You might ask: dogs can go vegetarian without much of a problem. Why not cats?
Cats are among the pickiest eaters on the planet; they require certain proteins and compounds found in animal flesh to survive. Their digestion systems are not like ours (or dogs', for that matter); ours are fairly flexible, enabling our livers to synthesize essential nutrients from plant proteins. "Cats can't do that with vegetable sources," says Colleran. Kornreich concurs: "Certainly cats have problems producing essential amino acids" from non-animal materials, he told me.
The one proper study on this topic, from 2006, appeared in the Journal of the American Veterinary Medical Association, and measured the health (mostly via questionnaire) of 34 cats fed a vegan or vegetarian diet for at least one year, and 52 cats fed a conventional diet for at least one year. Seventeen of the vegetarian or vegan cats were given a physical. Three of those were found to have low taurine levels. (Taurine deficiencies, according to that study, can cause a weak heart that has trouble pumping blood, and weak eyesight.) Makers of vegetarian cat food usually include taurine supplements, but these cats tested low anyway. Another problem: 35 percent of the cats surveyed were outdoor cats, and could have been supplementing their vegetarian home diet by hunting outside. (The three cats that tested low were exclusively indoor cats, fed only vegetarian food.) And, note the authors of the study, no research has been done into the long-term effects of a vegetarian feline diet.