Turmeric (Curcumin)

Curcumin is the main curcuminoid of turmeric and gives the spice its characteristic yellow colour. Chemically it is a diarylheptanoid polyphenol, two aromatic phenolic rings joined by a seven-carbon chain bearing an alpha,beta-unsaturated diketone; it undergoes keto-enol tautomerism, favouring the enol form in organic solvents and the keto form in water. Turmeric itself is the ground rhizome of Curcuma longa, a plant of the ginger family, and curcumin makes up only a few percent of it.

Turmeric has been used for centuries as a culinary spice, a textile dye and a component of traditional medicine, and curcumin was first isolated in the early nineteenth century and characterised chemically in the early twentieth. Today curcumin serves as a food colouring, designated E100 in the European Union, as well as a flavouring and a cosmetic ingredient, and it is sold in concentrated form as a dietary supplement.

A defining feature of curcumin is its poor drug-like behaviour. Little of an oral dose reaches the bloodstream because it is poorly absorbed, rapidly metabolised and quickly eliminated, and much is excreted unchanged; it is also chemically unstable and readily degrades ^[2]. Numerous strategies have been tried to raise its exposure, including combining it with piperine from black pepper to slow its breakdown, and formulating it as nanoparticles, liposomes or phospholipid complexes ^[2].

Curcumin has been the subject of a very large research effort and well over a hundred clinical trials, yet the results have been disappointing. In medicinal chemistry it is classified as a pan-assay interference compound and an invalid metabolic panacea, meaning it tends to produce false positive signals across many assays; reviewers conclude that it is an unstable, reactive and non-bioavailable molecule that is a highly improbable drug lead, and that no well-designed, double-blind, placebo-controlled trial has shown it to work ^[1]. Commentators have used curcumin as a cautionary example of how an apparently promising natural product can mislead ^[3].

As a food additive, curcumin is generally recognised as safe by United States regulators, and turmeric is a familiar dietary ingredient. It is not approved as a drug, and regulators have acted against unsupported disease-prevention claims for curcumin products. While usually well tolerated, concentrated turmeric and curcumin supplements have been linked to a growing number of cases of liver injury, in some instances serious, an effect associated with certain genetic backgrounds and sometimes with added piperine ^[4].

Curcumin is well tolerated, with occasional digestive upset or, rarely, a mild headache. It can have mild blood-thinning effects and may interact with anticoagulants, so caution before surgery is wise. High doses or certain formulations have rarely been linked to liver issues, and piperine can alter how other drugs are metabolized.