A recent editorial in the Journal of the National Cancer Institute (JNCI) addresses the issue of whether micronutrients, such as vitamins, have a clear role in cancer prevention. It is fairly common in broad epidemiological survey studies to look for associations between levels of risk of a particular disease and lifestyle variables. Casting such a broad net often snags a number of associations, some of which may later prove causal and important, many of which will turn out to be spurious or at least complex and not direct cases of the lifestyle variable being responsible for the greater or lesser risk of the disease observed.
The vitamin content of the diet is a common variable found to be associated with lower risk of some diseases, including cancer, and this tends to suggest that vitamin supplementation may be protective. However, the logic behind that assumption is quite shaky. For one thing, there is a huge difference between eating foods rich in folate or Vitamin C or omega 3 fatty acids or whatever, and taking these as supplements. Foods are complex mixture of nutrient and non-nutrient substances, and while in some cases individual nutrients are clearly critically important for health (such as Vitamin C for preventing scurvy), many times the overall pattern of the diet may be more important than any individual component.
But because of the historical success of vitamin supplementation eliminating a few common diseases that were due to vitamin deficiencies, we tend to have a warm and fuzzy feeling about vitamins. And, of course, in America if some is good more is always better! Except when it isn’t.
The editorial in JNCI points to many studies in which vitamin supplementation proved to be useless for cancer prevention, and even a few in which vitamin supplementation appeared to increase cancer risk. For example, beta carotene and Vitamin E have been shown to increase the rate of lung cancer, and overall mortality, in smokers. Diets high in folic acid have been associate with a lower rate of colorectal cancers, but some experimental trials on folic acid supplementation shown no benefit and even suggest they may increase the rate of this disease, and also prostate cancer.
Such research evidence reminds us that as much as we desires simple stories, and clear good guys and bad guys, health and disease are complex. Vitamins can be beneficial, useless, or harmful like any other potential preventative or treatment, and we must rely on solid evidence rather than assume the benefit or safety of such substances, whether “natural” or not.