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Kidneys have to work against osmosis. Osmosis is a process which exists in all membranes which can allow water though - essentially in all "wet" membranes, such as cell walls. Osmosis is a process whereby water tends to seep through such a membrane from a dilute solution to a more concentrated solution. Osmotic pressure tends to try and equalise the concentration on each side of the membrane.
Concentrated urine would then, by osmosis, tend to draw water from the less concentrated blood. The kidneys have to work against this process and retain water at the correct level in the blood, whilst extracting unwanted salts from the blood and excreting these salts into the already concentrated urine. So the kidneys are largely working against osmosis.
Of course if there is excess water in the blood, then the natural osmosis can be allowed to occur and this osmotic flow will, in theory, greatly reduce the work the kidneys have to do.
It should then follow that the more water the animal imbibes (within very wide limits), the more spare water here should be in the blood, so the more dilute will be the urine and the less hard the kidneys will have to work.
Furthermore, crystals form in a solution when the concentration of that chemical becomes too high. These crystals may be expressed normally in the urine, or under certain conditions they may form into a stone in the urinary system - a urolith. It should be obvious that the weaker the urine, the less likely are crystals, therefore uroliths, to form.
From the above, the more water we can encourage our cats to eat or drink, the healthier will be their kidneys. Now dried cat food contains very little water. Wet cat food is about 85% water. Dry food would only make sense if the cat drinks enough water to make up the deficiency - and then extra water to ease the burden on the kidneys. So the natural question is - do cats have an adequate thirst drive to know they are water deficient?
The ancestor of the modern domestic cat is the near-eastern or African wild cat, Felis silvestris lybica. This is an arid region where free water is scarce. Thus it is likely that the cats got most of their water from their prey. If so, they would not have needed much of a thirst drive. Wikioedia has an article on the African wildcat. At one point the article says:
This theory about cats' water sources is backed up by a Google search for cat sea water. Cats can drink sea water when necessary! There are also plenty of www sites that state that cats are opportunistic drinkers, drinking from streams, ponds, puddles or any suitable water source they find, when they find it.
So it seems very credible that cats need wet food. Even very wet food. It makes no sense at all to feed dry food.
In the case of kidneys, that means that the more water they pass, the easier is their work. So cats should be encouraged to take in as much water as possible. Dry food is not good! Renal dry food is an oxymoron!
In contrast our cats Flora and Fauna were raised in the breeder's kitchen, and fed on such human foods as chicken, meat and cheese as well as wet cat-food. They were given diluted milk to drink. So they now eat a much wider range of foods, and drink a lot. Certainly their urine output is voluminous.
A paper from the Journal of Small Animal Practice, Volume 33, Issue 6, pages 261—265, June 1992, entitled Effect of food intake on urine pH in cats concludes:
A paper from the Journal or Nutrition, June 1, 2002, vol. 132 no. 6 1754S-1756S, entitled Feline Reference Values for Urine Composition states:
A paper from the Journal or Nutrition, August 1, 2004, vol. 134 no. 8 2128S-2129S, entitled Dietary Sodium Promotes Increased Water Intake and Urine Volume in Cats states:
There is a paper entitled "The feline urolithiasis syndrome: a review and an inquiry into the alleged role of dry cat foods in its aetiology" by J. BARKER and R. C. POVEY - Journal of Small Animal Practice: Volume 14, Issue 8, pages 445—457, August 1973. This is not available on line - if you have a copy I would be interested in reading it. It apparently concludes that wet/dry food makes no significant difference.
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