Without vitamin D, you won’t absorb and use calcium properly, and the lack of both calcium and vitamin D will weaken the structure of your bones. Researchers are also starting to link vitamin D deficiency with all kinds of health problems and diseases, like asthma and cancer.
Meat eaters should be just as concerned here. The results of a 2009 study showed that the majority of both vegetarians (59%) and meat-eaters (64%) do not have sufficient blood levels of vitamin D.
Vitamin D is produced by our skin when it’s exposed to sunlight. It’s a hard thing to measure and rely on, since we produce different amounts depending on skin color and some other factors. In the winter, we usually don’t get as much sun exposure as we do in the summer. The further north you are, the more winter will affect your vitamin D levels. Even in the spring and fall, we would need more sun exposure than in the summer to produce the same amount of vitamin D since the sun is at a lower angle in the sky. A general recommendation of 20 minutes per day might not be enough.
My opinion is that since supplementing with vitamin D is very inexpensive, and the health cost of a deficiency could be huge, I would rather be safe than sorry.
The active forms of vitamin D are cholecalciferol (D3) and ergocalciferol (D2). Most supplements get D3 from the lanolin in sheep wool. D2 is derived from plant sources. There are some new supplements with plant-sourced D3, which is great because D3 is the more active form in your body and better for correcting a deficiency. They’re a bit more expensive, so the good news is that D2 is just fine for maintaining healthy levels.