WARNING: Some high-dose vitamins can increase cancer risk

Don’t assume mega-doses are harmless merely because they are vitamins.

That bottle of vitamins beside your breakfast may look harmless.

One capsule promises “antioxidant protection.” Another packs several hundred percent of your daily requirement. Add a multivitamin and another supplement recommended online, and you could be consuming doses far beyond what you would normally get from food.

But more vitamins do not necessarily mean more protection. Large clinical trials have found that specific high-dose supplements can increase the incidence of particular cancers in certain groups.

One of the clearest examples is beta-carotene.

The Beta-Carotene and Retinol Efficacy Trial, or CARET, studied 18,314 smokers, former smokers and workers exposed to asbestos. Participants assigned 30 mg of beta-carotene plus 25,000 IU of retinyl palmitate daily developed 28 percent more lung cancers and had 17 percent more deaths than those receiving placebo. The intervention was stopped 21 months early.

Another warning came from vitamin E.

The SELECT trial enrolled more than 35,000 men to determine whether selenium or vitamin E could prevent prostate cancer. Longer follow-up found that men assigned 400 IU of vitamin E daily had a statistically significant 17 percent higher incidence of prostate cancer than men receiving placebo.

Folic acid has also produced cautionary findings. In a randomized trial involving people with a history of colorectal adenomas, 1 mg of folic acid daily did not prevent new adenomas and was associated with more advanced lesions during later follow-up. The evidence does not establish that normal folate intake causes colorectal cancer.

That distinction is crucial.

These findings do not show that vitamins in general cause cancer, nor that everyone taking supplements faces increased cancer risk. The demonstrated risks involve particular nutrients, doses and populations.

The World Cancer Research Fund recommends against using high-dose supplements for cancer prevention and says there is convincing evidence that high-dose beta-carotene supplements increase lung cancer risk in current and former smokers.

Supplements can still be essential for people with diagnosed deficiencies and during circumstances such as pregnancy.

The warning is simple: do not assume mega-doses are harmless simply because they are vitamins.

Check your labels, particularly when taking several products with overlapping ingredients, and consult a doctor before using high-dose supplements long term.

Some high-dose vitamins have not prevented cancer. Under specific conditions, clinical trials found they increased the risk instead.

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