I recently had my vitamin D level tested. I'll get to that story later, but first I'll hit the highlights of the function, types, sources, etc. of this essential micronutrient.
Function/Purpose - why do we need it?
Vitamin D is important for the normal calcification of our bones and regulates our body's use of many minerals, especially calcium and phosphorus. Similarly, it is required for the proper mineralization of our teeth. There are basically 2 types: D2 which is plant derived, and D3 which is animal derived.
Sources - where do we get it?
When the UV rays of sunlight hit our skin, our bodies use a form of cholesterol to manufacture the D3 form of vitamin D - hence the "sunshine vitamin" nickname. In addition to the skin, the bloodstream, liver and kidneys are all involved. It is stored in the liver, spleen, skin and bones. And because our bodies can produce it, vitamin D is considered a hormone as well as a vitamin. Food sources of D3 include cod liver oil, egg yolks, butter, liver, and oily fish such as salmon, mackerel, sardines and herring.
The D2 form is the one most commonly added to milk, and is found in low amounts in mushrooms and dark leafy greens (because plants don't have cholesterol, they can't produce D3).
Both forms are available as supplements, but because our bodies make D3, it appears to be the more desirable form.
Deficiency - am I getting/making enough?
Several factors can contribute to our bodies not making enough vitamin D, including clouds, reduced daylight hours during winter, sunscreen use, clothing, smog and dark skin pigmentation. Also gallbladder, kidney or liver dysfunction will all hamper proper synthesis.
Low stomach acid (HCl) and other digestive issues can prevent us from being able to properly absorb vitamin D from our food or supplements.
Signs of deficiency can include muscle cramps, nearsightedness, osteoporosis (if levels are low, the body pulls minerals from the bones), insomnia and many others.
Excess - am I getting/making too much?
Because it is fat soluble, toxicity is also a concern. This can result from excessive intake, usually from supplementation, as well as overexposure to sunlight.
Signs of toxicity include excessive thirst, nausea, weakness and headaches. Over time, it can increase one's susceptibility to kidney stones.
My Vitamin D Story
For many years, my doctor noted that every Canadian should assume they are deficient in vitamin D. On some level, I agree with this in general, especially at the end of our long, dark winters. However, we are all individuals, and so this wouldn't necessarily be true for every Canadian.
She recommended that I take 4,000 IU of vitamin D per day - this is well in excess of any guidelines I have ever come across and it concerned me because of the potential toxicity of this vitamin. When I voiced those concerns, she told me a story about a man from Africa who was taking 10,000 IU per day (I can't remember why) and he was fine..... I remember thinking to myself "how does she think that because one person is fine at this level, I would be fine too?" I asked her if I could be tested, so that I would know my level and could supplement accordingly, but apparently our healthcare system will not test us for vitamin D, even if we are willingly to pay for it. Crazy.
So I decided that I'd do my own guesswork, and take 2,000 IU per day during the winter, and 1,000 IU/day during the spring/summer, when I'm spending much more time outdoors. If I was ever in the tropics, I just stopped taking them altogether. (In no way am I recommending anyone else to follow this same regimen - this was my personal experience and is not medical advice.)
This past winter, I went to a Naturopathic Doctor on another matter, and while I was there, decided to ask her about vitamin D levels, supplementation and testing. She didn't hesitate to suggest getting tested, which she could requisition for me and send to a lab in the USA. It was a quick, painless blood draw which didn't cost very much, and within 2 weeks, I had the results - I was right in the middle of the "optimal" range.
So I have confirmation that my approach is right for me, and proof that unless you get tested, you won't know, and neither will your doctor. You'll both just be guessing.