Curcuminoids

A narrow, almost entirely Zingiberaceae distribution:

Tetrahydrocurcumin, often sold as a supplement, is not really a plant compound at all — it is curcumin's principal human metabolite, colourless and more stable than its parent.

Curcumin is the archetype of a compound that does everything in a test tube and very little in plasma. It is poorly soluble, poorly absorbed, rapidly glucuronidated and sulfated in the gut wall and liver, and quickly excreted — free curcumin is barely detectable after an ordinary oral dose. This is why the market is full of phospholipid complexes, piperine co-administration and nanoparticle formulations, all attacking the same problem from different angles.

There is a second caveat the enthusiasm usually omits. Curcumin is a recognized pan-assay interference compound: it is a metal chelator, a redox cycler, membrane-disruptive and fluorescent, and it produces apparent hits across an implausible range of unrelated screening assays. A large share of the thousands of published curcumin "mechanisms" are artefacts of those properties. This does not mean turmeric does nothing — it means the mechanism literature should be read sceptically and the human trial data weighted far more heavily.

What survives the caveats is a reasonable body of randomized-trial evidence in osteoarthritis, where curcumin preparations show pain and function improvements broadly comparable to NSAIDs in several trials, and reasonable signals in ulcerative colitis maintenance and in metabolic and inflammatory markers.

Where the effect comes from is genuinely unresolved — the conjugated metabolites, tetrahydrocurcumin, direct local action in the gut, and effects on the microbiome are all plausible contributors alongside whatever little parent compound circulates.