Any ideas? Cat not eating.

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I rescued my beautiful black 2 1/2 year old kitty last November. Was told he needed to lose weight so have had him on a special weight reduction diet from the vet. He has always been massively greedy, badgering me for food and gobbling every last drop down at the speed of light at every mealtime. Have taken him back for regular weight checks only to find he has gained weight and the vet think me a liar but have since caught him going through my neighbour's cat flap to steal food...

Anyway, over the last week I have noticed him eating less and less and last weekend he ate almost nothing (but was choosing not to go out much and I'm pretty positive he hadn't been stealing food)

I took him to the vet yesterday who kept him in for the day (not eating in fat cats can be v dangerous apparently) to check him out thoroughly and run blood tests. They found absolutely nothing wrong with him except that he has lost a massive and shocking amount of weight since his last weight check. He ate a little when he got home last night but has not eaten a thing again today (and has chosen to stay in so again, not stealing food) He seems ok in himself but I'm quite worried. It is so out of character for him & I wonder when this massive weight loss will end.

Anyone got any ideas or had similar with their own cat?

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Thanks all. And for the cat weighing tip thecatneuterer