I think any animal, including cats (and humans) is capable of simply gorging themselves and being lazy - especially domesticates whose instincts have been dulled somewhat, and who have food on tap. Indeed, the one instinct remaining in some is probably to eat whatever is offered, in case there is famine tomorrow. In any population, there will those who are active and fit, and those who are lazy - it's just that in the wild the lazy ones would die.
We have a skeleton in our reference collection from a cat that died of vitamin A poisoning. Its owner, a little old lady, found that it liked liver, so indulged it with liver every day. The cat must have lapped it up, despite the fact that it was been slowly killed. Excess vitamin A over a long period leads to excessive bone formation around joints, fusing them, and eventually the whole cat just seized up. Now, it can't have known why it was getting stiff, although I think many animals do have an innate ability to self medicate if they can, but it happily ate a completely unnatural diet, rather than ever turn up it's nose at the liver....
