Turmeric and curcumin supplements: what has actually been tested

NCI describes it plainly: curcumin comes from the underground stem of Curcuma longa, an East Indian plant. It is the substance found in the spice we call turmeric.

That is the yellow in your curry powder, made into a concentrated capsule and sold with claims ranging from reasonable to wild.

This is not some fringe substance scientists ignored. Lab and animal work came first, and human trials followed. Researchers looked at curcumin for preventing cancer, treating it, easing side effects, and improving quality of life. The cancers studied include colorectal, oral, and liver cancers.

NCI's summary of all that work is blunt: there is not enough evidence to know whether curcumin products can prevent or treat cancer.

One trial in oral leukoplakia — white patches in the mouth that can turn into cancer — found that patients taking curcumin improved compared with placebo. That benefit leveled off after six months, though. Studies combining curcumin with cancer treatments gave mixed results. A topical turmeric cream was reported to reduce skin damage from radiation.

These are interesting findings. None of them is settled.

NCI states it plainly: the FDA has not approved curcumin as a treatment for cancer or any other medical condition.

That is the gap between a substance being studied and a substance being a proven treatment. Curcumin sits firmly in the first group. Anyone selling it to you as the second is going further than the evidence, or the regulator, supports.

Being studied and being proven are two different things. A lot of supplement marketing lives in that gap.

Even if the early findings made you want to try it, there is a practical snag. NCI notes that the amount of curcumin in products labelled as curcumin can vary. Two bottles on the same shelf may not give you the same thing.

Supplements in the US do not go through the same premarket approval that medicines do. There is no guarantee of consistency between batches or brands.

For ordinary products, the picture is mild. NCI says few side effects have been reported from curcumin products, and that digestive trouble is the most common complaint. In one study that combined curcumin with anticancer drugs, about a third of participants quit because of bloating.

But there is one caution worth taking seriously. It comes from the National Center for Complementary and Integrative Health. It says regular turmeric or curcumin, not altered to boost absorption, is likely safe in normal amounts for up to two or three months. But some people have reported liver damage after taking the enhanced-absorption versions.

The symptoms to watch for, according to NCCIH, are fatigue, nausea, poor appetite, dark urine, and yellowing skin or eyes. NCCIH's instruction is to stop taking the turmeric or curcumin product immediately and talk to your health care provider. Do not wait to see whether it passes.

NCCIH also states that turmeric supplements may not be safe to take during pregnancy.

Tell your oncology team. This is the single most useful thing you can do, and it is the step people skip most often, usually because they expect to get a lecture.

You will not. Your team needs the full list because supplements can interact with cancer drugs, and because a symptom like fatigue means something different if a liver-stressing supplement is part of the mix.

None of this means you should throw out the spice jar. These concerns apply to concentrated supplements taken every day, not to seasoning your food. Keep cooking with it if you enjoy it.

Here is a rule you can carry to every supplement question you meet: if something worked as claimed, it would be in trials, and someone would already be racing to get it approved.