As you are reading these sections please keep in mind that the science of measurement and definitions of normal vs. optimal levels for these substances is still evolving. Plus there is incomplete standardization of these tests in the US - meaning the reference normal range may be different from one lab to the other. But these few paragraphs will give you a good background with which to interpret new discoveries and opinions as they emerge!
The sunshine vitamin. The emerging science of vitamin D is beginning to paint D as a "master vitamin". In short, the effects of vitamin D go way beyond building and maintaining strong bones and normal calcium levels, but may even protect us against certain cancers and help keep our moods bright.
There is little naturally occurring vitamin D in vegan food (although, I have heard that UV exposed mushrooms which then contain vitamin D will soon be on the market), and only a few rare sources of D in animal foods (fatty fish for example). Get this - our skin makes vitamin D (well the precursor to D which is then activated in the liver and kidneys) in response to UV from the sun. Yay for the sun! and skin!
You see the problem. Many of us live where the sun don’t shine. And rates of vitamin D deficiency even in healthy young adults (especially those who live north of the 42th parallel, say in Boston for example, where this blog was written) are alarmingly high - 30% in some series. Vegan and non-vegan alike are at risk for vitamin D deficiency if you avoid or get little sun exposure throughout the year. For instance, it's thought that from November through February, the sun north of the 42th parallel is too weak to stimulate D production.
In the US, dairy milk is always fortified with vitamin D - so this remains the main source for non-vegans, but even this effort has not proven effective at boosting D levels for some northerners. (Granted, rickets (aka osteomalacia in adults) - a consequence of severe vitamin D deficiency - is much more rare than a century ago)
So here's what you should do:
1/ Make sure you have a regular source of D
Your two main options are fortified soy or rice milk plus a daily multivitamin (which will typically have 200 IU of vitamin D), or a vitamin D supplement (which is often coupled with calcium). Cereals and "cliff"/or equivalent granola bars are often fortified as well. Vitamin D is a "fat-soluble" vitamin so it will be better absorbed if taken with some fat containing food.
The exact amount of vitamin D to aim for is a matter of debate. But with the exception of B12 and perhaps a few others, more is not necessarily better and does have potential for harm. In extreme megadoses for example, vitamin D is toxic. Most experts suggest aiming between 400IU and 800IU total per day. That said, some have recently advocated for higher doses (up to 1200 IU)...so follow the news and your docs advice for updates to these guidelines. The IOM is expected to issue comprehensive revised guidelines mid 2010.
2/ Get your D levels checked once (preferably during the winter), if you live where there is a winter or don't get daily sunshine (you use sunblock or avoid the sun like the plague). The name of the vitamin D we test for in the blood is 25-hydroxyvitamin D.
Just like B12 there is some debate about whether having a low but "normal" level of 25-hydroxyvitamin D is good enough. Typically we say below 20 ng/mL is deficient, below 12 is severely deficient, and 20-30 is low normal. It will take more science for us to know what is "ideal", but based on what is known now I aim to be near 40 or above.
3/ Respect the sun. I detest the word moderation - that meaningless cop out - so I'll say respect. There are probably important benefits of sunshine (vitamin D production being one), but sun burning undoubtedly causes skin cancers, which kill many many people each year. So avoid the summer sun in the mid-day hours when it is strong, but enjoy the early morning and late afternoon rays. Obviously, if you have a high risk for skin cancer this advice does not apply to you - use sunscreen always (and more importantly avoid the strong sun) and get your vitamin D the modern way - through fortified food or vitamins.
Please note - light therapy (aka full spectrum) lights which are wonderful for SAD (seasonal affective disorder, aka the "winter blues") do NOT give off UV rays or stimulate vitamin D production.
Vitamin D Summary - find a daily reliable source - sunshine or a fortified item. This is especially important if you live north of the 42nd latitude (think of a line between northern California and Boston) or if you avoid the sun - and in either case: do get your levels checked one time (preferably in the winter).
Note: since a vitamin D test can cost up to $200, an alternative strategy to testing once is to take a relatively high-dose daily supplement (e.g. 800-2000 IU).
For those seeking a great and thorough medical review on vitamin D search the New England Journal of Medicine for "Vitamin D Deficiency" by Michael Holick : N Engl J Med 2007; 357:266-81.
As always, this blog does not provide individualized medical advice - for that you should find a good doctor.
If any of the above is unclear, please feel free to post related questions.