A: Yes. Indoor cats (which is what we're exclusively talking about here) will not, despite a common myth, find a way to get enough exercise on their own by chasing shadows and attacking shoelaces.
Cats, as we know, sleep a lot -- around 14 hours a day -- and in an indoor environment with no prey, people, or other cats around, they have a tendency to become sedentary, much like ourselves. Now, inactivity is natural for cats; in the wild, their behavior includes lots of downtime. But the domestic cat is fed by its owner, taking away that healthful and instinctive hunting activity. So do you need to, in part, assume the role of personal trainer? "Absolutely," says Dr. Elizabeth Colleran, former president of the American Association of Feline Practitioners and the owner of two west coast veterinary clinics focused on cats.
Without it, two big problems can arise, according to Colleran: Obesity and boredom.
A short list of potential effects of feline obesity, from a 2006 article in the Journal of Nutrition: Orthopedic disease, diabetes mellitus, abnormalities in circulating lipid profiles, cardiorespiratory disease, urinary disorders, reproductive disorders, neoplasia (mammary tumors, transitional cell carcinoma), dermatological diseases, and anesthetic complications.Then there's arthritis. Says the Cornell Feline Health Center: "Older cats frequently become less agile as arthritis develops and muscles begin to atrophy." They recommend "regular moderate play" to stave off arthritis.