Compared to other mammals, guinea pigs lack the enzyme needed to create their own vitamin C, so you should constantly give them supplements that contain it. Low calcium Timothy hay is the essential food in a guinea pig's diet because, being herbivorous animals, they need lots of fiber for healthy digestion, and hay can provide just what they need. Thanks to their nutrients, fruits and vegetables should also be part of a guinea pig's complete diet, but because they contain sugar, they should not represent more than 20% of their daily food intake.