Today, dry, commercial cat food is by far the most popular product to feed to companion cats. The attributes which all brands of this product have in common are : "convenient" and "inexpensive" when compared to other methods of feeding cats. In recent years commercial dry foods have been heavily promoted through advertising and by Veterinarians to be the choice of caring, health conscious care givers.

The truth is, dry commercial cat foods are anything but healthy for cats.
First and foremost, the nutritional composition of commercial dry foods does not compare to or reflect the cat's natural diet from which cats have evolved as absolute and true carnivores.

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.

An analogy:
It is not an exaggeration to compare a commercial dry cat food based diet fed to a cat with a fortified macaroni and cheese dinner diet fed to a human. Both products are overprocessed and based on refined carbohydrates. Added vitamins attempt to compensate for nutrient loss, but the food still lacks many other essentials including enzymes, complete amino acids and fatty acids. Neither reflect the natural diet or nutritional needs of either species. However, in the opinion of the individual consuming it, both taste good . For a more accurate analogy, the macaroni and cheese dinner would need to be modified such that the cheese flavoured sauce is a component of the noodles and most importantly these new noodles are served dry to the human!

Water is the most important nutrient. Of course, neither we nor our cats can live on water alone, but its importance is demonstrated by the fact that during the absence of food and water a creature will perish from thirst long before perishing from starvation.
That said, we don't claim that cats die of dehydration when fed on a commercial dry cat food diet, because most cats will have a supplementary source of water available of which they will take advantage. Or do they?

We mentioned previously how cats evolved as dwellers of the African plains and desserts, and their adaptation of stilling their needs for water with the moisture content of their prey. During the past 40 million years, the cat did not need to rely on supplementary water intake and, even if needed, the cat would not readily do so, because to her it is not natural.

1 cup (85 gm) of dry commercial cat food rehydrated with 225 ml water to contain a 75% moisture yields over 2 cups of food.

On average, natural foods contain 70% water. A cat fed a commercial dry food diet will consume approximately one cup of the product per day. For an adequate water intake, the cat would need to drink 225 ml (8oz) supplemental water per day! If she does not consume this adequate amount, dehydration will set in.
Once ingested, the commercial dry food will absorb moisture like a sponge from the cat's stomach, causing the cat to dehydrate from within. Because commercial dry cat food diets are very calorie dense, one cup of dry food, once ingested, will actually give the cat the equivalent of 2 cups of fresh food. Hence, cats on a commercial dry cat food diet are usually over-fed, because the care giver judges how much to feed by volume not caloric density. With the additional high carbohydrate content of dry foods, cats very quickly become obese.
Rehydrating dry commercial cat food, by soaking it in water before feeding, to the same moisture content found in natural foods dilutes protein and fat concentrations per serving to well below nutritionally adequate levels. More of the soaked food would need to be fed to meet daily protein and fat requirements resulting again in an over feeding of carbohydrates and calories.

Commercial dry cat food diet and FLUTD (Feline Lower Urinary Tract Disease):
Clinical disorders of the lower urinary tract of cats are not a new phenomena and have been observed as early as 1925. The frequency of its occurrence in the companion cat population is, however, on the rise, and it is now considered to be a common feline disease. The formation of struvite crystals in the urine, leading to obstruction of the bladder, is directly linked to the diet of the cat. Most research relating diet to lower urinary tract disease has focused on mineral content or more recently on the effect of diet on urinary pH; much less research has been devoted to the effect of diet on urinary volume or specific gravity. It has been predicted from theoretical considerations, increasing urine volume for a given solute load has a greater influence on the likelihood of struvite crystal formation than a reduction in urinary magnesium concentration through restriction of dietary magnesium. In addition, increasing urine volume may increase the frequency of urination which would hasten crystalloid and crystal transit time through the urinary tract, thus reducing the potential for crystal growth. It was demonstrated that haematuria (one type of FLUTD) induced in cats by feeding a high magnesium, low moisture-content diet could be abolished by feeding the same diet rehydrated, containing 80% moisture. The same observations have been made in the treatment of cats affected with lower urinary tract disease where the re-occurrence of the condition was significantly reduced by feeding the cats a canned food, compared to cats maintained on dry food. Consumption of dry food has since been implicated as a risk factor for lower urinary tract disease.
New commercial dry diets for the treatment and prevention of struvite crystals are formulated to contain low magnesium levels and are acidified to reduce urinary pH. A low dietary magnesium intake as well as excessive intake of acidifiers, such as ascorbic acid, however, interfere with proper calcium distribution in the body and result in calcium deposits in soft tissue in the form of calcium oxalate containing stones. These stones usually accumulate in the heart and upper urinary tract including the kidneys and, if not surgically removed, will cause death. The occurrence of oxalate containing crystals is now equal to the occurrence of struvite crystals.
Often overlooked is the significance of protein in the acid formation in the body. A high protein diet will assure natural acid levels in the body and a low urinary pH. Contrary to common belief, a diet high in protein does not cause kidney disorders or lead to renal failure, whereas dehydration is damaging to the kidneys and, as a result of feeding an all dry diet, the long term dehydration is a possible cause of chronic renal failure in cats.