Vitamin D - How Strong is the Evidence?

There's been quite a bit written about vitamin D lately, especially its association with heart disease, diabetes, Alzheimer disease, and even cancer.  I have to admit that this is all quite a shock when I recall my post-grad days working in a biochem lab under two of the pre-eminent vitamin D researchers in the world.  Of course, youth is wasted on the young, thus I didn't realize the amazing opportunity I had been given.  So what have we learned in the ensuing quarter century?

The proof behind vitamin D and bone strength is good.  It's strong.  We have demonstrated cause & effect.  Low vitamin D leads to weak bones and replacement of vitamin D makes bones strong again.  This is the level of proof that is required on a public health basis before we can tell everyone to start doing something.

On the other hand, the studies linking vitamin D insufficiency to just about every other human condition is just that, a link, an association.  That's why the Institutes of Medicine didn't make a stronger recommendation last November when they had a chance.  A new commentary was just published in JAMA reviewing the studies linking vitamin D to prevention of both cardiovascular disease and diabetes.  As I've tried to point out time & again, association does not prove causation.  All the studies supporting a link or an association are observational in nature.  None are gold-standard, randomized, double-blind, placebo-controlled trials that have been repeated & shown able to withstand intense scrutiny.

The authors note that we are at least 5-6 years away from demonstrating cause & effect.  In the meantime, what do we do?  We can tell our patients to stick w/just 400-800 units daily up to 1,000 units maximum.  That's what evidence-based medicine would tell us.  Or we can suggest taking more to achieve higher levels while monitoring closely to minimize the risk of hypercalcemia & kidney stones in hopes that we'll be proven correct 5-6 years from now.  Whichever way we decide to roll the dice, we need to beware of the possibility of another medical black swan, something akin to the Women's Health Initiative that rocked our world just shy of 9 years ago.