Vitamin D deficiency?

Slashdot posted a link to the following:
http://www.theglobeandmail.com/servlet/story/RTGAM.20070428.wxvitamin28/BNStory/specialScienceandHealth/home
where researchers believe a significant source of cancer is a Vitamin D deficiency. There is a study that points in that direction. Now we all know (at least here) that nothing is proven by one study, but it sounds interesting.
So, what is the straight dope on Vitamin D and cancer?

By and large, the evidence is still circumstantial.

I think the idea of vitamin D as being involved in protecting against cancer arose from the observation that sunlight exposure is inversely proportional to the rates of many cancers (the Globe & Mail article is not new news. Rather, it’s reporting a more in-depth study of this phenomenon). Since sunlight exposure is a surrogate marker for vitamin D levels (i.e. sunlight exposure leads to vitamin D production by the body), the notion that low levels of vitamin D leads to higher rates of cancer was born.

Since then, a number of other lines of evidence have emerged to strengthen the link between low vitamin D and high rates of cancer. To wit:

  1. there is an inverse associations between vitamin D and calcium intake and various cancers

  2. levels of vitamin D metabolites (breakdown products) are inversely associated with cancer risk (in some studies)

  3. as noted, high sunlight exposure, presumably reflecting vitamin D synthesis by the body (in the skin), has been associated with a reduced risk of cancer

  4. in breast cancer specifically, vitamin D and calcium intakes have been inversely related to breast density, a step along the path to breast cancer

  5. again, in beast cancer specifically, calcium levels (a marker for vitamin D levels) are associated with a reduced risk of of precancerous states of breast tissue

In terms of mechanisms, it’s known that vitamin D functions as a potent inhibitor of normal and cancer cellular proliferation. Indeed, vitamin D deficiency in mice led to a 60% increase in colon tumor growth, compared to vitamin D-sufficient mice. Not too surprisingly perhaps, in this light, vitamin D is also a key regulator of apoptosis (with apoptosis referring to the deliberate “suicide” of cells, especially those whose presence can lead to cancer (abnormal apoptosis is felt to be an important component in the steps leading to cancer).

Wowser! My lifelong addiction to milk may be a Very Good Thing! Yay!