One of the greatest triumphs of marketing over evidence was the incredible rise of vitamin supplement use in the 20th century. Supplement makers successfully created a “health halo” around vitamins, and taking your vitamins became a virtue, something mothers told their children to do. The evidence, however, does not tell such a simple story.
In recent years it has become increasingly apparent that there are unintended consequences to taking vitamin supplements, and in fact there may be a net negative health effect. This is especially true for those who are healthy and don’t need vitamins, and for those who exceed the recommend dosages.
A recent review of the last 20 years of literature on the subject, presented at the American Association for Cancer Research 2015 meeting, found an overall increased risk of cancer among vitamin users. Dr. Tim Byers presented the study, which echoes the result of a 2012 review that he and others published. He specifically refers to two famous studies showing an increased risk of cancer from vitamins.
The 2011 SELECT trial found an overall increased risk of prostate cancer among men taking vitamin E.
It has also been shown, first in a 1994 NEJM study, that high doses of beta carotene increase the risk of lung cancer in smokers and those exposed to asbestos.
In fact, antioxidants in general lack evidence for an overall health benefit and in high doses may increase cancer and overall mortality.
Folic acid and cancer risk is a more complex topic. There is evidence that taking folate reduces the risk of breast and possibly colon cancer. There is a “U” shaped relationship between dose and cancer risk, however:
Women with daily dietary folate intake between 153 and 400?µg showed a significant reduced breast cancer risk compared with those <153?µg, but not for those >400?µg.
There is preliminary evidence that higher doses of folate may increase cancer risk.
There are several possible mechanisms by which excess vitamin intake may promote cancer. Cancer cells are metabolically very active. Taking more vitamins than is necessary for healthy cells may only serve to feed cancerous cells, promoting their growth. Elevated levels of vitamins, exceeding what the body need for normal metabolism, may also result in the creation of alternate downstream metabolites that have negative health consequences. Further, taking megadoses of a vitamin may alter the body’s natural homeostasis with negative consequence. This is likely true with antioxidants – oxidative compounds are used by the immune system and act as signals for the creation of protective proteins. There is a balance between oxidants and anti-oxidants, and shifting that balance is not beneficial.
There are also possible unintended consequences. The evidence is fairly clear that having a healthful and well-rounded diet has health benefits. Taking vitamins does not replace having a good diet. However, taking vitamins may lead to a false sense of security, justifying a less healthful diet with net negative health consequences.
Full: https://www.sciencebasedmedicine.org/vitamins-and-cancer-risk/