Whats the best food for my cat who possibly has irritable bowel (and getting treatment for giardia)

One of my cats, Davis, had his (9 year) senior check up and blood work. His general exam and blood panel look great. Davis has appeared to have a sensitive stomach at times over the years. He eats, drinks, has clear eyes, normal behavior and a great coat.

He has bloody, loose stools and has lost 1 pound in two months. He tested + for giardia 5 days ago. Our vet gave him an injection to calm his intestine and sent home 5 doses of anitbiotic. After 3 days of medication, there was still blood in his stool. The vet was surprised the injection didn't help. He said to check in on Monday and let him know how Davis is doing.

The vet is wondering if Davis may have irritable bowel and possibly need prescription food.

Aside from the Giardia we are treating, do you have ideas on what food is best for his sensitive system?

I free-feed 6 cats. So they would all be on the same food. Two are asthmatic and get flovent daily. Otherwise, we are all in great shape. ; )

I am having almost the same problem with my cat Jasmine at the moment except she is only 6 months old. She has been treated for Giardia 3 times in the last month and it just kept coming back. She is still having diarrhea but her most recent fecal exam finally came back negative for giardia. The vet thinks she may have IBS.

The most frustrating thing about Giardia is that it is highly contagious to humans and other animals! Davis needs to be separated from his buddies until he is better. We also had to disinfect her box daily and wipe her paws and butt every time she went so that she didn't reinfect herself when grooming.