(from wiki)

Dry food

Dry food (8-10% moisture) is generally made by extrusion cooking under high heat and pressure. Fat may then be sprayed on the food to increase palatability, and other minor ingredients, such as heat-sensitive vitamins, which would be destroyed in the extrusion process, may be added.

Major brand-name dry cat food manufacturers often use primarily grain-based ingredients with animal protein by-products or animal digest to cut cost. Higher end foods offered as ‘premium’, ‘natural’ or ‘holistic’ formulas contain less or no grain, and a higher percentage of animal protein. However, grain-free dry diets still contain carbohydrates, from sources such as potato or tapioca. The starch in those ingredients allows the kibble to hold its shape. Cats have no metabolic need for carbohydrates as the feline system prefers to create glucose from protein.

Dry cat food has an advantage over wet in convenience and price – besides usually being significantly cheaper, dry cat food can also be left out for the cat to eat at will over the course of several days, whereas canned or raw cat food spoils or becomes unappetizing after several hours. It should be noted, however, that even dry food, since it is sprayed with fats as noted above, becomes rancid and stale as it oxidizes. Using a free feeding practice can also contribute to overeating, and ultimately obesity.

Dry food is recommended by some based on the idea that cats break apart dry foods with their teeth, which causes the food to scrape off dental calculus. The degree of benefit this provides has been disputed in recent years.

Since the occurrence of BSE infection through contaminated meat and bone meal, the use of meat meal in pet foods has been prohibited in parts of the world (e.g. Japan and France) but is still common practice in other parts (e.g. USA).[citation needed] Comparative studies conducted by Japanese researchers have shown that meat meal is superior to the other protein meal sources in terms of dry-matter digestibility and nutritional value for cats, while corn gluten meal is the least nutritional.

The same studies showed that cats fed with these dry food diets excreted alkaline urine. Urine pH has been implicated in the formation of struvite crystals in feline urolithiasis, and many dry food manufacturers address this by adding urine-acidifying ingredients to their food. However, this practice may lead to the formation of calcium oxalate stones, therefore water intake rather than urine pH appears to be the most crucial diet related factor for the prevention and treatment of feline stones.

Wet food

Wet (Canned) cat food example (Fish flakes in jelly)

Canned or wet food (75-78% moisture) generally comes in common can sizes of 3 oz (85 g), 5.5 oz (156 g), and 13 oz (369 g). It is also sold in foil pouch form by some manufacturers.

Owners and veterinarians who recommend a diet consisting largely or entirely of canned, homemade or raw cat food point to higher water content of such food and the increased total water consumption in comparison to a dry food diet as an important health benefit. Wet food also generally contains significantly less grain and other carbohydrate material. Many foods are made with fish, however an excessive consumption of fish (which contains high levels of unsaturated fatty acids) can cause yellow fat disease. In comparison to dry food, canned food is thought to either help treat or noticeably reduce the likelihood of numerous health issues including urinary tract disorders, diabetes, chronic renal failure, constipation (sometimes leading to megacolon), and obesity.[citation needed]

Canned cat foods in pop-top containers may play a role in the development of hyperthyroidism in cats. This may be due to bisphenol A used in the pop-top can coating leaching into the food, so it is suggested to place unused food in reusable containers to prevent contamination.