Full mechanism, in text

Why it can't supply B12

1 · Real B12 meets receptors

Real B12 carries DMB as its lower axial ligand, its chemical ID, and three carriers move it toward the cell, but they are not equally strict about look-alikes.True vitamin B12's chemical core is cobalamin — a corrinoid ring around a central cobalt (Co) atom, with one axial ligand above and one below:

· Upper axial ligand: methyl (methylcobalamin) / 5'-deoxyadenosyl (adenosylcobalamin) / cyano (cyanocobalamin) / hydroxo (hydroxocobalamin) — determines which reaction B12 works in
· Lower axial ligand: 5,6-dimethylbenzimidazole (DMB) — the chemical ID of 'real B12'

Three carriers move real B12 toward the cell:

1. R-binder / haptocorrin (HC): saliva + stomach, protects B12 through the gastric acid environment
2. Intrinsic factor (IF): secreted by gastric parietal cells; hands B12 to the next stage in the proximal small intestine
3. TCII / transcobalamin II: the last-mile blood transport to inside cells

The three are not equally strict about look-alikes: haptocorrin (HC) binds the various B12 analogs about equally well; intrinsic factor (IF), the one responsible for absorption, is far pickier; and the two enzymes that finally put B12 to work (MTR / MUT) need true B12. Next: where the B12 look-alike in spirulina differs.

2 · Pseudo-B12 with adenine bottom

Pseudo-B12, the main look-alike in spirulina, has adenine where real B12 has DMB, so the body cannot use it, yet the common microbiological assay counts it as B12.The most abundant B12 look-alike in spirulina is pseudovitamin B12 — it shares with real B12:

· The same corrinoid ring
· The same cobalt center
· The same possible upper axial ligands

The one critical difference:

· The lower axial ligand is not DMB, but adenine

The whole molecule differs by that one part, yet the enzymes that actually put B12 to work in the body cannot use it: the Watanabe 2014 review states plainly that pseudo-B12 is biologically inactive in humans.

How the B12 on the label gets counted (Watanabe 1999, commercial spirulina tablets):

· The commonly used microbiological assay counts a whole family of B12 look-alike molecules
· For the same batch of tablets, the microbiological reading was 6-9 times the intrinsic-factor chemiluminescence reading; most of the excess was B12 substitutes and inactive analogs
· The researchers then purified the fraction that binds intrinsic factor: 83% was pseudo-B12 and only 17% was true B12

So:

· The Vitamin B12 on the label is mostly an artifact of how it was counted
· The natural B12 source claim counts mainly look-alikes, not B12 the body can use

3 · The intrinsic-factor gate is only partial

Intrinsic factor only partly filters out pseudo-B12, the enzymes cannot use whatever gets through, and so relying on spirulina alone for B12 is as good as taking none.Along the B12 absorption route there are three binding proteins, and they do not treat look-alikes the same way (the Stupperich 1991 binding study):

· Haptocorrin (HC): binds the various analogs about equally well — it takes pseudo-B12 and true B12 alike
· Intrinsic factor (IF): the one responsible for absorption, with only moderate affinity for pseudo-B12 — it does bind it, just not as tightly as true B12
· Transcobalamin (TCII): the last stretch, carrying B12 from the blood into cells

This gate only partly filters: what Watanabe 1999 purified was the fraction that binds intrinsic factor, and 83% of it was still pseudo-B12.

The link nobody has measured: whether pseudo-B12, once in the gut, occupies the absorption route and crowds out true B12 has not been measured directly in people; the often-repeated competitive occupancy story is a guess, not a measurement.

What is certain is the outcome: even if some pseudo-B12 gets through the gate, the enzymes that use B12 cannot use it (Watanabe 2014). For a vegan relying on spirulina for B12:

· Most of the number on the label is look-alikes counted by the microbiological assay
· Of the fraction that binds intrinsic factor, 83% is still pseudo-B12
· A plant-based diet carries almost no true B12 of its own
· → Little or no B12 actually reaches the enzymes; it is as good as taking none

Next: what happens to MTR / MUT, the two enzymes that genuinely need B12.

4 · The enzymes cannot use it

MTR and MUT need true B12, and when it falls short (Hcy) rises and (MMA) piles up, so read both together with serum B12.Real B12's two core intracellular jobs:

1. MTR (methionine synthase) uses methylcobalamin to convert (Hcy) → methionine → , the universal methyl donor → methylation of DNA / myelin / neurotransmitters (the methylation cycle covered in the vitamin B12 story)
2. MUT (methylmalonyl-CoA mutase) uses adenosylcobalamin to convert methylmalonyl-CoA → succinyl-CoA → into the cycle — clearing breakdown products of odd-chain fatty acids and certain amino acids

Pseudo-B12 is of no use here: both enzymes need true B12, and pseudo-B12 is biologically inactive in humans (Watanabe 2014). When true B12 falls short:

· MTR stalls → plasma Hcy rises
· MUT stalls → (MMA) piles up in blood and urine — a relatively sensitive and relatively specific marker of functional B12 deficiency

How to test:

· Serum B12 measures the total amount of B12 in the blood; on its own it cannot say whether the part your body can use is enough
· When B12 deficiency is suspected, read serum B12 + MMA + Hcy together; a serum B12 that looks normal alongside a raised MMA suggests a functional shortfall, worth tracking down with a doctor
· Hcy is less specific: low folate, low B6 and reduced kidney function also raise it

Clinical consequences:

· Early: fatigue / poor attention / numb hands and feet — atypical, and it can take months to years before anyone notices
· Late: megaloblastic anemia / nerve damage; the longer it goes on, the more likely the nerve damage is not to recover fully
· Take special care: strictly vegan pregnant and breastfeeding mothers — the infant gets its B12 from the mother, and a shortfall harms the developing nervous system

If you or someone close to you is vegan or a long-term vegetarian:

· Do not rely on spirulina for B12
· The reliable sources are a B12 supplement (cyanocobalamin or methylcobalamin) or fortified foods — for example, one 2000 µg tablet once a week, or fortified nutritional yeast and fortified plant milk every day; the full plan and doses are in the vitamin-b12 story
· If you eat a strict vegan diet and develop numb hands or feet or an unsteady walk, see a doctor and ask for B12 and MMA tests

Full mechanism, in text

1 · Real B12 meets receptors

2 · Pseudo-B12 with adenine bottom

3 · The intrinsic-factor gate is only partial

4 · The enzymes cannot use it

The pseudo-B12 trap

Full mechanism, in text

1 · Real B12 meets receptors

2 · Pseudo-B12 with adenine bottom

3 · The intrinsic-factor gate is only partial

4 · The enzymes cannot use it

The pseudo-B12 trap