Thirst regulation in cats

Hi all,

I've done a bit of reading recently on the ever controversial topic of dry food and cats. I've read the paragraphs pertaining to moisture in Dr Pierson's much referenced article on catinfo.org, as well as various people's views in this mammoth post:

http://www.thecatsite.com/t/252221/moistening-dry-food

Whilst I feel fairly convinced of the arguments against dry cat food on a moisture basis I'm still yet to see any of the supporting studies, which makes me nervous. In addition, I have a question regarding one of  Dr Pierson's arguments. 

Regarding cat hydration, the argument runs: as cats are obligate carnivores and are adapted to get the bulk of their water requirement from meat, feeding them a food with only 10% water will dehydrate them. Whilst they still drink water, studies show that they don't make up the deficit. Also referenced is the high incidence of urinary and renal issues that affect cats.

Studies aside, I find a cats lack of ability to correctly regulate its fluid requirements odd. Does the thirst mechanism of cats work in the following two ways outlined here:

http://en.wikipedia.org/wiki/Thirst: "There are receptors and other systems in the body that detect a decreased volume or an increased osmolite concentration."

Or, is it the quantity (and 'expected' moisture content of) the food they eat?

The later would have to be true for low the moisture content of dry food to effect the drinking habits of cats. And it's this that I find hard to believe.

I suppose looking at an actual study will quickly prove me wrong in this! But it doesn't stop it being weird. Also, are the high incidence of urinary / renal disease reliably correlated to a cat's hydration?

Thoughts / links  / information appreciated.

Tom

ldg I am on my tablet and can't link anything right now, but there's a great article in the Articles on tips for increasing water intake that talks about the low thirst drive of cats and how much water is needed to make up for the lack of moisture in dry food. I think the amount will surprise you. Many vets do now recognize how important adequate moisture is to keep the system flushed out and functioning properly.

This should address your question(s). I can't figure out how to provide a direct link, but at least the relevant pages of the Nutrition Research Council's "Nutrient Requirements of Dogs and Cats" are available on Google books (the book costs $255). This is the book on which the AAFCO (in the U.S.) and FEDIAF (in Europe) base their "complete and balanced" recommendations for pet foods.

http://books.google.com/books?id=aq...ms, and feeding and drinking patterns&f=false

This should produce search results.

The first result should be p. 23; the second result should be p. 22.

Click on the 2nd link (p. 22), so you can read the section "Diurnal Rhythms, and Feeding and Drinking Patterns."

Please note in the 2nd paragraph where it says.

Although fresh water is always recommended for both dogs and cats, the amount required varies greatly with type of diet and environmental conditions. Dogs will spontaneously drink more water per kilogram of body weight than cats (Anderson 1981), which is thought to be the result of the cat evolving as a desert animal (Chew, 1965). Cats can concentrate urine more than either dogs or humans and can rehydrate drinking seawater (Wolf, 1959). Cats are slower to initiate drinking or to drink enough for complete rehydration than dogs, who, when dehydrated, will drink enough to replenish 6 percent of their body weight in an hour compared to the 24 hours that it takes for cats (Adolph, 1947). This weak thirst drive of cats results in an intake of only about 2 mL of water for every gram of dry food ingested and in increased risk of urolith formation as compared to cats fed food containing more water, such as canned food containing 78-82 percent water. Nevertheless, both dogs and cats are able to maintain water balance when fed meat or fish containing 67 - 73 percent water, without drinking any water (Caldwell, 1931; Danowski et al., 1944; Prentiss et al., 1959). However, even cats could not maintain water balance with meat desiccated to a moisture content of 61 percent (Prentiss et al., 1959), possibly because of the volume of urine required to excrete urea....

Click to expand...

Bold, my emphasis - the parts addressing your questions.

Urolith = bladder stones (crystals).

Cats are thought to have a weak thirst drive because they evolved not to drink water, but to derive moisture from the food they eat.

tombates said:

The later would have to be true for low the moisture content of dry food to effect the drinking habits of cats. And it's this that I find hard to believe.

Click to expand...

Why is it hard to believe?

Well - anyway, "the latter" is the explanation.

So from that discussion in the Nutrient Requiremenst of Dogs and Cats, you can see that to maintain minimum adequate hydration, a cat requires the equivalent moisture of a food that is 63% moist (as at 61%, they are already dehydrated).

So for MINIMUM hydration, the math is as follows. Assuming the kibble has 10% moisture:

2.5 ounces of kibble with 10% moisture (2.5 - (2.5 x 10%)) is 2.25 ounces of the nutrition excluding water in kibble.

For 63% moisture, (2.25 ounces of kibble, divided by 100%-63% (which is 27%), the equivalent is 6.1 ounces of moist food (at 63% moisture).

6.1 ounces of moist food equivalent minus 2.5 ounces of food fed as kibble = 3.6 ounces of water.

A cat eating 2.5 ounces of kibble must drink a minimum of 3.6 ounces of water to maintain a MINIMUM hydration level. If you want your cat to have the same moisture intake of a cat eating a canned food that is 75% moisture, your cat would need to drink 6.5 ounces of water (using the same example of 2.5 ounces of dry food).

If you'd like to read the TCS article that includes this information and tips on increasing your cat's water intake, it is here: http://www.thecatsite.com/a/tips-to-increase-your-cat-s-water-intake

Hope this helped.