One of the most striking findings regarding cancer incidence is that for many forms of the disease, cancer incidence is higher the further you live from the equator. Also, studies have found that the most serious cases of cancer are diagnosed in the winter.
This is true, for example, of colorectal cancer.
The trend doesn’t just apply to colorectal cancer. It also applies to breast cancer. (See graphic below.)
Breast cancer and colorectal cancer are distinctly different cancers, so in order for these graphs to be as similar as they are, there must be a common denominator of extremely broad applicability underlying the latitude trend. And there is. It turns out the common denominator is vitamin D.
More than 2,500 research studies have been published in biomedical journals investigating the inverse association between vitamin D (and its metabolites) and cancer, including almost 300 epidemiological studies. For a good overview, I recommend the review article by Garland et al. (2009). As you review the literature, you might notice (as I did) a certain amount of hesitancy on the part of big-name researchers to come right out and pronounce vitamin D a bonafide cancer-preventive agent, due to the relative dearth of prospective (intervention-based) randomized controlled trials. (One intervention study worth reading is the 2007 trial by Lappe et al. in Am J Clin Nutr.) After the CARET disaster, no one wants to get caught recommending a vitamin regimen based on epidemiological happy-talk, and I can understand that.
Nevertheless, I think the weight of the evidence in favor of vitamin D, at this point, is substantial enough (and any down side negligible enough) that people should start thinking about taking substantial amounts of vitamin D as prophylaxis against cancers of all kinds (not just CRC and breast). If you can get adequate sun exposure, your body will make ample vitamin D on its own. (As a rough guide, 20 minutes of exposure over 40% of your body gives the equivalent of 10,000 IU, although this can vary considerably depending on skin color.) Absorption of supplemental vitamin D varies. Your doctor can suggest guidelines applicable to your body type.
My advice is: Read the literature and decide for yourself whether or not to start a vitamin D regimen. Don’t wait for FDA, CDC, the National Cancer Institute, or anyone else to give you the green light on this one. They’ve got their own agendas to worry about.
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