Do vitamins cause cancer?

Among believers in alternative health, it is an article of faith that vitamin supplements prevent all manner of serious diseases including cancer. Yet the reality is almost exactly the opposite. According to Kristal and Lippman, writing in the Journal of the National Cancer Institute (Nutritional Prevention of Cancer: New Directions for an Increasingly Complex Challenge):

The prospects for cancer prevention through micronutrient supplementation have never looked worse. Several large, randomized cancer prevention trials have recently reported no reduced risk from micronutrient supplementation, and [there is] a growing body of evidence that micronutrient supplementation may be harmful…

The authors are commenting on a paper that appears in the same issue of JNCI that investigated whether folate supplementation decreased the risk of precancerous growths in the colon. Folate did not decrease the risk; it increased it by 67%. Further analysis revealed that folate supplementation increased the risk of prostate cancer by 167%.

The authors note:

… Among studies addressing micronutrient supplementation for the prevention of cancer, only a single randomized trial, testing 1200 mg of calcium for preventing the recurrence of colorectal polyps, has reported a statistically significant and positive result for its primary cancer outcome, whereas large trials testing supplementation with multivitamins, folate, selenium, β-carotene, and vitamins E, C, D, B 6 , and B 12 have found no benefits.

In fact:

… Even clinical trials designed to test agents that were found to reduce cancer risk in secondary analyses of previous trials, such as vitamin E and selenium for prevention of prostate cancer, have failed to find benefit from supplementation. The harmful effects of β-carotene supplementation in heavy smokers are well established, and it now appears that folate supplementation may increase cancer risk as well…

The scientific rationale for testing vitamin supplements for cancer prevention was sound. A variety of studies have shown that people who don’t have cancer have higher levels of certain micronutrients. Unfortunately, the assumption that these vitamins and minerals prevent cancer was unjustified.

… the notion that some is good and therefore more is better has been proven wrong; it is more likely that for any given micronutrient, there is an optimal range of intake.

That’s not surprising, considering how vitamins and minerals function within the body. Micronutrients are like nails in a house. Without enough nails to hold the various parts together, a house will fall apart. However, once the optimal number of nails has been reached, adding more will not increase the stability of the house and in large amounts, might even decrease stability.

That explains why vitamin supplements fail to prevent cancer. How would supplements act to cause cancer?

… using the folate supplementation trial as an example, it is not unreasonable to assume that optimal levels of folate are associated with more fidelity in DNA replication and thus a lower risk of spontaneous mutations, but high folate may also support more rapid cell growth and promote carcinogenesis in previously initiated cells.

Another possibility is that large quantities of specific vitamins or minerals may be consumed by particular types of cancer. In that case, low levels of that vitamin or mineral in cancer patients reflect the fact that the cancer needs the micronutrient. The level has dropped not because high levels of the vitamin or mineral prevent cancer, but because the cancer has used up what is available. Far from preventing cancer, supplements might actually feed the cancer and promote rapid growth.

Whatever the reason, it is clear that supplements do not represent the next frontier in cancer prevention. As the authors acknowledge:

… It is safe to conclude that cancer prevention is not going to be as simple as recommending high-dose micronutrient supplements for middle-aged and older adults.

In fact, the opposite may turn out to be true. Vitamin and mineral supplements may promote cancer growth.