Guinea pig diet is one of the most straightforward in the small pet world — and one of the most frequently wrong. The commercial pellet diet, sold as “complete,” is not. Guinea pigs are one of the few mammals besides humans and non-human primates that cannot synthesise their own vitamin C. They depend entirely on dietary sources, and a guinea pig on an inadequate vitamin C intake develops scurvy-like symptoms within 2–3 weeks. The other common dietary error is underfeeding hay — which makes up 80% of the correct diet and is the one thing most owners feed least. This guide covers the exact proportions, the fresh food rotation, the daily vitamin C requirements, and the complete toxic food list.
Guinea pigs need unlimited timothy hay (80% of total diet by volume), 1 cup of fresh vegetables per day per guinea pig (focused on vitamin C-rich leafy greens), and a small amount of pellets (1/8 cup per day, hay-based, plain — no seeds or dried fruit). Vitamin C requirement is 10–30 mg per day (higher during illness or pregnancy). Fresh bell pepper and leafy greens provide this reliably. Vitamin C degrades rapidly in water — do not supplement via water bottle. Hay must always be available — it is the digestive and dental health foundation of the diet.
The Hay Foundation — Why 80% Is Not an Exaggeration
Guinea pig digestive and dental anatomy makes hay non-negotiable:
Digestion: guinea pigs are hindgut fermenters. Their digestive tract requires constant movement of fibrous material to function. Without adequate hay, gut motility slows, which can progress to gastrointestinal stasis (similar to rabbit GI stasis — a life-threatening emergency). See our Rabbit GI Stasis guide for the signs that apply equally to guinea pigs.
Dental health: guinea pig teeth grow continuously throughout life (hypsodont dentition). Chewing fibrous hay keeps teeth worn down correctly. A guinea pig on an inadequate hay diet develops dental malocclusion — overgrown molars that cut the tongue and cheeks, cause drooling, weight loss, and eventually the inability to eat. Dental malocclusion is the leading cause of deterioration and euthanasia in guinea pigs and is almost entirely diet-preventable.
Hay Type Comparison
| Hay Type | Protein % | Calcium % | Appropriate For | Notes |
|---|---|---|---|---|
| Timothy Hay | 8–11% | 0.4–0.6% | All adult guinea pigs as the primary hay | Gold standard; most widely available |
| Orchard Grass | 7–10% | 0.3–0.4% | All adults; good rotation alternative | Lower calcium; good for stone-prone pigs |
| Meadow Hay | 6–10% | Variable | All adults; variety | Mixed grasses; provides dietary variety |
| Alfalfa Hay | 15–18% | 1.2–1.5% | Juveniles under 6 months, pregnant/nursing sows | Too high in calcium and protein for adults — causes bladder sludge |
| Botanical/Herb Hay | Variable | Variable | Supplementary only | Chamomile, rose, lavender additions — enrichment, not a primary hay |
Rule: adult guinea pigs get timothy, orchard grass, or meadow hay as primary. Alfalfa only for young guinea pigs and pregnant/nursing females.
Vitamin C — The Critical Requirement
Guinea pigs have a genetic mutation in the enzyme (L-gulonolactone oxidase) that enables most mammals to synthesise vitamin C from glucose. Without dietary supplementation, they cannot maintain normal connective tissue, immune function, or wound healing.
Daily requirement: 10–30 mg per guinea pig per day. Pregnant, nursing, growing, or sick guinea pigs need the higher end (25–30 mg/day).
Signs of vitamin C deficiency (scurvy):
– Rough, bristled coat
– Reluctance to move; crying when touched (joint pain from subcutaneous bleeding)
– Swollen joints
– Poor wound healing; spontaneous bleeding from gums
– Weight loss, lethargy
– Timeline: symptoms appear within 2–3 weeks of inadequate intake; critical illness within 4–8 weeks
Vitamin C Content of Common Foods
| Food | Vitamin C per 100g | Notes |
|---|---|---|
| Red bell pepper | 128–190 mg | Highest common source; universally accepted by guinea pigs; feed daily |
| Green bell pepper | 80–120 mg | Slightly lower than red but still excellent |
| Kale | 93 mg | Feed 2–3× per week; high calcium — rotate with lower-calcium greens |
| Parsley | 133 mg | Very high vitamin C; high calcium — use as a supplement, not a daily staple |
| Strawberry | 59 mg | Moderate C; high sugar — 1–2 per week maximum |
| Broccoli | 89 mg | Good vitamin C source; feed 2× per week; some pigs experience gas |
| Romaine lettuce | 4 mg | Low vitamin C; good volume leafy green but not a C source |
| Iceberg lettuce | 2 mg | Nutritionally poor; high water content; not recommended as a staple |
Why not vitamin C drops in water? Vitamin C (ascorbic acid) is unstable in water, especially in light. A bottle of vitamin C-supplemented water loses most of its activity within 2–4 hours of preparation. Providing fresh C-rich vegetables is far more reliable than water supplementation.
Supplementation for sick or recovering guinea pigs: veterinary oral vitamin C supplementation (ascorbic acid tablet dissolved in a small syringe of water, given directly) provides a precise dose. Discuss with a vet if a guinea pig is showing signs of deficiency.
Fresh Vegetables — The Daily Rotation
Volume: approximately 1 cup of mixed fresh vegetables per guinea pig per day. Split into two servings if possible.
