Nutritional Quirks of the Cat
A mantra that was repeated over and over again when I attended veterinary school was "Cats are not small dogs." This may seem obvious to both dog and cat owners, but it was a worthwhile reminder to veterinary students who are faced with absorbing mountains of data and may be tempted to make generalizations to make the Sisyphean task more manageable. The saying is especially true when it comes to nutrition.
Dogs are omnivores. I've seen some dogs thrive on high protein diets that consist primarily of animal-based ingredients while others who eat vegan do just as well. Of course, every dog is an individual and what works for one may be detrimental to another, but looking at them as a group, meeting the nutritional needs of dogs is really not that hard.
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Cats are different (in oh so many ways). They are carnivores and as such have some unique physiological quirks that make providing them with a complete and balanced diet more challenging. First of all, cats need to eat a lot of protein. The National Research Council's (NRC's) recommended allowance for protein in adult cats is 50 g per 1, 000 calories of metabolizable energy (ME) in the diet (a measurement that takes into account how much volume of a particular food a cat should be eating due to its size, activity level, etc.). Compare that to the recommendation of 25 g per 1,000 calories of ME for adult dogs. It can truthfully be said that on average, cats need to eat about twice as much protein as do dogs.
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The significance of protein continues down to the level of amino acids, the compounds that combine to form larger protein molecules. Dogs can make the amino acid taurine by modifying other amino acids that they get in their diets. Cats do not have the physiological mechanisms to do this and therefore require that taurine be directly supplied by their food. The NRC recommends that cats get 0.10 g of taurine per 1,000 calories of ME, and makes no recommendation for dogs. Dogs are also much more efficient at manufacturing the amino acid arginine in comparison to cats. According to the NRC, dogs should receive 0.88 g of arginine per 1,000 calories of ME while cats need 1.93 g, more than twice as much.
The nutritional quirks of cats are not limited to protein. Arachidonic acid, a type of fatty acid, is also important. Again, dogs can make their own, but cats need to get it from their diet. The NRC has set the feline recommended allowance at 0.015 g per 1,000 calories of ME. Cats also have special requirements when it comes to vitamins A, B, and D.
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What many of these nutrients have in common is that they are found in high concentrations in animal tissues but not plants. Not too surprising, eh? Why would carnivores invest in physiological mechanisms to make nutrients that they should get in ample amounts from their diets? While it is very difficult for owners to determine whether or not a particular food contains 1.93 g arginine per 1,000 calories of ME (for example), a quick glance at the ingredient list can tell you whether or not it is made primarily from animal-based ingredients.