My old cat has lost a lot of weight, but it is happy and alert. Is there any high calorie cat food I can offer it to help it gain weight?

There is likely an underlying disease that has caused your cat to lose weight. The most common cause of weight loss in older cats is an overactive thyroid. The disease often also makes the affected cat overactive and hungry, but despite the increased food intake, still results in weight loss.

Cats that lose weight unintentionally should always be examined by a veterinarian.