Vitamin D is the second supplement that I have on my "favorites" list, right after calcium. In fact, I could probably combine it with calcium as it really is part of calcium physiology. Vitamin D facilitates the absorption of calcium from the digestive tract, it maintains optimal blood calcium levels, and it regulates bone growth and remodeling. So if you're supplementing with calcium, there's a good chance you should be supplementing with Vitamin D as well.
Vitamin D gets its own post because it does have other functions besides its role with calcium. In fact, many organs and tissues in the body have Vitamin D receptors, suggesting that there are many other functions for Vitamin D that may be unrelated to calcium although these functions have not been fully studied and understood. A few that have been studied, even if they aren't fully understood, are Vitamin D's anti-cancer properties as well as the role it may play in preventing auto-immune diseases. I expect as more studies are done we will discover more and more roles of Vitamin D.
While the RDA (recommended daily allowance) for Vitamin D is 600IU, I have found that more that 10 times that much appears to be required simply as a maintenance dose. A group of Canadian researchers found that 8000IU seems to be the dosage required to maintain optimal levels measured with blood testing. Remember, that is a total of 8000IU, sunlight, food and supplementation. As a physician, I have found that some individuals require much higher dosages if they come to me highly deficient. However, I use higher dosages in conjunction with laboratory testing to ensure that my clients don't get too much Vitamin D.
The primary mechanism for humans to get Vitamin D is, of course, the sun. There are cells in our skin that are triggered by sunlight to produce Vitamin D. However, a certain concentration of a specific type of ray from the sun is required to trigger this process. Adequate levels to produce sufficient Vitamin D is not present year round in all parts of the world. Specifically, the further you get from the equator, the less time the sun will trigger Vitamin D synthesis. For specific information about your Vitamin D production in your location, I recommend using an app that takes your location as well as the time of year, and estimates how much Vitamin D you can get from sun exposure. The app that I use is called dminder. There are probably several apps that do similar things, this is simply the one that I found and it gets the job done.
If you are unable to get sufficient Vitamin D through sunlight, or you are unwilling to get the necessary sun exposure, there are food sources for Vitamin D. The best food sources are oily and fatty fish. This would include sardines, salmon, mackerel, and cod liver oil.
Finally, there are supplements. Vitamin D can be found in both D2 and D3 forms. Research suggests that Vitamin D3 is a more effective Vitamin D supplemental form. When supplementing with Vitamin D there are a few things to keep in mind. First, Vitamin D is a fat soluble vitamin. So if you are supplementing with a capsule and the Vitamin D is in powder form, it will be important to take the supplement with some fatty foods. It could be taken with yogurt (full fat), cheese, avocado, nuts, or other fat containing foods. There are also Vitamin D supplements that are already emulsified in fats. The second thing to consider is with these fat emulsified supplements. Although I have found no research on this, I have found that Vitamin D in soybean oil doesn't seem to have the same outcomes as the supplements that use other oils. Consequently, I always look for a Vitamin D supplement in olive oil or another (non-soybean) oil.