My cat just fell out the window...

since i changed the diet of my dad's cats (he used to buy whiskas type food, now they get hills or iams) the 2 fat ones have lost weight, and the 2 thinner ones have gained weight.

The vet told me not to feed my cat Iams.

Has anyone kept their cat (other pet) to the end rather than get the vet to kill them (I find the "put to sleep/down" expression very silly). I am seriously thinking of never choosing that option but make my cat as comfortable as possible until he is ready to die.