Guinea Pig Diet Composition
| Food Type | Amount | Frequency | Notes |
|---|---|---|---|
| Timothy Hay | Unlimited | Daily — always available | 80% of diet, essential for GI & teeth |
| Bell Peppers | 1-2 rings | Daily | Best vitamin C source |
| Leafy Greens | 1 cup per pig | Daily | Romaine, cilantro, parsley |
| Vitamin C Pellets | 1/8 cup per pig | Daily | Check manufacture date — C degrades |
| Carrot | 1-2 small pieces | 2-3× weekly | High sugar — limit |
| Fruit (apple, berry) | Small piece | 1-2× weekly | Treat only — high sugar |
Guinea Pig Diet Decision Flow
Guinea Pig Diet Do's and Don'ts
| Do | Unlimited timothy hay — 80% of diet |
| Do | Daily bell pepper for vitamin C |
| Do | 1 cup fresh leafy greens per pig daily |
| Do | Timothy pellets — 1/8 cup per pig (check date) |
| Do | Fresh water changed daily |
| Avoid | Alfalfa hay for adults — too high calcium |
| Avoid | Iceberg lettuce — no nutritional value, causes GI upset |
| Avoid | Vitamin C drops in water — degrades too fast to be reliable |
Guinea pigs cannot synthesize vitamin C. This single metabolic fact -- shared only with humans and a handful of other mammals -- makes guinea pig nutrition categorically different from other small pets. A guinea pig receiving insufficient vitamin C develops scurvy within 2-4 weeks: swollen, painful joints; rough coat; anorexia; reluctance to move; and eventually bleeding gums and weight loss. The dose is small and the fix is simple, but owners who skip it create preventable suffering.
Hay: The Non-Negotiable Foundation
Timothy hay (and other grass hays -- orchard grass, oat hay, meadow hay) should make up 75-80% of a guinea pig's diet by volume. Hay provides the indigestible fiber that maintains the delicate cecal bacterial balance essential for guinea pig digestion. Without constant hay access, cecal dysbiosis develops rapidly, causing diarrhea, bloating, and potentially fatal enteritis. The chewing action of hay also wears down continuously erupting teeth -- malocclusion (misaligned teeth causing pain and anorexia) is dramatically less common in hay-fed guinea pigs than those fed predominantly pellets and soft foods.
Alfalfa hay is appropriate for guinea pigs under 6 months old (higher calcium and protein for growth), but adult guinea pigs should eat grass hay exclusively. The higher calcium in alfalfa contributes to bladder sludge and uroliths (bladder stones) in adults. Unlimited hay access 24 hours daily is the standard; hay should never run out.
Vitamin C: Dose, Sources, and Delivery
Guinea pigs require 10-30 mg of vitamin C daily (higher during pregnancy, illness, or stress -- up to 50 mg/day). The most reliable sources: fresh vegetables high in vitamin C, particularly bell peppers (red bell pepper is the highest common source at approximately 190 mg per 100g), leafy greens (kale, parsley, cilantro, dandelion greens), and fresh herbs. A single tablespoon of red bell pepper daily provides adequate vitamin C for most adult guinea pigs.
Vitamin C in water drops is the least reliable method -- ascorbic acid degrades rapidly in water, particularly in light. A vitamin C solution that is 12+ hours old has lost significant potency. If water supplementation is used, change the solution twice daily and use opaque water bottles. Vitamin C in pellets also degrades with storage -- pellets more than 90 days past manufacture date may have inadequate vitamin C. Best practice: rely on fresh vegetables as the primary vitamin C source, supplement pellets as secondary, and use water supplementation only as a backup.
Pellets
High-quality guinea pig pellets (Oxbow Cavy Cuisine, Small Pet Select) with added vitamin C (ascorbic acid) should comprise 1/8 to 1/4 cup per guinea pig per day. Adult guinea pigs should eat timothy-based pellets (not alfalfa-based, which has excessive calcium). Pellets are a supplement to hay, not the primary food. Common mistake: over-feeding pellets because the guinea pig eats them preferentially over hay. Pellets are calorie-dense; pellet overconsumption reduces hay consumption, disrupts cecal fermentation, and causes obesity.
Fresh Vegetables and Foods to Avoid
Appropriate vegetables: bell peppers (all colors), leafy greens (romaine, leaf lettuce, kale -- not iceberg, which has minimal nutrition), cucumber, zucchini, small amounts of carrot, fresh herbs (basil, dill, cilantro, parsley). Feed 1-2 cups of varied vegetables daily per guinea pig. Introduce new foods gradually (one new food at a time, small quantity) to prevent digestive upset.
Foods to avoid: iceberg lettuce (near-zero nutrition, high water content causing diarrhea), potato (high starch, low nutrition), onion and garlic (toxic), rhubarb (oxalic acid toxicity), beans and lentils (flatulence and digestive upset), fruit in large quantities (high sugar causing cecal dysbiosis and obesity -- limit to 1-2 tablespoons per day maximum of low-sugar fruits like blueberries), cabbage and broccoli in excess (gas-producing). Dairy, meat, and eggs should never be offered -- guinea pigs are strict herbivores with no ability to process animal protein.
Sources: NRC Nutrient Requirements of Laboratory Animals, 4th edition (guinea pig vitamin C data); Quesenberry and Carpenter, Ferrets, Rabbits, and Rodents, 3rd edition; ASPCA guinea pig toxic food list; Harkness and Wagner, Biology and Medicine of Rabbits and Rodents, 4th edition; Oxbow Animal Health nutritional research publications.