Hi again kitty ,
you asked whether you might possibly have an absorption problem. Well there is a possibility ... but not a probability. As holdthewine says , autoimmune problems tend to cluster.
Some diseases associated with autoimmunity problems can lead to poor absorption , i.e crohn's disease , coeliacs disease, pernicious anaemeia etc. The latter can lead to B12 deficiency.
Yes , you are right , in my opinion , it would be a good idea to check your Vit D blood level say 3 months after you have been DAILY supplementing, whether gel capsule,a spray or tablet.
The possibility of poor absorption would be a consideration if you have a diagnosis of any of the above , or even IBS or gut problems of a similar nature.
There is quite a lot of evidence that higher levels of vitamin D , are potentially helpful, in many of those conditions. IF you have any reason to suspect a gut absorption problem , then , in my opinion , a spray is better , because it is absorbed directly into the bloodstream , bypassing the digestion. It does tend to be a little bit more expensive. BUT , it tastes nice ..!
With regards to magnesium , the least absorbable form is Magnesium Oxide ( MgO ) . If you are taking the supplement route , then magnesium citrate or glycinate , or malate may be preferable.
I think I have mentioned that veg have little or no Vit D , so as a vegetarian , you are limiting yourself to the amount you might get from diet. And as previously described , sunlight in the UK , is too weak to provide any Vit D from sunlight after mid October , or before mid April. ( So you will be having a lower and lower blood level for 6 months , unless you supplement. ) The supplementation of 800 IU , is just too low to raise your level significantly.
When your blood level reaches 100 , it is then that your liver slows down it conversion of VitD3 supplement , to 25 hydroxyD3. This enables your body to store some VitD3 for autocrine use , whereby your cells convert the VitD3 internally in the cell, when required, to the hormone form 1,25 dihydroxyD3, ( for gene transcription) and then the cell can respond as it needs to do . i.e. say for instance a white blood cell can attack an infiltrating bacteria. You can get the picture ... A good blood level of Vit D , ( over 100 ) will enable your cells to do their job without struggling. Suffice it to say , that this has been shown in numerous scientific and medical papers.
Sorry to go on a bit .... but at least you and any other readers of this thread have the information needed.
hope this is helpful
BTBH
PS. As you are a vegetarian , if you are having blood tests , then check B12 , many vegetarians are low , as this vitamin is almost exclusively found in animal products. A deficiency can cause all sorts of problems. In addition - It is sometimes not appreciated , that low Vit D , can lead to autoimmune problems , one of which can be pernicious anaemia , which leads to B12 deficiency.
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