The best diet for your guinea pig is one specifically formulated for guinea pigs.
Hay and pellets
Guinea pigs should have hay available at all times. The best hay to feed your guinea pig is timothy or oat hay. Alfalfa hay is too rich in calcium and protein and should be given only sparingly. Fortified guinea pig pellets can be fed in small amounts in addition to hay.
Guinea pigs have a unique requirement for vitamin C. Adult guinea pigs require 20-25 mg/day while pregnant guinea pigs need 30-40 mg of vitamin C per day. Since they usually do not get sufficient quantities from their pellets and hay, their diet needs to be supplemented with vitamin C. Therefore, a variety of fresh vegetables rich in vitamin C should be offered daily. Excellent choices would include red pepper, kale, turnip greens, mustard greens, other dark green leafy vegetables and sugar snap or snow peas, . It is always best to feed a mixture of vegetables, instead of just one kind, to prevent mineral imbalances. Feed only small amounts of fruit - papaya, strawberries and kiwi are some of the fruits with the highest vitamin C content.
Especially for guinea pigs who may not be eating well, vitamin C supplementation in the form of drops or tablets should be provided. Generally, adding vitamin C to the drinking water is not advised for two main reasons. First, if the guinea pig does not like the taste of the supplement, he may not drink the water, and could become dehydrated. Secondly, vitamin C is not stable, and will degrade in light, heat and moisture. There are some human pediatric vitamin C products available as tablets and syrups, that can be given directly to the guinea pig. The Oxbow company has a vitamin C tablet specifically for guinea pigs (GTN-50C®) that most seem to enjoy.
When to feed
Unlimited amounts of grass hay (they will eat a lot!) and fresh water should be available at all times. Small amounts of quality pellets (around 1/8 cup) can be given. Provide fresh vegetables several times a day, always removing any leftovers from the previous feeding. It is a good idea to monitor weight with a small gram scale so you will know if your guinea pig is losing or gaining weight.
Guinea pigs develop dietary preferences very early in life, so it is important to acquaint them with many types of food while they are young. Outside of this basic, staple diet of hay, pellets, and fresh vegetables and fruits, anything else should be considered a treat and fed sparingly. This diet will ensure a long, trim, and healthy life for your favorite pig.