If you think about supplements and cancer, you probably assume that the former can help prevent the latter. I think most people believe that pills containing antioxidants (which include vitamin A, vitamin C, vitamin E, and selenium) can protect them from the dreaded disease. And it probably would never occur to you that popping vitamins could do you serious harm.
But an eye-opening recent commentary in the Journal of the National Cancer Institute details the research on vitamin supplements and makes it clear that there is absolutely no proof that taking vitamins, including antioxidants, prevents cancer, but there is evidence that some supplements may play a role in causing cancer.
The idea that antioxidants prevent cancer was born out of the observation that diets high in fruits and vegetables are associated with lower risk of cancer, and out of lab studies showing that antioxidants prevent growth of abnormal tissue. But while these substances likely play a role in cancer prevention when consumed as food, there is no evidence that taking them in pill form prevents cancer. What's worse, for those of us who pop vitamin supplements, there is a fair amount of evidence that these pills can do the body harm.
The JNCI article cites findings linking antioxidant supplements with increased risk of death for people with gastrointestinal cancer, calcium supplements with heart attacks, beta-carotene (vitamin A) pills with increased risk of death from all causes, selenium supplementation with type 2 diabetes, long-term folic acid supplementation with colorectal, prostate and breast cancers, calcium with prostate cancer, and vitamin D with prostate cancer.
Now, there's no need to freak out if you've been taking any of these supplements, or multivitamins that contain them. The article emphasizes that it's difficult to prove a causal effect of supplements on health outcomes, and much is still unknown.
However, it is safe to say that you shouldn't pop vitamin supplements under the assumption that they'll prevent cancer, and you definitely shouldn't take megadoses (meaning amounts far above 100% of the RDA) because there's evidence that they can do harm.
Now, this advice contradicts that of most of the experts I interview for HGS, such as Never Fear Cancer Again author Raymond Francis, who say that it's very difficult to get all of the nutrients we need from food, and that supplements are essential to health. Who to believe—the supplement-promoting "experts" or the scientific evidence touted by the cancer journal? I wish I knew!
Personally, I don't take multivitamins (I stopped after reporting on studies like the one linking multivitamins with breast cancer). I think it makes more sense to focus on eating a balanced diet and only take supplements when you suspect your diet is failing you. For instance, I do take vitamin B12, because it's only found in animal products and I don't eat many of those (and because this recent interview I wrote about for HGS convinced me of B12's importance).
All I do know is that you should do as much research as possible, and take into account what your body probably needs, before taking supplements, and remember that even though these pills don't require a prescription, they may still have potent effects.
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