I was actually doing some research into kibble last week.
No matter what vitamins are added (and they are the cheapest synthetic versions of the vitamins) it is all destroyed by the cooking process which is known as extrusion.
Pet food producers use all the throw aways that are considered unfit for human consumption.
This includes intestines, udders, esophagi, and possibly diseased and cancerous animal parts.
You know that strong smell that dry dog food has....... its from Animal and Poultry Fat
It is most often rendered animal fat, restaurant grease, or other oils too rancid or deemed unfit for humans.
"Fat blenders" or rendering companies pick up this used grease from restaurants etc. and mix the different types of fat together, then use stablizers with powerful antioxidants to retard further spoilage, and then sell the blended products to pet food companies and other users.
These fats are sprayed directly onto dried kibbles or extruded pellets to make an otherwise bland or distasteful product palatable. The fat also acts as a binding agent for other things to be added to the food. Dog food companies are masters at getting a dog or a cat to eat things they would normally refuse to eat.
Then there are the preservatives, sequestering agents (thats what they add to make the food clean the teeth).........Dont get me started!!