Vitamin D is a fat-soluble substance that we mostly make in our skin in reaction to UV-B rays and is modified by our liver and kidneys to a version called ‘cholecalciferol’ or D3. It plays an important role for calcium and phosphorus absorption across the GI tract, and thus is integral in bone growth and stabilization. It’s also indirectly implicated in immunity as well as protection from heart disease, cancer, some neurological disorders, and likely other functions we don’t yet know about.
We only get a smallish amount (10%) from our diet and it’s found mainly in fish oils (remember cod liver oil?), egg yolks, animal products, and fortified foods.
Fortified foods have mostly D2, and we need D3, so its better to take a D3 supplement than rely on processed fortified foods.
Dr Weil recommends 2000 IU a day and I believe him. It’s also a good thing to get some sunshine. Summertime is great for us to make Vit D, but from September to May,the skin makes little if any from the sun at latitudes above 37 degrees north in the United States —>
SO, a couple of 1000IU vitD3 capsules a day after Labor Day to Independence Day should do it. And add some sunshine time (10-30 minutes, a few times a week at least) in the summer – less if you are lighter skinned, and more if you are darker.
UPDATES 9/12/17: Beth asked about absorbability: Vit D is absorbed in the small intestine downstream from the stomach without any extra cofactors needed. However, Magnesium is also an important player in the mineral->bone pathway and needs to be consumed in adequate amounts for VitD3 to have it’s desired effects.
Jaynie spoke about taking a liquid form vs a capsule, so I had a look at that. Seems like oil-emulsified VitD3 drops are an efficient way of getting 2000IU/d, but oil caplets and chewable tablets were effective too. There is also a sublingual spray available according to an MD commenter – I didn’t follow up on that. These are options to ask your health practitioner if your blood Vit D is low.