Vitamin D3 is a steroid hormone manufactured from the irradiated skins of animals [which could include pig], which is processed and added to milk. Not all milk with Vitamin D added is from D3, so check with the dairy as to which it is, D1, D2, or D3.
Added Vitamin D3 in Milk
The remaining of the vitamin D content of the milk, 320 to 350 IU per quart, comes from the fortification process. Vitamin D3 can be produced chemically. First, 7-dehydrocholesterol is extracted from animal skins--usually cow, hog or sheep. After purification and dissolution in a solvent, the 7-dehydrocholesterol is irradiated with UV light to form vitamin D3. It is impossible to determine the origin of the vitamin D3 in milk. Natural or added, it has the same molecular structure. Moreover, it is also impossible to determine the animal source for the chemically synthesized vitamin D3. F. Hoffman La Roche of Switzerland and BASF of Germany, are the main companies providing vitamin D3 for milk fortification.
This full article can be found at http://www.livestrong.com/article/182519-source-of-vitamin-d3-in-milk/
The following are others sources:
"Vitamin D3 can be produced photochemically by the action of sunlight or ultraviolet light from the precursor sterol 7-dehydrocholesterol which is present in the epidermis or skin of most higher animals." This found here
"Windaus isolated three forms of the vitamin: two derived from irradiated plant sterols, which he called D1 and D2, and one derived from irradiated skin, which he called D3."
The term vitamin D actually refers to a group of steroid molecules. Vitamin D3, also known as cholecalciferol is generated in the skin of animals when light energy is absorbed by a precursor molecule 7-dehydrocholesterol. Vitamin D is thus not a true vitamin, This found here
"produced in human and other animal tissue by photoactivation (normally sunlight on skin) of 7\-dehydrocholesterol.(Source: CRISP)" This found here