NADH.

NADH hands its electrons to complex I, and complex I passes them to coenzyme Q10, which ferries them on to complex III. The two sit next to each other on the same electron path, so supplying both supports the handoff at either end.

Complex I uses a flavin mononucleotide group, built from riboflavin, as the site that accepts electrons from NADH. Without adequate riboflavin the flavoprotein entry point is limited no matter how much NADH is present.

After the flavin accepts electrons from NADH they travel through a chain of iron-sulfur clusters inside complex I. Iron status therefore sets how well the NADH entry point can move electrons forward.

Nicotinamide riboside is phosphorylated and adenylylated to build NAD+, the oxidised partner of NADH. One raises the pool from the salvage side while the other supplies the reduced form directly.

NMN is the immediate precursor that NMNAT converts to NAD+. It feeds the same pool that NADH belongs to, one step upstream.

Nicotinamide enters the salvage pathway through NAMPT and becomes NAD+. It is the classic dietary route to the pool that NADH cycles within.

Dihydrolipoamide dehydrogenase uses NAD+ to reoxidise reduced lipoamide, and the same NAD couple regenerates lipoic acid in cells. The two are linked through one redox reaction.

Tyrosine hydroxylase needs tetrahydrobiopterin, and dihydropteridine reductase uses NADH to regenerate BH4 after each turn. One supplies the amino acid substrate while the other supports cofactor recycling at the same step.

Ribose forms the sugar backbone of both halves of the dinucleotide and is the slow step in building nucleotides from scratch. It supplies raw material for the same pool NADH occupies.

Excess nicotinamide is cleared by methylation using SAM, which draws on the body's methyl pool. Betaine restores methyl groups through the BHMT route, which is why it is a standing pairing with NAD-pathway ingredients.

Magnesium complexes ATP and is required by the kinases and the ATP synthase that sit downstream of NADH oxidation. Electron delivery from NADH only produces usable energy if those magnesium-dependent steps run. This is textbook enzymology, not a combination trial.

Cytochrome c oxidase carries two copper centres and is the terminal step that lets electrons entering at complex I reach oxygen. Without adequate copper the chain backs up and NADH cannot be reoxidised efficiently. The dependency is structural.

Thiamine pyrophosphate is the cofactor of pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase, the two complexes that generate much of the mitochondrial NADH. Thiamine sits upstream of the same electron flow that NADH feeds. They are sequential, not interchangeable.

Coenzyme A, built from pantothenic acid, delivers acetyl groups into the citric acid cycle where NAD+ is reduced to NADH. Adequate pantothenate is part of what keeps that cycle supplied. The link is stoichiometric.

Aromatic amino acid decarboxylase, the enzyme that converts L-DOPA and 5-hydroxytryptophan onward, is pyridoxal-5-phosphate dependent, and the hydroxylase step just before it depends on tetrahydrobiopterin recycling that uses reduced pyridine nucleotide. B6 and NADH therefore act at consecutive steps of catecholamine and serotonin synthesis. This is established pathway biochemistry.

Phenylalanine hydroxylase converts phenylalanine to tyrosine using tetrahydrobiopterin, and the oxidised pterin is returned to its active form by dihydropteridine reductase running on NADH. Supplying the amino acid and maintaining the reduced nucleotide pool address different halves of the same reaction. The pairing is mechanistic, not clinical.

Tryptophan hydroxylase is the same class of biopterin-dependent enzyme as the tyrosine and phenylalanine hydroxylases, and its cofactor is regenerated by an NADH-dependent reductase. Amino acid supply and cofactor recycling are complementary constraints on that step. No human combination trial has measured the pair.

Ascorbate helps keep tetrahydrobiopterin in its reduced form and is itself regenerated by a semidehydroascorbate reductase that uses reduced pyridine nucleotide. The two therefore share the same recycling economy around biopterin-dependent hydroxylases. The relationship is enzymatic rather than an outcome.

Alcohol and aldehyde dehydrogenases are zinc enzymes that interconvert NAD+ and NADH during ethanol handling, and the shift in the NAD+ to NADH ratio they cause is the classic example of nucleotide redox constraining metabolism. Zinc status is part of that machinery. Standard biochemistry.

Carnitine shuttles long-chain fatty acids into the mitochondrion, and each beta-oxidation cycle there generates NADH. Fat oxidation is therefore a major producer of the reduced nucleotide rather than a consumer of it. The two act at consecutive points of the same route.

Creatine phosphate buffers ATP over seconds while NADH-driven oxidative phosphorylation replenishes it over minutes, so the two support cellular energy on different timescales. They are frequently combined in energy formulas on that reasoning. No trial has tested them together.

PQQ is a redox-cycling quinone that can be reduced and reoxidised by cellular reductants, which places it in the same electron-handling space as the pyridine nucleotides. Whether that interaction is meaningful at supplement doses in people has not been measured. Read it as chemistry rather than clinical evidence.

Manganese superoxide dismutase clears the superoxide that leaks from the electron transport chain as NADH is oxidised. Adequate manganese therefore supports normal handling of the by-products of that electron flow. The link is a cofactor requirement, not an additive effect.

Alpha-ketoglutarate is the substrate for a dehydrogenase complex that reduces NAD+ to NADH inside the citric acid cycle. Supplying the substrate and supplying the nucleotide touch the same step from opposite sides. The relationship is stoichiometric biochemistry.

Menaquinones are structurally quinones and have been reported in preclinical models to act as electron carriers in mitochondrial membranes, which is the same compartment where NADH gives up its electrons. Human relevance of that role has not been established. This is an early mechanistic observation only.