Daily Staples (every day or 5–7 days per week)
- Red or orange bell pepper — primary vitamin C source; most guinea pigs eat it eagerly
- Romaine or green leaf lettuce — volume leafy green; well-tolerated daily
- Cucumber — hydrating; well-tolerated; low nutritional density but adds variety
Regular Rotation (3–4 days per week)
- Zucchini — well-tolerated, low calcium, mild
- Carrot — moderate sugar; feed in small amounts; very popular with most guinea pigs
- Kale — high C and nutrients but also high calcium; rotate with lower-calcium greens
- Cilantro — very high in vitamin C; excellent addition 3× per week
- Parsley — very high C but also high calcium — use as a supplement, not daily
Occasional Treats (1–2 times per week)
- Strawberry, blueberry, or raspberry — high sugar; small amounts only
- Broccoli — good nutrition but can cause gas in some guinea pigs; test with a small amount first
- Tomato — moderate C; remove seeds and green parts (leaves and stems are toxic)
Pellets — Less Than Most Owners Feed
Commercial pellets are a vitamin and mineral supplement to a hay-and-fresh-food diet, not the foundation of it. The correct amount is small.
- Amount: 1/8 cup (approximately 30g) per guinea pig per day
- Type: plain timothy hay-based pellets without seeds, dried fruit, coloured pieces, or “treats” mixed in. Oxbow Essentials Guinea Pig Food is the most widely recommended.
- Avoid: muesli-style mixes with seeds and dried fruit — guinea pigs select the high-sugar components and leave the nutritionally complete pieces, resulting in selective malnutrition
- Vitamin C in pellets: commercial pellets contain added vitamin C, but it degrades over time. Pellets stored for more than 3 months have significantly reduced vitamin C content — buy in small quantities and store in a cool, dry place.
Complete Toxic Foods List
Toxic Foods
| Food | Reason |
|---|---|
| Iceberg lettuce (large amounts) | Near-zero nutrition; watery; can cause diarrhoea |
| Onion, garlic, leek, chives | Thiosulfates — cause haemolytic anaemia |
| Potato (especially raw, skin, eyes) | Solanine; also high starch |
| Rhubarb | Oxalic acid — highly toxic; can cause kidney failure |
| Tomato leaves and stems | Solanine and tomatine — toxic; ripe flesh is fine in small amounts |
| Avocado | Persin — toxic to most small animals |
| Mushrooms | Variable toxicity; avoid entirely |
| Dairy products | Guinea pigs are lactose-intolerant; no milk, cheese, yoghurt |
| Meat or animal protein | Guinea pigs are strict herbivores; protein from animal sources causes organ damage |
| Seeds and pits from fruit | Most stone fruit pits and apple seeds contain cyanogenic compounds |
| Cabbage (large amounts) | Goitrogenic; causes bloat in large amounts; small amounts occasionally are fine |
| Chocolate | Theobromine toxicity |
| Nuts and nut products | Too high in fat; peanut butter is a common cause of choking in guinea pigs |
| Cereals, bread, crackers | No nutritional role; high starch and sugar |
| Raw beans | Lectins — toxic |
Water Requirements
Guinea pigs drink 80–100 ml of water per kilogram of body weight per day — more in warm weather, during illness, and when eating dry hay without supplementary fresh vegetables.
- Provide fresh, clean water daily via a water bottle (less contamination risk) or a heavy ceramic bowl (prevents tipping)
- Clean the water bottle or bowl daily — algae, bacteria, and biofilm accumulate quickly
- Guinea pigs that are eating adequate fresh vegetables (high water content) will drink less from their water source — this is normal
- Monitor water intake: a guinea pig drinking significantly more or less than usual is a health signal worth investigating
Frequently Asked Questions
Can guinea pigs eat fruit?
Yes, in small amounts. Fruit is high in sugar and should be a treat (1–3 times per week, small portions) rather than a dietary staple. Strawberries, blueberries, and small pieces of apple (no seeds) are well-tolerated. Avoid citrus (too acidic for most guinea pigs) and any stone fruit pits.
Why is my guinea pig not eating its pellets?
If hay and fresh vegetables are adequately available, some guinea pigs prioritise these over pellets. This is nutritionally acceptable and normal. Concern arises if the guinea pig is also eating less hay and vegetables, or if it was previously eating pellets and has stopped — appetite loss is a veterinary symptom.
Can guinea pigs eat rabbit food?
No — rabbit pellets do not contain added vitamin C (rabbits synthesise their own). A guinea pig fed rabbit pellets will develop vitamin C deficiency. Species-specific food is non-negotiable.
How do I know if my guinea pig is getting enough vitamin C?
The most reliable sign is absence of deficiency symptoms: smooth coat, good movement, no joint swelling, healthy gums, normal wound healing, normal weight. If you are providing half a red bell pepper daily per guinea pig plus leafy greens, vitamin C intake is almost certainly adequate. If in doubt, a 10 mg vitamin C supplement tablet crushed into a small piece of bell pepper twice weekly provides a reliable safety margin.
Sources
- Hoefer, H.L. (1994) — Guinea pig husbandry and medicine. Veterinary Clinics of North America: Exotic Animal Practice
- Oxbow Animal Health — Guinea Pig Nutrition Guide: oxbowanimalhealth.com
- RSPCA — Guinea Pig Diet Guidelines: rspca.org.uk
- National Research Council (2003) — Nutrient Requirements of Laboratory Animals. National Academies Press