I am guessing that the GP gave a prescription for D-2 not D-3. As far as I know there is no prescription form of D-3.
I copied a post from another forum, as it pertains to what is referred to as "Vitamin D Resistance".
To sum it up, absorption or conversion problems can be caused by several factors. Magnesium deficiency, poor fat absorption, or not taking it with fats.
I doubt I am Mg deficient since I have taken a Mg supplement before and it had no effect on my D levels. I also presently get 500mg from a whole food multivitamin. I also take vitamin K2 since I was on Prednisone for a longer time, and do not want my bones to become brittle.
Poor fat absorption is likely in my case.
Here is the post I ripped off.
Dan
Quote:
(NaturalNews) As more people begin to realize the amazing health benefits that Vitamin D has to offer, more people are naturally beginning to get more vitamin D, either through supplements or sunshine. The downside of this increased intake, however, is that more people are having adverse reactions from Vitamin D. What most people are not aware of, though, is that most of these so-called 'Vitamin D Side Effects' are not problems with taking the vitamin itself, but are actually problems with not getting enough magnesium.
Vitamin D, just like all other nutrients, works in harmony with several other nutrients to perform its many functions. Most importantly, vitamin D requires and 'uses up' magnesium to convert from supplements or sun into its active form in the blood.
As such, it is a big mistake to simply take large doses of Vitamin D without taking the need for magnesium into consideration. Yet this is exactly what is happening in most cases and it is causing a lot of people to have problems that they believe are due to side effects of Vitamin D- or even worse they believe they are experiencing an overdose. Such a huge number of people have subtle magnesium deficiency that some researchers and doctors are calling magnesium deficiency an epidemic, and anyone with even a mild or 'subclinical' magnesium deficiency will have this deficiency amplified when Vitamin D is taken. This is creating some uncomfortable 'Side Effects of Vitamin D' that are actually symptoms of an induced magnesium deficiency! Some of the magnesium deficiency symptoms being attributed to Vitamin D are:
Headaches
Insomnia
Jitteriness
Muscle Cramps
Anxiety
Heart Palpitations
Constipation
While there are always going to be those who simply can't tolerate taking Vitamin D supplements for one reason or another, the good news is that the vast majority of these problems can be prevented and even reversed by getting clinically significant amounts of magnesium - while you are getting your Vitamin D from pills or from the sun.
Vitamin D Absorption Problems
Conversely, it's also true that taking Vitamin D may not raise blood levels in those who are magnesium deficient. In many cases, both the Vitamin D deficient person and their doctor believe that they are having 'absorption' problems. This lack of knowledge about the need for magnesium ends up causing serious issues such as:
1) A lot of fear being generated that an underlying serious medical problem exists
2)Unnecessarily high dosages of Vitamin D that further worsen the magnesium deficiency
3)Thousands, and sometimes tens of thousands of dollars, being spent on unnecessary medical testing to find the 'absorption' problem
4)The underlying magnesium deficiency not being found because testing for magnesium levels is not useful in determining need for the nutrient
This leaves many people still low in Vitamin D and believing that they are toxic or allergic to Vitamin D. This belief is being encouraged by practitioners and websites that are unaware of this intimate connection between these two nutrients and who don't have a solution for those who are suffering. Unlike drugs, nutrients are interconnected with one another and rarely does someone have only One nutrient deficiency completely in isolation. But as Vitamin D testing is becoming more common, people are being treated with large and sometimes massive doses of Vitamin D without taking into consideration their need for other nutrients. In particular, the need for sufficient magnesium is critical to avoid some of the uncomfortable problems that are often falsely attributed to being Vitamin D side effects or overdoses.
Resources
http://www.springerlink.com/content/n92g57j9k00ul822/
http://www.easy-immune-health.com/vitamin-d-absorption.html
http://www.ajcn.org/cgi/reprint/29/8/854.pdf