Guinea Pig Food & Vitamin C Calculator

Guinea Pig Nutrition Guide

What do I need to know about Hay — 80% of the Diet?

Timothy hay (or other grass hays — orchard grass, meadow hay) must make up 80% of a guinea pig's diet and be available at all times. Hay provides: the fibre needed to maintain correct gut motility (a guinea pig's digestive system must never stop — GI stasis is rapidly fatal). Dental wear — guinea pig teeth grow continuously; hay grinding is essential to prevent overgrown teeth. Essential micronutrients. Never reduce hay access. Lucerne/alfalfa hay is too high in calcium for adult guinea pigs and can contribute to bladder stones over time — timothy or meadow hay is the better everyday choice for adults.

What should I know about The Vitamin C Requirement?

Guinea pigs cannot synthesise Vitamin C (like humans, they lack the enzyme L-gulonolactone oxidase). Vitamin C deficiency (scurvy) develops within 2-4 weeks without adequate intake and is a common cause of illness in pet guinea pigs. Symptoms: lethargy, reluctance to walk (joint pain), rough coat, dental problems, slow wound healing. Daily requirement: 10-30mg/kg body weight (pregnant females need 30-40mg/kg). A 1kg guinea pig needs 15-30mg/day minimum. Best food sources: bell peppers, with red peppers containing around 190mg of vitamin C per 100g — one of the richest natural sources available for a guinea pig's diet.

What do I need to know about Fresh Vegetable Feeding?

Fresh vegetables provide Vitamin C, hydration, and variety. Serve approximately 1 cup (100-150g) of fresh vegetables per guinea pig per day. Ideal vegetables: bell peppers (red, yellow, orange — highest Vitamin C), fresh herbs (parsley, coriander, dill), dark leafy greens (romaine lettuce, kale, spinach in moderation — high oxalates). Introduce new vegetables gradually to avoid digestive upset. Avoid high-sugar fruits as regular treats. Always provide fresh vegetables rather than relying on vitamin C supplement drops added to water, since guinea pigs often don't drink enough treated water to get a reliable dose, and the vitamin degrades quickly once dissolved.

What do I need to know about Foods Toxic to Guinea Pigs?

Absolutely never feed: onions, garlic, leeks, and chives (toxic to red blood cells). Potatoes and all nightshade family plants. Rhubarb (extremely toxic). Buttercups and foxglove. Mushrooms. Processed human foods. Dairy products. Meat. Iceberg lettuce (not toxic but causes diarrhoea due to high water content). Avocado. Seeds and pits of most fruits (apple seeds, cherry pits). Items to limit: high-sugar fruits (grapes, apple, pear) to occasional treats — no more than 2-3 times per week, in small quantities, to avoid digestive upset and unnecessary sugar intake.

Why Can't Guinea Pigs Produce Their Own Vitamin C the Way Most Mammals Can?

Guinea pigs, along with humans and a small number of other species, lack the functional liver enzyme (L-gulonolactone oxidase) needed to synthesise vitamin C internally from other nutrients, a trait most other mammals retain, which means dietary vitamin C is an absolute daily requirement for guinea pigs rather than a nutrient their body can produce as needed. Without adequate dietary vitamin C (generally recommended at around 10-30mg daily depending on life stage and health status, more during pregnancy or illness), guinea pigs can develop scurvy, causing joint pain, poor wound healing, lethargy, and a weakened immune system — a potentially serious and sometimes fatal condition that is entirely preventable with consistent dietary intake. This is why fresh vegetables high in vitamin C (such as bell peppers, which are unusually rich in it) are considered an essential daily component of a guinea pig's diet, not an optional treat, and why guinea pig-specific pellets are fortified with added vitamin C that degrades over time, meaning old or improperly stored pellets can no longer be relied upon as a sole vitamin C source.

Why Is Sudden Dietary Change Particularly Risky for Guinea Pigs?

