Guinea pigs have unique dietary requirements particular to just them.
80-90% of their diet needs to be good quality grassy hay (meadow hay or non-lucern based hay) for good gut and dental health. Spending hours chewing hay also keeps them occupied and prevents the development of boredom issues. A variety of fresh vegetables should make up the rest of their diet (10-20%). Fruit should only be given in small amounts, and considered a treat due to the high sugar levels which can cause digestive issues and obesity.
Contrary to popular belief, it is not necessary to feed commercial pellets as long as other requirements of hay and vegetables are met. However, if they are to be fed, they should constitute up to only 10% of their diet. Guinea pigs need to be fed a specific guinea pig pellet and not rabbit pellets.
One of the most important things to consider in a guinea pig’s diet is the vitamin C content. Guinea pigs (like humans) are unable to manufacture this important vitamin within their body and rely on obtaining vitamin C solely from their diet. Vitamin C deficiency (commonly called scurvy) is a serious disease where they may become weak, innappetant, lose weight, have a poor coat, delayed wound healing, swollen joints, lameness, diarrhoea and death (if untreated).
The daily vitamin C requirement for an adult guinea pig is around 20-25mg (and up to 30-40mg in pregnant guinea pigs). It is necessary to supplement with vitamin C because a diet high in hay is actually low in vitamin C. Moreover vitamin C degrades in commercial guinea pig food after 90 days.
The easiest way to provide enough vitamin C is to ensure your guinea pig eats fresh vegetables. Below is a list of some fruit and vegetables and their Vitamin C content. Remember not to feed too many fruits, due to their high sugar content.
Amount needed to provide 25mg Vitamin C
¼ - ½ orange
Chewable Vitamin C tablets that are given directly to your guinea pig can also help ensure they get enough, if fresh fruit and vegetables aren’t readily available.
Supplementation of vitamin C into the water is not recommended as it degrades in sunlight and can change the taste of the water.