Vitamin C (not specified).

Non-heme iron has to be in the ferrous form to cross the enterocyte via DMT1, and rising pH down the small intestine pushes it toward the insoluble ferric state. Ascorbate keeps it reduced and soluble. It also partly overrides the inhibition from phytate, polyphenols and tannins in the same meal. This is one of the most reliably documented nutrient interactions there is.

When alpha-tocopherol quenches a lipid radical it becomes a radical itself, sitting in the membrane. Water-soluble ascorbate meets it at the interface and hands over an electron, regenerating the tocopherol. This is why the two are described as a redox couple rather than as two separate antioxidants. The reaction is well characterised in vitro. How much it constrains status in a normally fed person is less settled.

Oxidised ascorbate is recycled back to the active form partly by glutathione-dependent reductases. The two pools spare each other, which is why depleting one raises demand on the other. That is documented cell biochemistry. It does not follow that oral glutathione raises the intracellular pool, which is a separate and less settled question.

Ascorbate is a reducing agent and copper is redox-active, so the two interact chemically in the gut lumen. Sustained gram-level intakes have been associated with lower measures of copper status. At ordinary supplemental doses this is not usually a practical concern. It becomes one for people taking several grams a day for long periods.

High ascorbate concentrations can degrade cobalamin in vitro, and older work raised concern about co-ingestion. Later work found the effect largely an artefact of the assay conditions rather than a meaningful in-vivo loss. The pairing is still separated in some products as a formulation precaution. Read it as a stability question, not a demonstrated deficiency risk.

Flavonoids donate electrons and become phenoxyl radicals. Ascorbate can reduce them back. The pairing appears in most quercetin products for this reason. Quercetin also inhibits some intestinal transporters, which cuts both ways for other co-ingested compounds. The recycling chemistry is solid. The downstream benefit in people is not established.

Prolyl-4-hydroxylase needs ascorbate to keep its iron centre in the ferrous state. Without hydroxyproline the collagen triple helix is unstable and the molecule is degraded rather than exported. That is why the two are combined. Whether adding ascorbate to a normally fed person's collagen supplement changes anything measurable is a separate question, since the enzyme is not usually ascorbate-limited outside deficiency.

Carnitine is built from lysine and methionine through hydroxylation steps that require ascorbate. When ascorbate runs out those steps stall, and the resulting carnitine shortfall contributes to the fatigue seen in deficiency. The relationship is a cofactor requirement, not a dose-dependent boost. Supplementing above adequacy does not push the pathway faster.

Converting dopamine to norepinephrine requires a copper-dependent hydroxylase that ascorbate keeps in its reduced active state. Ascorbate also supports the enzyme that breaks tyrosine down. Both are established cofactor roles in catecholamine handling. They describe a requirement rather than a stimulant effect.

Zinc is required for thymic function and for hundreds of enzymes. Ascorbate accumulates in leukocytes at high concentration. They act through separate routes on overlapping tissue. The pairing is near-universal in the category and rests on each nutrient's own requirement rather than on a demonstrated combination effect. Correcting a low intake is what does the work.

The reduced form of lipoic acid has a low enough redox potential to regenerate ascorbate, which in turn regenerates tocopherol. That chain is the reason lipoic acid is described as a network antioxidant. The chemistry is characterised. The clinical consequence in well-nourished people is not.

A share of ingested ascorbate is converted to oxalate and excreted in urine, and gram-level doses raise urinary oxalate measurably. Dietary calcium taken with meals binds oxalate in the gut and reduces its absorption, which cuts the other way. Anyone with a history of calcium oxalate stones should know that high-dose ascorbate raises a relevant marker. Marker, not demonstrated stone formation in most people.

Tetrahydrofolate oxidises readily, and ascorbate in the same environment slows that loss. This is why folate stability in foods and formulations tracks with ascorbate content. It is a stability effect on the delivered molecule, not an effect on folate metabolism itself.

Bioflavonoids travel with ascorbate in the fruits it was originally isolated from, and the pairing has persisted in products ever since. Claims that they improve ascorbate absorption are not well supported at ordinary doses, since absorption is transporter-limited rather than solubility-limited. Regard the pairing as convention with a plausible antioxidant rationale.

Glutathione peroxidase requires selenocysteine and consumes glutathione. Ascorbate helps keep the glutathione pool functional. The two nutrients therefore support the same peroxide-handling network from different positions. High-dose ascorbate has also been reported to reduce absorption of selenium in the selenite form specifically, by reducing it to elemental selenium in the gut. Organic selenium forms are not affected the same way.