This comes close to answering your question, I hope!
I am sorry I cannot remember the exact source of this information, but I
believe it was from Columbia U School of Vet Medicine:

The natural prey diet of the cat contains between 65%-75% water. The cat,
having evolved on the plains of Africa, has adapted to quench her water
requirements entirely on the moisture content in her prey.
Due to its nature, commercial dry cat food contains no more than 10%
moisture.

Cereals create the base of dry commercial foods and make up over half of the
foods weight. Cereals frequently used in commercial dry cat foods like corn,
rice, and wheat, give the food bulk and structure and represent a cheap
source of calories. Cereals are primarily made up of carbohydrates, a
nutrient nearly absent in the cat's natural prey diet. The liver and other
organs store small amounts of carbohydrates and the cat may receive
additional minute amounts of this nutrient through the stomach and
intestines of her prey; this however, would never total more than 1-2%
carbohydrates compared to the total weight of the prey. However, commercial
dry foods may contain as much as 45% carbohydrates. A diet high in
carbohydrates will result in obesity, because excessive amounts of this
nutrient are converted by the liver to body fat. Since a cat metabolizes
primarily fat and protein for energy, most of the carbohydrates in the diet
are then stored as body fat.