A guinea pig's diet should center on unlimited timothy hay, which provides the fiber essential for healthy digestion and continuous dental wear. Hay should constitute roughly 80 percent of daily intake and must be available at all times.

Plain timothy-based pellets formulated specifically for guinea pigs are the second dietary pillar. Choose pellets fortified with vitamin C, as guinea pigs cannot synthesize this nutrient on their own and deficiency leads to scurvy. Offer approximately one-eighth cup of pellets per pig per day. Avoid muesli-style mixes with seeds, dried fruit, and colored pieces, which encourage selective eating and nutritional imbalance.

Fresh vegetables should be provided daily, roughly one cup per pig. Prioritize leafy greens like romaine lettuce, green leaf lettuce, cilantro, and parsley. Bell peppers, especially red and yellow, are excellent natural sources of vitamin C. Rotate vegetables to prevent calcium buildup from high-oxalate greens like spinach, which should be offered sparingly.

Fruit is a treat, not a staple. Small pieces of strawberry, blueberry, or apple once or twice a week are fine, but the sugar content makes daily fruit inappropriate.

Fresh, clean water must always be available via a drip bottle or a heavy ceramic bowl that resists tipping. Change water daily.

Avoid feeding iceberg lettuce, potatoes, onions, garlic, rhubarb, and any processed human foods. These range from nutritionally empty to outright toxic.

If your guinea pig is under six months old, pregnant, or nursing, alfalfa hay and alfalfa-based pellets are appropriate due to higher calcium and calorie content. Transition to timothy-based products once the pig reaches adulthood.