Cats are obligate (strict) carnivores. They satisfy their nutritional needs by the consumption of animal-based proteins. Their bodies aren't able to derive the nutritional support they need from plant-based proteins (grains & vegetables) because Cats lack specific metabolic (enzymatic) pathways and cannot utilize plant proteins as efficiently as animal proteins.
Proteins derived from animal tissues have a complete amino acid profile. (Amino acids are the building blocks of proteins.) Plant-based proteins do not contain the full complement of the critical amino acids required by an obligate carnivore. The quality and composition of a protein is also referred to as its biological value.
Taurine is one of the most important amino acids that is present in meat but is missing from plants. Taurine deficiency will cause blindness and heart problems in cats. In the 1980's, it was discovered that cats are sensitive to taurine deficiency. There were many cases found at that time that were going blind and dying due to this deficiency. There are several situations that can lead to a diet being deficient in taurine but one of them is using a diet that relies heavily on plants as its source of protein.
The protein in dry food, which is often heavily plant/grain-based, is not equal in quality to the protein in raw food or canned food, which is animal-protein-based. The protein in dry food, therefore, earns a lower biological value score. Another important issue with regard to the protein contained in canned and dry food is that it has been cooked at very high temperatures for a long period of time. This extensive cooking required to dry the product significantly decreases the biological value of the protein sources as well. Harsh cooking also destroys other nutrients and enzymes, which are utilized by the body to assist in digestion and absorption, among other roles.
With regard to the overall protein amounts contained in a food, do not be confused by the listing of the protein percentages in dry food compared to canned/raw food. At first glance, it might appear that the dry food has a higher amount of protein than the raw or canned food—but this is not true on a Dry Matter Calculation which is the accurate way to compare the two foods. Most canned foods, when figured on a dry matter basis, have more protein than dry food and a raw food diet has even more. And remember, even if this were not the case, the percentage numbers do not tell the whole story. It is the protein’s biological value that is critical.
Obligate carnivores are designed to eat animal protein – not grains - and they need to consume water with their food as explained below.
Another extremely important nutrient with respect to overall health is water. It is very important for a cat to ingest water with its food, as the cat does not have a very strong thirst drive . This is a critical point. This lack of a strong thirst drive leads to low-level, chronic dehydration when dry food makes up the bulk of their diet .
Cats are designed to obtain most of their water with their diet since their normal prey contains approximately 70 - 75 percent water. Dry foods only contain 7-10 percent water whereas canned and raw foods contain approximately 78 percent water. Raw food diets and Canned foods therefore more closely approximate the natural diet of the cat and are better suited to meet the cat’s water needs.
A cat consuming a predominantly dry-food diet does drink more water than a cat consuming a raw diet or canned food diet, but in the end, when water from all sources is added together (what’s in their diet plus what they drink), the cat on dry food consumes approximately half the amount of water compared with a cat eating canned foods. This is a crucial point when one considers how common kidney and bladder problems are in the cat.
Side note: Please keep in mind that when your cat starts eating a more appropriately hydrated diet of raw food or canned food, his urine output will increase significantly.
A Raw diet is the best diet for a Cat because the protein in the food has not been altered by the high temperatures due to cooking and the amount of water needed in their diet is also naturally contained in the food they are consuming.