Saturday, August 31, 2013

The Geography of Cancer

Yesterday I wrote about the geography of colorectal cancer (CRC) and showed a map of CRC mortality in the U.S. The striking thing about the map was (is) that CRC is much more of a problem in the northern states than in the southern states. It turns out this is not an anomaly and in no way limited to the U.S. Colorectal cancer tracks latitude, worldwide. The further you live from the equator, the greater your chances of dying of colorectal cancer.

The above graph comes from a 2005 paper by Mohr et al. (no longer online; but see the 2013 paper by Cuomo, Mohr, et al.) that correlates cloud cover and distance from the equator with cancer rates in 175 countries. It shows quite clearly that colorectal cancer incidence varies with latitude. The countries with the lowest CRC rates are near zero degrees latitude (the equator).

This effect doesn't just apply to colorectal cancer, though. It also applies to breast cancer:

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. It turns out the common denominator is vitamin D.

I'll spare you the book-length treatment. Suffice it for now to say: More than 2,500 research studies have been published in biomedical journals investigating the inverse association between vitamin D, its metabolites, and cancer, including almost 300 epidemiological studies. For a good overview, I recommend the review article by Garland et al. (2009). 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). My advice is: Read the literature and decide for yourself. 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.