Originally Answered: Why/how bland food helps with gastroenteritis (vomit/diarrhea) in cats?
Thanks for your question and I hope your cat Boba is feeling better.
Many vets recommend chicken and rice as a short term solution for vomiting/diarrhoea as it's a bland diet. When a cat has an upset tummy, there is usually inflammation of the gut, increased peristalsis and nausea. A bland diet is easier to digest than richer foods and will help the tummy get back to normal quicker. If cats are feeling nauseous they may not want to eat their normal food but they may eat chicken so it helps to keep them eating too.
There are bland commercial foods available that you can feed instead of chicken and rice if you prefer, these are called 'gastrointestinal' or 'sensitive' diets and are available in most vets or pet stores. I often recommend these in cases that will need a bland diet for more than 2-3 days as they're complete diets and have plenty calories. This will prevent your cat from losing weight.
It doesn't mean that there's anything wrong with your cat's normal food, it's just that a bland diet helps their tummy to recover faster. In some cases, cats continue eating their normal food during a mild upset tummy and they recover fine. But it's important not to feed any treats or additional food alongside their normal diet. If their normal cat food is a new type, brand or flavour it may be that this is the cause of a tummy upset so in this case I recommend to feed a bland/gastrotinestinal diet.
I hope that helps,