Each person has their own personal dose of vitamin D that must be learned by testing the D blood level. An example: A mother is dosing three of her kids with 2,000 IU/day for one month in the winter, Nathan goes from 20 ng/ml to 50 ng/ml, Samantha goes from 18ng/ml to 25ng/ml and Alex goes from 25ng/ml to 95 ng/ml, each are taking exactly the same dose for same period of time.
The maximal amount of vitamin D made on the skin of a light skinned person, fully sun exposed, middle of the summer, is said to be 20,000 IU, so that is the largest dose that I will start even in someone whose level is very low (undetectable –10ng/ml). That does not mean that everyone gets into the right range with that dose, however. Some of my patients required much bigger doses, up to 50,000 IU (D3 not D2) per day for several years. They had usually been very sick for a very long time and we worked up to that dose over many months as their D level failed to increase on lower doses. Though it is documented that there are people who have specific problems with absorption, I have tried various types of replacement in those who needed larger doses: on the skin, sublingual etc, and I did not find that those methods were any more reliable than oral pills.
I disagree with the proposed idea that fat people have lower D blood levels because they’re fat. It has been suggested that they dilute their vitamin D by storing it in their fat, instead of their blood. Obese people may indeed have lower D levels, and require higher doses, but I think that is because they have been sick longer. Their low D level caused their obesity, not the reverse. 5-6 years of dosing the same obese person also revealed that the dose requirements fell over time, into a more routine range, even while the weight remained the same.
There are several other variables that affect vitamin D blood dosing. Based on my experience with over 5000 patients, about one in 50 people needs much smaller doses. These people operate in a dose range of 1/10th of what everyone else needs. (This probably suggests that they also make less vitamin D per sun exposure, though that has yet to be proven.) It is also impossible to predict who will need these smaller doses; they can have any initial D blood level, can be any color and any age. If you are one of these few you will feel terrible within a week of starting a D dose that is ten times too big, and this is one of the reasons I suggest that you start slowly.
I did not attempt to measure any of the genetic mutations that affect skin production of vitamin D, vitamin D binding protein, GI absorption, or the vitamin D receptor. These are all variables that do indeed exist and need to be sorted out once the “healthy” D level is actually agreed upon. Until then I don’t think it makes sense to blame all differences in D dosing and blood levels on “problems with absorption”. What I learned over time was that if the level wasn’t going up usually the dose being given was just too low. Please see the chapter on “Vitamin D Controversies” for more details.