For most dry pet foods, the last production step is to spray the food with a substance to make it more appealing.
As you would expect, putting frosting on food will make it more appealing but it doesn't make it more nutritional.
You are well advised to be wary of commercial pet food (including that sold by vets).
A bit of Googling will show that:
"What most consumers dont know is that the pet food industry is an extension of the human food and agriculture industries. Pet food provides a convenient way for slaughterhouse offal, grains considered unfit for human consumption, and similar waste products to be turned into profit. This waste includes intestines, udders, heads, hooves, and possibly diseased and cancerous animal parts.
...
The pet food industry loves to say that its more highly regulated than human food, but thats just not true. Pet food exists in a bit of a regulatory vacuum; laws are on the books, but enforcement is another story."
Best of luck,
Uno
Sentry Dog Handler
US Army 69-71