We make vitamin D in our bodies when our skin is exposed to sunlight. About 15 minutes a day seems to be enough, but the catch is, it has to be outside, not through a sunny window. From what I've read, glass filters out all but a fraction of the Vitamin D producing UVB rays, leaving us with the skin damaging UVA rays. Although my south facing windows are warm and cozy to sit by, they aren't going to do the trick as far as vitamin D goes, and darn it, the light might actually be bad for my skin. Going outside with sufficiently exposed skin to produce the vitamin myself isn't a good option either on sunny, but 20 degree days.
So what about supplements? Most are made from either pig or sheep skins. Not exactly vegan. There are, however, vegan D supplements on the market, so do a little googling and read labels if this seems like a good idea for you.
Another option, not in pill form, which pleases me, is mushrooms. As I understand it, skin and mushrooms are the only things that can manufacture vitamin D from sunlight. Mushrooms are a pretty good vegan source of the vitamin as they come from the store, especially if they're grown outside, but you can boost the levels enormously by placing your mushrooms in sunlight. Again, it has to be outdoors, not through a window. I'm giving it a try as we speak.
I will confess that I have a history of being "seasonally affected," but it never occurred to me that I might be low on vitamin D, especially in New Mexico! I have a tray of nice fat crimini mushrooms out on my patio now, and will give the slices another dose tomorrow before storing them in a paper bag in the refrigerator. Fresh mushrooms should be eaten within 5-7 days, so this will be an ongoing process for me. You can also make good use of summer sun next year by sunning, dehydrating, and storing dried mushrooms for the winter.
And by the way, never store fresh mushrooms in plastic or air tight containers. They need to breathe. Ever had mushrooms develop a "fishy" smell? Don't eat them! Food poisoning is really not fun. Store fresh mushrooms in a paper bag, and eat them within a few days.
For more on how to add sunny vitamin D to your mushrooms, visit Fungi Perfecti.
Other sources for this post are:
Fresh Mushrooms - Nature's Hidden Treasure
Mark's Daily Apple
About.com - Frugal Living