Is the absorption of vitamin d3 guaranteed when taken as a supplement? what brand of vitamin d3 is best in the market?

Malabsorption: If there are problems with nutrient absorption in the GI tracts, Vit D is sometimes not absorbed well. There are many causes, including GI surgery, inflammatory bowel disease, sprue, etc. In normal people, it is well absorbed.

Vit D3 Well-Absorbed: Nothing is guaranteed. But I think you can feel comfortable that you are absorbing your Vitamin D3. I believe most brands are acceptable, but the best-tested brand is Bio-Tech. Bio-Tech D3 is the form that has been used in most of the major clinical studies. I recommend that D3 levels be maintained in the 60-70 ng/dl range. Usual dose to be in that range is 5-7,000 IU per day.

Depends on the brand: Vitamin D3 is a fat-soluble vitamin, so remember to take it with food. I personally use USANA or GNC Vitamin D3. I see that you live in California, so make sure you get 15 minutes of sunlight every day so that your own body can produce Vitamin D.