Guinea pigs can eat a wide variety of produce and are most happy when provided with leafy greens and colorful bell peppers. Carrots, apple slices, berries and other fruits should be used sparingly due to their high sugar content. High-quality hay and pellets round out the guinea pig diet.
Guinea pigs, like humans, cannot create their own vitamin C and require it in their diet to prevent scurvy. Many fruits contain vitamin C, and bell peppers and some dark greens are rich in the vitamin. There are also fortified pellets that contain the nutrient, in order to ensure that a guinea pig gets the required daily dose. Avoid water-soluble vitamin C drops, as it can be difficult to ensure that a guinea pig drinks enough to prevent health issues.
There are a few fruits and vegetables that should be avoided. Broccoli can cause stomach upset, and iceberg lettuce can trigger diarrhea due to its high water content. Grapes, kale, spinach and mustard greens can be toxic in significant amounts and may trigger bladder stones that could be life-threatening. While celery may seem like a suitable treat, it has no real nutritional value for guinea pigs and the long strings can present a choking hazard.