In the wild, guinea pigs feed on fruits and veggies. It's important to feed a guinea pig as close as possible to its natural diet. Fresh fruits and veggies are an important source of Vitamin C as well as other nutrients. Chewing on hard veggies also helps keep their teeth at a good length. If possible, buy organic fruits and veggies, otherwise wash them well to get rid of any pesticide residue. Perhaps cut down on the other food and try offering small bits of a variety of fruits and veggies.