Sudden dramatic change in attitude/behavior in my newly adopted cat

Hello! This community has been so helpful in answering the many questions I'm having as a first time cat owner, so I thought I'd bring you guys another.

I adopted my 8-year-old cat, Skimble, from the local humane society two weeks ago. I picked him because he was affectionate right off the bat. He immediately rolled onto his back for belly rubs the minute we opened the door to the little room they had him in. When I first brought him home, he kept that affectionate behavior. He was always excited to meet new people. He purred constantly and loved being held and pet. He greeted me at the door when I came home. He's always had a bit of a love-nip problem. He would nip when he was being pet or held or to signal when he was done with being pet, but it never hurt and he never drew blood.

About a week ago I started to change the food I was giving him because I ran out of the food they had him on at the shelter. He had been eating Purina Pro Plan Chicken and Rice there and they gave me a sample of Hill's Science Diet Adult Cat, so I figured I wouldn't let that go to waste. My intention was to feed him the Science Diet until it ran out and then switch him to a mix of Nature's Variety Instinct wet food and Orijen Cat and Kitten dry food, both of which I already have. He doesn't seem to like the Science Diet very much. I feed him a quarter cup morning and night, but he never finishes it. He always finished the Purina and ate it at such a rate that I ended up splitting the half cup he eats daily into 3 or 4 feedings. (I'd really rather not go back to the Purina as it's not a very high quality food and I'd like to get him on a low-grain and more natural diet.)

I've noticed a pretty significant change in behavior since switching. He's much more touchy. He doesn't like to be pet as long as he used to. He doesn't purr as much and he's not nearly as excited to visit with the guests that come to our apartment. He spends a lot more time under my bed than he used to. His bites have also gotten a lot harder and more painful (although the little nips he does when he's happy don't hurt). It also seems like he changes moods incredibly quickly. A few minutes ago he was happy and purring while my roommate and I pet him. I picked him up for some cuddles and he continued purring and started kneading my shoulder. Then, all of a sudden, he started meowing and biting my shoulder hard. I put him down on the floor and he started attacking and biting my leg, ears pinned back.

I'm very confused and disheartened by this change in behavior. I would like to attribute it to the food change, but I'm afraid it's because he's settling into his new home here and this is actually his real personality. I'd really like to believe that my sweet and affectionate cat is still in there. Could a change in food really cause this? 

Thank you for your help in advance. I really need it.

Cassafrass710 said:

 
Hello! This community has been so helpful in answering the many questions I'm having as a first time cat owner, so I thought I'd bring you guys another.

Thank you for your help in advance. I really need it.

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This could be really easy to fix.  He doesn't like the food change and he's unhappy and showing it behaviorally with you.  I would put him back immediately on the original food.

StephenQ is right.  If you want to change food it must be switched over slowly.  For now I would just go back to the old food.  Thank you so much for adopting an older cat & giving him a nice home.  Everyone seems to want kittens which is usually a mistake for first time cat owners.

Science Diet isn't actually much (or any) better than the Purina he was on before. Vets will keep stock of Hill's food because Hill's tends to sponsor them, but it isn't a good quality food. All of my cats, whether my own or fosters, have hated Science Diet. Your cat is hungry, and he's acting out to show you that he needs more food.

I'd advise that you switch back to his favorite food, and then slowly introduce a better quality new food if you wish. But remember, slow is key, especially with older cats, who aren't always going to be as open to diet changes as a younger cat might be.

Thank you for all your great replies! (And sorry for my delayed response.) I ended up switching him over to the high quality food I bought, and he seems to like the dry Orijen quite a bit, but not the wet stuff. I'm still on the hunt for a wet food that he likes and maybe a slightly cheaper dry food to have on hand as well, but he seems to be doing better behaviorally now that he's actually eating something. Thank you for all your suggestions!