Vitamin B9 (methyltetrahydrofolate).

Methionine synthase transfers the methyl group from 5-methyltetrahydrofolate to homocysteine, and the enzyme cannot run without cobalamin as its cofactor. When B12 status is low, folate accumulates in the methylated form and cannot re-enter the folate cycle. Giving methylfolate on its own can normalise blood folate while the underlying B12 gap stays hidden. The two are almost always formulated together for this reason.

MTHFR is a flavoprotein and holds FAD, which riboflavin supplies. People carrying the 677 T variant produce an enzyme that loses its FAD more readily, so riboflavin status affects how much methylfolate the body makes for itself. Supplying methylfolate directly bypasses that step, but riboflavin still supports the rest of the flavin-dependent one-carbon machinery.

Folate handles the remethylation arm of homocysteine metabolism. B6, as pyridoxal-5-phosphate, runs the transsulfuration arm through cystathionine beta-synthase and cystathionine gamma-lyase. The two arms clear the same substrate by different exits, which is why homocysteine work almost always pairs them. This is metabolic reasoning, not a claim about any endpoint.

Betaine donates a methyl group to homocysteine through a folate-independent enzyme found mainly in liver and kidney. That gives the body a second remethylation route running in parallel with the folate route. The pairing matters most when folate-dependent flux is limited. Neither route substitutes for the other in every tissue, since the betaine enzyme has restricted tissue distribution.

Choline is oxidised to betaine, which feeds the same folate-independent methyl donation step. Folate and choline requirements move against each other: when one is short the body draws harder on the other. Formulators pairing them are covering the whole methyl pool rather than one branch of it.

Methionine synthase carries a catalytic zinc that binds and activates the homocysteine thiol before the methyl group is handed over. Betaine-homocysteine methyltransferase is also a zinc enzyme. Adequate zinc is therefore part of the machinery that uses methylfolate, not an add-on to it.

Reduced folates including 5-methyltetrahydrofolate oxidise readily in solution and in the stomach. Ascorbate holds them in the reduced state, which is why food-folate analysis and some liquid formulations include it deliberately. The effect is on molecular stability rather than on any physiological endpoint.

Folic acid and 5-methyltetrahydrofolate use the same proton-coupled folate transporter and reduced folate carrier at the gut wall, so large folic acid doses compete for uptake. Folic acid also has to pass through dihydrofolate reductase, a step with limited capacity in humans, which is where unmetabolised folic acid in plasma comes from. Stacking both forms is duplication with a competitive edge rather than an addition.

Green tea catechins inhibit dihydrofolate reductase in cell-free and cell systems at concentrations above what ordinary tea drinking produces. Because methylfolate skips that enzyme, the interaction matters less for this form than for folic acid. Read it as mechanistic rather than clinical, and it has not been shown to change folate status in people.

The glycine cleavage system strips a carbon from glycine and loads it onto tetrahydrofolate, which is one of the main ways the one-carbon pool is refilled. Serine hydroxymethyltransferase runs the same chemistry in the other direction. Glycine is therefore a substrate feeding the cycle that methylfolate exits.

Folate supplies the thymidylate needed for DNA synthesis in dividing marrow cells while iron supplies haem. Low status in either shows up in red cell indices, and correcting only one can leave the other limiting. This is a shared-outcome pairing, not a direct chemical interaction between the two molecules.