Spermidine.

Ornithine decarboxylase converts ornithine to putrescine, and putrescine is the backbone that spermidine synthase extends into spermidine. Ornithine supply sits one enzymatic step upstream of the polyamine itself.

Decarboxylated S-adenosylmethionine donates the aminopropyl group that spermidine synthase attaches to putrescine. Polyamine synthesis therefore draws on the same methionine cycle that SAM-e feeds.

Spermidine inhibits acetyltransferase activity while resveratrol favours the deacetylase side through SIRT1, and both shift the same acetylation balance that governs autophagy induction. The entry points differ, and the shared readout has so far been described in laboratory models rather than human trials.

Arginine is converted by arginase to ornithine, which ornithine decarboxylase then turns into putrescine and onward to spermidine. Raising the arginine pool feeds the same pathway spermidine sits at the end of.

Citrulline escapes first pass metabolism and is converted to arginine in the kidney, lifting systemic arginine more reliably than oral arginine does. That arginine is the substrate for the ornithine to putrescine to spermidine sequence.

Ornithine decarboxylase, the rate-limiting enzyme that makes putrescine from ornithine, is pyridoxal-5-phosphate dependent. Without adequate B6 the step upstream of spermidine runs slowly.

Spermidine is built by transferring an aminopropyl group from decarboxylated S-adenosylmethionine onto putrescine. Betaine remethylates homocysteine back to methionine, keeping the SAM-e pool that supplies those groups topped up.

5-methyltetrahydrofolate donates the methyl group that regenerates methionine from homocysteine, and methionine becomes the SAM-e used in aminopropyl transfer. Folate status therefore sets part of the ceiling on polyamine synthesis.

Methionine synthase needs B12 to move the folate methyl group onto homocysteine. A shortfall stalls SAM-e regeneration and with it the aminopropyl donor step of spermidine synthesis.

Lysine and arginine share the y+ cationic amino acid transporter, so a large lysine dose lowers arginine uptake at the same time. That reduces the ornithine supply feeding polyamine synthesis, which is worth spacing apart.

Ornithine can go down the decarboxylase route toward polyamines or through ornithine aminotransferase toward glutamate semialdehyde and proline. The two therefore draw on the same node in different directions.

Polyamine oxidase and spermine oxidase release hydrogen peroxide as they catabolise spermidine and spermine. Glutathione peroxidase clears that peroxide, so glutathione status shapes how cleanly polyamine turnover runs.

Spermidine lowers the activity of the EP300 acetyltransferase, shifting the cell toward deacetylated, autophagy-permissive states. Stilbenes such as pterostilbene push on the sirtuin side of the same acetylation balance.

Sirtuin deacetylases need NAD+, which NMN supplies, and spermidine reduces the opposing acetyltransferase activity. The two reach the same autophagy and mitochondrial turnover setpoint from opposite sides of the acetylation balance.

The aminopropyl groups used to build spermidine come from decarboxylated S-adenosylmethionine, which is made from methionine and ATP. Methionine supply therefore feeds polyamine synthesis through the same node that feeds methylation. No combination trial is needed to state the connection; it is standard pathway chemistry.

Choline is oxidised to betaine, which remethylates homocysteine back to methionine and so helps maintain the S-adenosylmethionine pool that donates aminopropyl groups to polyamine synthesis. The link is indirect but well described. It supports normal methylation and polyamine turnover rather than acting on spermidine directly.

Diamine oxidase and the other copper-containing amine oxidases that degrade dietary polyamines require copper at the active site. Copper status therefore influences how quickly ingested spermidine is broken down in the gut wall and plasma. The direction of the net effect in people has not been measured; the cofactor requirement itself is settled.

Riboflavin is the precursor of FAD, the flavin cofactor used by the polyamine oxidases that run the back-conversion arm of the pathway. Without adequate flavin these steps slow. This is a cofactor relationship, not a demonstrated outcome in supplemented people.

Acetylated polyamine oxidation generates hydrogen peroxide as a stoichiometric byproduct. N-acetylcysteine supplies cysteine for glutathione synthesis, and glutathione peroxidase is the main route for clearing that peroxide. The pairing is mechanistic; no trial has measured the two together.

A large share of the polyamine pool reaching the gut wall is bacterial in origin, and fermentable substrates such as inulin shift which organisms dominate the colon. That makes fibre intake a plausible modifier of luminal spermidine supply. The relationship is inferred from microbial physiology rather than measured in a combination trial.

Several bifidobacteria produce putrescine and spermidine from arginine-related substrates and release them into the lumen. Adding such organisms alongside spermidine addresses the same pool from two directions, one dietary and one microbial. This is an association drawn from microbial metabolism, not a measured human outcome.

Galactooligosaccharides enrich bifidobacteria, which are among the colonic polyamine producers. The chain from substrate to bacterial polyamine output to host exposure has each link described separately, but not end to end in supplemented people. Read it as a plausible route rather than a demonstrated one.

Spermidine is associated with autophagy-related signalling while nicotinamide riboside raises NAD availability for sirtuin and redox chemistry. The two act through different entry points, so co-formulation is a reasonable design choice rather than a demonstrated interaction. No study has measured the pair together in people.