Guinea pigs are unable to produce their own vitamin C, much like humans, and therefore need an outside source to avoid developing a disease called scurvy. To prevent this from happening, you should feed your guinea pig fresh fruits and vegetables daily. Dark leafy greens such as kale, collard greens, mustard greens and dandelion greens are particularly good sources of vitamin C, but you can also try offering sweet peppers, apples, pears, berries, broccoli, cucumbers, parsley and basil. You should feed your pig around ¼ to ½ packed cup of fresh foods daily.