According to Dr Pete Wedderburn, a vet writing for the Daily Telegraph, it is dangerous for cats to lose weight quickly, as this can lead to hepatic lipidosis, which is a potentially fatal liver disease.
Dr Wedderburn was contacted by an owner whose cat is obese. Their vet has put him on a special cat food diet but after a month, he still looks enormous, they wrote.
They said the cat, Ralph, becomes agitated at meal times and has even attacked his brother.
In response, the vet wrote: "A steady decrease in body weight is the aim; it may take up to a year for Ralph to reach his ideal weight. Regular visits to your vet for weigh-ins are important. You should step up his exercise by increasing play or movement around the house."
He added that the owner could also try providing frequent and smaller meals and that the other cat should be kept away at feeding time.