They start out by saying if you are the typical child you should get 1,000 IU of vitamin D per 25lbs of body weight and test in three months to see where they are. If you are a adult you should take 5,000 IU a day and check in three months. Then they say, if you are black, obese, or old you need more than others. There is no recommendation of what to do if you are in that group until tested.
After 40 years of giving people medications I realize dosing is weight and age related and so my recommendation for adults is the same as children. 1,000 IU per 25lbs. I think people who follow this schedule will be at a more OPTIMUM place in three months.
Additionally if you are a person who eats a unhealthy diet please consider that you may need a little more MAGNESIUM in your diet. Magnesium is vegetables, nuts; you know the healthy stuff! When you start taking vitamin D magnesium is used to make the hormone vitamin D turns into. This will lower your magnesium level and could be a problem as it has to do with maintaining a regular heart beat. Magnesium is worth "googling" and reading up on. While you are at it look up vitamin K also. If you know some one that says, " I started taking vitamin D, but I did not feel so good so I stopped. This is likely to be the reason why". Diet is not a benign thing.
Also, if you are taking a multi vitamin please look at the label. If you are getting up around 70% of vitamin A and your diet brings you over 100% of the RDA, this could be a problem. Vitamin A and vitamin D are both fat soluble and A can block D. Not a good thing. IMHO, multivitamins, which have never been proven to improve health, could be made beneficial by a very simple change. How? Switch the amounts of vitamin A and D.