Obesity is a serious problem in cats which can lead to premature death. Sadly it is the most common nutritional disorder seen in cats. Approximately 40% of cats in the USA are obese.
A cat is considered to be obese if it's weight is around 20% over the ideal weight.
What are the causes of obesity in cats?
The most common problem is overfeeding. If the calories consumed exceed calories burned then the cat will put on weight.
Dry food diets are extremely popular these days and can lead to obesity. The problem lies in the fact that the cat by nature requires a high protein diet and most dry foods are high in carbohydrates. Cats synthesize protein and fat to use as energy, carbohydrates are converted to fat.
Ageing - As the cat moves into his senior years his metabolic rate slows down, joints may become more painful and the senior cat will be less active. A slowed down metabolic rate and decrease in activity can both lead to obesity in cats.
What problems does obesity cause in cats?
How can I tell if my cat is obese?
It is always best to ask your veterinarian if he/she believes your cat is obese. Cats come in all shapes and sizes and therefore it is difficult to determine a standard weight which covers all cats and their differences. For example, a Singapura would be expected to weigh less than a Maine Coon.
However, a rough indicator is to feel along the side of the cat. You should be able to feel the individual ribs.
Stand over the cat, you should be able to see a waist, and have an hourglass figure.
There is a noticeable bulge on either side of the tail head 
What will my vet do?
Your veterinarian will perform a physical examination of your cat. He/she will also want to run blood tests to rule out a medical cause of obesity such as an underactive thyroid (hypothyroid).
Treatment of obesity in cats:
- Weight loss is something which requires close veterinary supervision. If it is done too quickly it can lead to hepatic lipidosis, which is life threatening. Therefore NEVER attempt a weight reduction diet on your own.
- Increase exercise: There are plenty of interactive toys on the market which will encourage your cat to exercise.
- Decrease caloric intake: There are prescription diets on the market specifically for cats, your veterinarian will be able to recommend the right diet for your cat. As stated above, it is now well known that many dry foods contain excessive quantities of carbohydrates which can lead to obesity. So switching to either canned or raw food should be discussed with your veterinarian. Weight loss needs to occur gradually to avoid hepatic lipidosis.
- No treats: It's easy to slip the odd treat to your cat, but this needs to be stopped completely.
- Feed 4-6 small meals per day instead of filling the bowl and leaving it down for your cat to graze on.
- If the cause is medical and not diet related, treating the condition should hopefully resolve the obesity.
 The Cornell Book of Cats