D3 Serum offers 1,000 IU per serving of natural-source Vitamin D3 as a fat-soluble liquid in a base of extra virgin olive oil.  D3  Serum is the preferred form of vitamin D is D3 (not vitamin D2) because it is better metabolized by the body. D3 Serum is also without those chemical tagalongs such as magnesium stearate (a questionable excipient) or other chemical no-no’s.