, I bought a 100 tablets of 100mcg Solgar cyanocobalamin. I had a follow up blood test after I had taken the 100 tablets and my levels had risen to 311 ng/L.

I'm going to take 2 more courses of 100 mcg tablets and see if my levels rise to a healthy medium of around 500ng/L. I'll then switch to a B complex.

I asked some questions about oral treatment of B12 in this post:

healthunlocked.com/pasoc/po...

On the specific question of my raising from 201 to 311 as a result of taking 100x100mcg tablets, PAS_ADMIN made this observation: "If your B12 has only gone up that little in 6 months you either take to little or should take another supplement I think, or you have very clear absorption problems (any cause) which should be looked at as well (esp as then there might also be other deficiencies/if there are other)".

One GP I spoke to said that he thought that the human body can only absorb 10mcg of B12 a day. I've no idea whether this is true or not.

In my case, it's very likely I have gut absorption problems. I've historic low levels of Vit D for which I take high dosage prescription D3. An NHS Dietitian investigating why I have chronic diarrhoea was the person who requested the B12 test. It's too much of coincidence that I have chronic diarrhoea and low levels of both Vit D and B12.

[Katanaqui in reply to mickeymouse42]

Thanks so much, I read your whole thread and it was interesting. I hope you figure out what's going on too.

If it's any help, I spent the afternoon digging through papers, and I think cyanocobalamin tablets should show on a holoTC (active B12) blood test 24hrs after taking them (possibly less, ~6hrs).

academic.oup.com/clinchem/a...

sciencedirect.com/science/a...

[Katanaqui in reply to mickeymouse42]

I'm also not sure if 10mcg per day is accurate without looking it up, but since we do only absorb a small percentage of B12 tablet doses we take (even healthy patients), limited/further limited by how much Intrinsic Factor we have available in a day, I can believe that we have an upper limit of 10mcg