PSA: Do you have a fat cat? This is your solution.
Constant Readers may recall that Snape's leg was mysteriously broken some years ago and that he had surgery to fix it. Because part of his hip was removed (it's held together with ligaments, the body's duct tape), it's important that he not become overweight and stress the load-bearing capacity of his non-joint. Nonetheless, because Snape is history's greatest and most ravenous monster, he got fat. But Lily is and always has been slender. Diet food is exactly what she doesn't need. Male cats also need lots of water to avoid UTI problems.
Enter canned food. Snape lost the 3-5 pounds that put him in the roly-poly category and has not had any other health problems. (Lily is her usual svelte self.) Any cat owner with a pudgy pet should consider this.