In 2013, I wrote about the burgeoning popularity of the spice turmeric as a medicinal herb. At the time, my conclusions were:
Overall, there is no compelling clinical evidence in humans supporting any use of curcumin or other turmeric compounds…There is virtually no clinical research in companion animals, and what there is does not support claims of benefit from turmeric compounds. Finally, the limited research to date suggests a few potential risks but the significance of these is unclear.
Since then, there have been many additional in vitro or lab animal studies, but no significant clinical trials in companion animal species. The pre-clinical research continues to find interesting biological activity of curcumin and other turmeric compounds which might, or might not, lead to clinically useful effects. At this point, there isn’t much new evidence that supports altering my previous conclusions.
However, one new review has looked at the biologic plausibility of curcumin, which is one factor in assessing the potential medicinal applications. This paper, somewhat surprisingly, suggests that the basic biochemistry of curcumin makes it unlikely to be a clinically useful remedy.
Nelson KM. Dahlin JL. Bisson J. et al. The Essential Medicinal Chemistry of Curcumin J. Med. Chem. 2017;60:1620?1637.
The authors review the pre-clinical and clinical trial literature for curcumin with an eye to features that would make the compound a better or worse candidate medicine. They conclude that its basic biochemical features make it unlikely to be useful but highly likely to generate false positive results if not tested with a clear understanding of its properties:
The likely false activity of curcumin in vitro and in vivo has resulted in >120 clinical trials of curcuminoids against several diseases. No double-blinded, placebo controlled clinical trial of curcumin has been successful. This manuscript reviews the essential medicinal chemistry of curcumin and provides evidence that curcumin is an unstable, reactive, nonbioavailable compound and, therefore, a highly improbable lead.
Curcumin…has shown excellent promise in early testing (in vitro), even though this testing may have been bedeviled by design problems that led to several misfires. The structure of 1 suggests that it might be unstable in a biological setting, and in fact, it is: both its in vitro and in vivo stabilities are abysmal…relative to commercial drugs.
To our knowledge, [curcumin] has never been shown to be conclusively effective in a randomized, placebo-controlled clinical trial for any indication. Curcumin is best typified, therefore, as a missile that continually blows up on the launch pad, never reaching the atmosphere or its intended target(s).
While these failures would normally end further research on its use as a therapeutic, they apparently have not deterred researchers interested in its development.
Given its low systemic bioavailability, we remain highly skeptical that an oral dose of 1 can ever be effective in human clinical trials that are translated from reports of in vitro activity… the lack of any observed efficacy of oral curcuminoids in clinical trials where it was given in high doses does not bode well for these alternative hypotheses of therapeutic efficacy.
Unfortunately, no form of curcumin, or its closely related analogues, appears to possess the properties required for a good drug candidate (chemical stability, high water solubility, potent and selective target activity, high bioavailability, broad tissue distribution, stable metabolism, and low toxicity). The in vitro interference properties of curcumin do, however, offer many traps that can trick unprepared researchers into misinterpreting the results of their investigations.
While such an analysis does not entirely preclude curcumin eventually being a useful remedy, it does reduce the likelihood of this, especially given the failure of any dramatic clinical trial results suggesting a significant real-world benefit.