Randall Parker says that it could be as easy as ABC...D:

60% Cancer Drop From Vitamin D Supplements

As regular readers know, I've been after you for years to raise your body vitamin D levels. If you haven't gotten off your duff yet to do anything about it how about this as something to get you going? A study coming out in June will report a more than halving of the incidence of cancer by taking vitamin D supplements.

The linked article goes on to describe the study which has yielded these astounding results in greater detail, noting that a drop of 60% indicates twice the impact on cancer of smoking. However, Randall urges caution about mega-doing on vitamin D:

I would suggest refraining from doses above 2000 IU, at least for now. Vitamin D research has become such a hot topic that we should expect more clarification on the risks and benefits of higher doses. But my guess from what I've read so far is that a 2000 IU dose daily is enough to provide the vast majority of the benefit.

Of course, the other way to get vitamin D is through exposure to the sun, but then that raises the risk of cancer. So we have a lot to learn. Still, this is potentially a huge development. And I don't think it's out of line at this stage to suggest that we should all at least be getting our RDA of vitamin D.