Fiber is the foundation of guinea pig health. Their digestive system is built for continuous processing of high-fiber plant material — without it, gut motility slows, fermentation balance shifts, and GI stasis can set in within hours. Fiber also provides the abrasive texture that wears down continuously growing teeth. Timothy hay should make up 80% or more of a guinea pig's diet.
Guinea pigs need unlimited timothy hay available at all times. Roughly 80% of their diet should be hay. A healthy guinea pig eats approximately their body size in hay each day.
GI stasis (the gut stops moving — a life-threatening emergency), bloating, soft or absent droppings, overgrown teeth and dental spurs, loss of appetite, hunched posture. GI stasis is the leading cause of death in guinea pigs after dental disease, and both are directly linked to insufficient fiber.
Fiber excess from hay is essentially impossible — guinea pigs self-regulate hay intake. However, a diet that is only hay with no vegetables would lack sufficient Vitamin C and variety.
Dietary fiber from hay fuels cecal bacteria that synthesize several B vitamins — including B12, biotin, thiamine, and folate. Without adequate fiber, the cecal microbiome collapses, and with it the guinea pig's primary internal source of these vitamins. This makes fiber not just a macronutrient but the engine that drives B vitamin production.
What this means: Unlimited timothy hay is non-negotiable. When hay intake drops — due to dental problems, illness, or a guinea pig being selective about food — B vitamin status can deteriorate within days. If a guinea pig stops eating hay, treat it as an emergency.
Simple sugars and structural fiber compete for dominance in the cecal ecosystem. When excess sugar reaches the cecum, it feeds fast-growing harmful bacteria (especially Clostridium species) that outcompete the slower-growing fiber-fermenting bacteria. This dysbiosis can trigger gas, bloating, soft stools, and potentially life-threatening GI stasis. Fiber from hay, conversely, sustains the beneficial bacteria that keep pathogens in check.
What this means: Always maintain a high fiber-to-sugar ratio. Fruit and sweet vegetables should be tiny, infrequent treats — never daily staples. If a guinea pig develops soft stools, the first step is to eliminate all sugary foods and ensure unlimited hay access to reset the cecal balance.
Cecal bacteria produce Vitamin K2 (menaquinone) as a byproduct of fiber fermentation. When guinea pigs eat their cecotropes, they absorb this bacterially produced Vitamin K alongside the dietary Vitamin K1 (phylloquinone) from leafy greens. Adequate fiber intake supports both the bacterial production of K2 and the overall gut health needed for proper absorption.
What this means: Unlimited hay supports Vitamin K2 production by cecal bacteria. Dark leafy greens (kale, parsley, romaine) provide Vitamin K1 directly. Together, these ensure adequate Vitamin K for blood clotting and bone health.