Like rabbits, guinea pigs rely on a delicate balance of fermenting bacteria in their cecum to digest high-fibre plant material, and this bacterial population is adapted to a relatively consistent diet over time — introducing a new food, especially in a large quantity, too quickly can disrupt this bacterial balance and trigger digestive upset or, in more serious cases, GI stasis, a potentially life-threatening slowing or stopping of gut motility. This is why any new vegetable or food should be introduced gradually, in small amounts over several days while monitoring for soft stool or reduced appetite, rather than added to the diet all at once, and why guinea pigs being transitioned between different brands or types of hay or pellets should have the new food mixed in gradually alongside the old rather than switched abruptly, giving the gut bacteria time to adjust to the dietary change.

What should I know about Why Guinea Pigs Need Unlimited Hay Access?

Guinea pigs' teeth grow continuously throughout their lives, and constant chewing on hay is what naturally wears those teeth down at a matching rate — without near-constant access to hay, a guinea pig's teeth can become overgrown (malocclusion), leading to painful dental problems and difficulty eating that often requires veterinary correction. Beyond dental health, hay's high fiber content is also essential for keeping the guinea pig's digestive system moving properly; a guinea pig eating primarily pellets or vegetables without adequate hay is at higher risk of gastrointestinal stasis, a serious and potentially life-threatening slowdown of gut motility. This is why veterinary guidance consistently places unlimited, good-quality hay as the single largest component of a guinea pig's diet, well above pellets and fresh vegetables combined, despite hay looking far less nutritionally "exciting" than the other options.

Worked Example: Daily Feeding Plan for a 900g Adult Guinea Pig

For a healthy adult guinea pig weighing 900g (0.9kg), vitamin C needs are calculated at roughly 10-30mg/kg bodyweight: 0.9 × 10 = 9mg (minimum) up to 0.9 × 30 = 27mg (higher end, e.g. for a pregnant or unwell guinea pig).

A quarter of a red bell pepper (roughly 25g) provides well over 20mg of vitamin C on its own, meaning a varied daily vegetable mix easily meets the target without needing a separate vitamin C supplement for most healthy adult guinea pigs, provided fresh vegetables are included consistently.

Vitamin C Content of Common Guinea Pig Foods

Because guinea pigs cannot synthesise their own vitamin C, knowing which everyday vegetables and fruits are the most efficient sources helps build a diet that reliably meets the requirement:

Bell peppers and dark leafy greens are consistently the most efficient everyday vitamin C sources for guinea pigs, which is why they feature so heavily in standard feeding guidance, while watery vegetables like cucumber, despite being safe, contribute little to the vitamin C requirement and shouldn't be relied upon as a main source.

Pellets: How Much and Which Kind?

Commercial pellets play a supporting role in a guinea pig's diet, not a primary one, and the type chosen matters as much as the amount. Veterinary and welfare guidance consistently recommends a plain, uniform pelleted food specifically formulated for guinea pigs (fortified with stable vitamin C, since the vitamin degrades over time and pellets alone shouldn't be relied upon as the sole source) over 'muesli-style' mixes containing visibly different seeds, flakes and dried pieces. Muesli-style mixes allow selective feeding — guinea pigs can pick out the tastiest, often less nutritious components (seeds, dried corn) and leave the rest, leading to an unbalanced diet and increased risk of obesity and dental problems despite the bag's overall nutritional label looking adequate. A typical daily pellet portion for an adult guinea pig is modest — commonly around 1/8 cup (roughly 15-20g) per guinea pig — reflecting pellets' role as a vitamin C and nutrient top-up alongside the much larger hay and vegetable components of the diet, not as the main calorie source.

Weight Monitoring as an Early Illness Signal

Because guinea pigs mask illness well and hide pain instinctively as a prey species, a weekly weigh-in is one of the most reliable early warning signs an owner can track at home, often catching problems before any other visible symptom appears. A healthy adult guinea pig's weight should stay broadly stable week to week, typically within about 2-3% either way. A drop of more than roughly 5-10% of body weight over one to two weeks — even with no obvious change in appetite — warrants a vet check, since it can signal dental problems (overgrown molars making eating painful), early digestive stasis, or a developing internal issue well before the animal stops eating altogether. Keeping a simple written log of weekly weights, taken on the same kitchen scale at the same time of day, turns a vague 'she seems a bit off' into a concrete, trackable trend a vet can act on immediately.