Every now and then some vitamin or dietary supplement becomes all the rage.  A couple of generations ago vitamin C was the miracle drug that could prevent all diseases, despite lots of evidence to the contrary.  Lots of my patients still take it for colds, demonstrating its persistent mythology.  Vitamin B12 became the wonder-drug a few decades ago, leading to a whole generation of patients getting monthly injections for reasons that remain scientifically mysterious.  And many lesser stars can be added to this grab bag, including glucosamine, folic acid, zinc and selenium, all of whom were taken for all sorts of putative benefits which were later debunked.

Now it’s vitamin D’s turn in the spotlight.  Vitamin D has been receiving increasing attention in the last few years as its role in bone health has been better understood and as more people are found who are deficient in vitamin D.  Unfortunately, with more understanding comes more hoopla, and there are now shaky claims that vitamin D helps prevent myriad diseases.  This week, the Institute of Medicine (IOM), the government body that decides how much of each nutrient we need, revisited their recommendations for vitamin D.  (See the link below for their report.)  This received much (frequently confusing) media attention and a whole bunch of my patients (thanks!) emailed me links to various articles about this story.

Let’s parse this issue and separate what is known from what is speculated.

Benefits of vitamin D

Vitamin D has only two proven benefits.  The first is that vitamin D is essential to maintain strong bones.  Another way to say this is that vitamin D deficiency eventually predisposes to osteoporosis.  The only other benefit of vitamin D which has been proven in randomized trials is that vitamin D supplementation in men and women over 60 helps decrease the frequency of falls.  How does it do that?  Does it help balance?  Does it help muscle strength?  Does it turn off gravity?  No one knows, but I’m pretty sure it doesn’t affect gravity.

Many other supossed benefits of vitamin D are frequently mentioned, from prevention of heart disease to prevention of cancer.  None of these claims are supported by a shred of evidence from randomized trials, so for now we should assume that they are false.  Anecdotally, many patients (including my patients) report a dramatic reduction in aches and pains and improvement in energy after starting vitamin D supplements.

How much vitamin D is enough?

This is where the IOM made waves.  They increased the daily recommended amount of vitamin D from 200 IU to 600 IU.  But many doctors (including me) have already been recommending daily supplementation of 1,000 or 2,000 IU.  So why is the IOM taking such baby steps?  Or, conversely, why are doctors prescribing so much more than the IOM?  The heart of the dispute is a disagreement about what vitamin D levels are normal.  The IOM defines a vitamin D blood level of over 20 nanograms per milliliter (ng/ml) as normal.  By that definition most North Americans have normal levels and 600 IU daily is all that most people need to achieve those levels.

But the Endocrine Society and the International Osteoporosis Foundation have concluded that a level of 30 ng/ml is necessary for optimum bone health.   They call levels above 30 ng/ml normal, levels between 20 and 30 as “vitamin D insufficiency”, and levels below 20 “vitamin D deficiency”.  They have some data that supports their conclusion involving hormonal markers of osteoporosis improving until vitamin D levels climb to 30 ng/ml.  By this more strict definition, many more people have low vitamin D levels (as many of my patients know) and to reach this higher level of 30 ng/ml 600 IU is frequently inadequate.

Risks of vitamin D

What happens if you take too much vitamin D?  Again, many of the reported risks are as unfounded as the reported benefits.  There are loose associations with all sorts of possible diseases, but none of these are from randomized trials, so they should be ignored.  The known risks of vitamin D toxicity all relate to causing abnormally high calcium levels and include kidney stones and decreased kidney function.

How much vitamin D is too much?

The simplest way to answer that question is to have your vitamin D level checked.  The risks listed above occur with vitamin D levels above 80 ng/ml.  Levels that high are very hard to reach with 2,000 IU of vitamin D daily.

So the take-home message is that there is no reason to believe that vitamin D will prevent cancer, make you famous, or lower interest rates.  It will keep your bones healthy and decrease your likelihood of falling when you’re older than 60.  The most accurate way to assess whether you are deficient and how much vitamin D you should take is to have your doctor check your vitamin D level.  Daily supplementation with 1,000 to 2,000 IU daily is unlikely to be harmful.

Learn more:

Institute of Medicine report:  Dietary Reference Intakes for Calcium and Vitamin D

Wall Street Journal article:  Triple That Vitamin D Intake, Panel Prescribes

MSNBC article:  How much vitamin D is enough? Report sets new levels

New York Times article:  Report Questions Need for 2 Diet Supplements

The best recent review I’ve seen in the medical literature about vitamin D deficiency is a New England Journal of Medicine article from 2007:  Vitamin D Deficiency

Tangential miscellany: Happy Hanukkah!