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2 posts from October 2006


Dry Cat Food: Should Carnivores Eat Corn Meal?

Feline nutrition is truly a fascinating topic. I could talk forever about how cats evolved, and how important an understanding of their evolution is to feeding them properly. But for now, let's just focus on the topic of dry cat food.

Domestic cats evolved in the desert. Their ancestors lived entirely on the prey that they caught, and got their liquids from the bodies of the prey as well. Needless to say, they had to develop the ability to exist without much water. And they did: cats are extremely good at concentrating their urine so as to conserve water. A cat's urine is much more concentrated (meaning it has a lot less water in it) than a dog's or a person's, for example. They also have a less sensitive thirst mechanism, since living in the desert there was no point to going around feeling thirsty all the time. So cats don't get as thirsty as other animals when they are dehydrated either. (That's why older cats with kidney problems often need their owners to give them fluids under the skin).

How does this connect to feeding them dry cat food?

Well, cats who eat dry cat food take in a lot less water than cats who eat moist food. Because they evolved to get liquid from their food, they don't make up for the lack of water in the food by drinking enough to compensate. So their urine becomes very very concentrated, and guess what - this causes crystals to form (remember chemistry class? Or think how you can only dissolve so much salt or sugar in a little water- if there is not enough water, the crystals don't dissolve).

So cats who eat dry food are more likely to form crystals and stones in their bladder, and in boy cats who don't have a very big opening (ahem), this makes them more likely to develop a life-threatening urinary obstruction. Ouch!

What about corn meal? Well, dry cat food is often composed mainly of corn meal. But wait, cats are CARNIVORES. Their bodies evolved to utilize animal proteins, not grain. They are not designed to run on carbs, and their bodies don't know what to do with all the carbs in dry the carbs are converted to fat, and voila!  before you know it you've got a fat, potentially diabetic cat on your hands. Corn meal is very inexpensive, so it's a nice cheap ingedient to put in dry cat food, but it is a completely unnatural diet for a cat.

The solution? Ideally, cats should be feed non-dry food (such as canned, pouch, or a <em>balanced</em> homemade diet) at specific mealtimes, such as twice a day. Another problem with dry food is that carnivores also aren't designed to nibble constantly, and this may also contribute to the development of diabetes since cats who do this need to manufacture a lot more insulin every day. You need to be careful though- cats can get very upset if you change their diet, and a hunger strike can lead to liver problems. It's important that cats like their food. The more non-dry food in their diet, the better, but plenty of cats live long, happy lives on dry food, so if your cat is addicted, don't despair!


Thirsty Dogs: Canine Cushing's Disease

Cushing's disease, more properly called hyperadrenocorticism, is a common hormonal condition in dogs. It occurs in people too, although it is more rare in humans.

The disease is caused by an excessive amount of hormones produced by one or both of the adrenal glands, most commonly a hormone called cortisol. Dogs with this problem can have a variety of symptoms, including increased  thirst and urination, hunger, skin or urinary tract infections, a pot belly, thin or darkened skin, and panting. They can develop diabetes or high blood pressure, as well as other serious health problems. The symptoms and health consequences will continue to worsen over time unless the condition is treated.

What causes canine Cushing's disease?

To explain what causes canine Cushing's disease, I need to first tell you a little about how the body works. Dogs (and people) each have 2 adrenal glands, one by each kidney. The adrenal glands produce a number of hormones, including cortisol. Cortisol may sound familiar to you because we use related cortisone-type medications for many purposes in human and veterinary medicine; cortisol is the natural hormone usually produced by the body in minute amounts. The body needs cortisol to remain healthy, but too much of it, whether produced by the body or given as a medication, can have side effects.

The "boss" of the adrenal glands is another gland called the pituitary gland, located at the bottom of the brain. The pituitary gland tells the adrenal glands how much cortisol to manufacture at any given time. Therefore, Cushing's disease can be caused by a malfunction of either the adrenal glands themselves, or their boss the pituitary gland.

In most dogs with Cushing's disease, it's the pituitary gland's fault. The pituitary gland develops a little tumor (technically a small brain tumor), and this tumor tells the adrenal glands to make too much cortisol. They obediently follow instructions, and then the dog suffers from the effects of too much of the hormone. Both adrenal glands become enlarged from all the extra work they are doing. In some dogs, the pituitary tumor becomes large enough to cause neurological signs.

In a minority of dogs, one of the adrenal glands develops a tumor, and it's this adrenal tumor that produces too much cortisol. In this case, one adrenal gland will be big, and the other one will be small, since it has nothing to do with its buddy doing all the work. A skilled ultrasonographer can perfom an ultrasound exam of the adrenals, and see which situation is occurring in a particular dog. It is very important to determine which type of Cushing's disease a dog has, pituitary or adrenal, because the treatment is different.

If there is a tumor on the adrenal gland the best plan is to remove it surgically if safely possible. These adrenal tumors are malignant about half of the time. If it is the pituitary gland at fault, most dogs are treated with medication  that decreases the adrenal glands' production of cortisol (such as a drug called Lysodren or mitotane; or another called trilostane), since surgery on the pituitary gland is very difficult due to its location under the brain. This surgery is being developed for dogs however, so stay tuned. Some dogs actually require radiation therapy if their enlarged pituitary gland is causing trouble.

Therapy of Cushing's disease is very rewarding but can be quite involved, so many general veterinarians choose to refer these patients to a veterinary internal medicine specialist for treatment.

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