During the time that I worked for an alternative medicine veterinarian, this idea of making pets vegetarians came up more often that not. Homemade diets were always our number one choice for the pets we would see but as soon as we would mention the idea of raw meat or even preparing meat for the pet, noses were turned up.  Most of the clients that we saw were those that sought out alternative medicine for their pets because they themselves were practicing natural health and medicine for themselves and knew the benefits of this lifestyle. Unfortunately our biggest obstacle was that most of these people were vegetarian or vegan.  This made things a bit difficult sometimes (no offense to vegetarians or vegans!).  The problem being, although we often think what is good for us is good for our pets, it is not always the case and diet is one of those times.

Cats and dogs both need protein in their diet and not through the vegetarian ways (beans and nuts), they need meat.  Here is the low down on our meat eating counter-parts:

Cats - these finicky felines are even finicky in their diet - they are obligate carnivores:

"Obligate or true carnivores depend solely on the nutrients found in animal flesh for their survival. While they may consume small amounts of plant material, they lack the physiology required for the efficient digestion of vegetable matter and, in fact, some carnivorous mammals eat vegetation specifically as an emetic. The domestic cat is a prime example of an obligate carnivore, as are all of the other felids. The ability to produce synthetic forms of nutrients such as taurine in the lab has allowed feed manufacturers to formulate foods for carnivores (zoo animals and pets) with varying amounts of plant material." Wikipedia

In a nutshell - they need meat!  Tofu eating kitties will just not be healthy.  Be sure to check that the food you are providing your cat is loaded with meats.  All the veggies in cat foods seem like a great addition to the diet but in general, cats actually can't digest vegetables. A cat's diet should be at least 75% meat based protein (unless otherwise directed by your veterinarian - some diseases may constitute a smaller amount)

Dogs - our "garbage eating" companions are actually omnivores:

"Omnivores (from Latin: omni all, everything; vorare(infinitive) to devour) are species that eat both plants and animals as their primary food source. They are opportunistic, general feeders not specifically adapted to eat and digest either meat or plant material exclusively. Pigs are one well-known example of an omnivoreCrows are another example of an omnivore that many people see every day. Humans are omnivores." Wikipedia

Although this means that they are able to digest vegetables and use their nutrients, it doesn't mean that is all they should have.  Meats are high in Vitamin B (particularly B12 and folic acid) which allow the body to function correctly in day to day situations.  These B vitamins are essential for detoxifying the liver, adding energy and keeping the eyes healthy.  Dog's diets should consist of 40 to 60% meat based protein. (unless otherwise directed by your veterinarian)

So although a human's healthy lifestyle may contain a diet low in meat or lacking in meat, pet's need this ingredient in their life.