Guinea pigs can eat oranges, apples, blueberries and bananas. However, fruit is high in sugar so fruit should only be given to guinea pigs once a day as a special treat.
With fruit given only as an occasional treat, the main parts of a guinea pig's diet should consist of pellets, hay, fresh vegetables and plenty of fresh water. Guinea pigs should be given about 1/8 cup of good-quality commercial pellets once a day. They also should be given an unlimited amount of timothy hay each day. Hay aids in guinea pigs' digestion and helps control the growth of their teeth.
Fresh vegetables are an important part of a guinea pig's diet. A guinea pig should eat the equivalent of 1 cup of fresh vegetables once a day. Suggested vegetables are romaine lettuce, kale, spinach and parsley. Occasionally, other vegetables such as carrots, zucchini and sweet potato can be offered, but these should be given no more than once or twice a week. Commercial treats marketed for guinea pigs are high in artificial sweeteners and have no nutritional value, so limit or avoid a guinea pig's consumption of these items. For a special treat, guinea pigs can have rolled oats along with their pellets.