For humans, being vegetarian or vegan is a personal choice, and often one that a person reaches from their love and empathy for animals. If you chose to change the diet of a deer, horse, or cow to a strictly meat diet, you would not expect it to thrive since it is not designed by nature to eat meat or digest and obtain nutrition from meat. There is a trend to make carnivorous pets into vegetarians or vegans and many cannot flourish despite our good intentions.
Cats evolved quite differently from us in how they process nutrition. While they do well in arid environments getting moisture from the animals they catch and getting all their essential nutrients, their bodies lost or never developed the ability to synthesize certain amino acids from plant-based proteins as we and other animals can, nor the ability to breakdown pharmaceuticals nearly as well.
For cats, an inadequate amount of meat-based protein creates deficiencies or imbalances in certain vitamins and amino acids, among them vitamin A, prostaglandins, and the amino acid taurine.
We will look at taurine as an example.
It is an essential amino acid for cats because they require it but cannot produce it at all! Cats need high quality protein to provide them the taurine required to build and repair tissue, have healthy heart muscle, healthy eyes, good platelet function and protect skeletal muscle function. Taurine which is found in meat, fish, and dairy is not found in sufficient quantity in plant proteins to support a healthy cat.
Taurine deficiency severely affects cats over time and can lead to blindness, dilated cardiomyopathy, the inability to digest fats, poor coat health, and reproductive issues among other things.
Supplements are certainly a consideration, but then you are adding synthesized chemicals to replace what is in a feline appropriate meat-based diet. In addition to what your cat requires, a supplement may contain additional vitamins etc. that are not needed, potentially causing a detrimental effect.
If you choose to not feed meat to your cat, then there is a definite need to check health values through bloodwork more frequently, as physical signs of a problem may show up too late to be corrected. If you choose to use supplements, not all are created equal in value or safety. Choose one that has the USP symbol on the bottle. These have been tested for standards of strength, quality, and purity by an independent non-profit organization.
Bottom-line, our cat friends are not designed to be vegetarians or vegans, and their body’s ability to function in that capacity long-term, is not provided for by nature.
*Sources WebMD and Scientific American