Hi!
Please read our diet guide with its picture of a balanced sample diet; that is not exactly a balanced diet with or without lettuce - for once, it is too low in vitamin C and too fattening, too high in calcium and vitamin A, which accumulates in the liver.
Here is our diet guide and our sample diet - no carrots, no spinach and very little lettuce if at all (it stands in for your own choice of extra veg. When introducing new veg, you need to be very careful with a piggy whose mother has obviously not had any, nor has the baby; so its digestive system is not yet adapted.
Hay should make over 80% of the food intake and pellets only 1-2 tablespoons per day (2 for babies up to 4 months; 1 for adults)
Long Term Balanced General And Special Needs Guinea Pig Diets
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Stop as soon as you get funny poos to allow the guts to rebalance and then go careful when re-introducing veg again to not tip it over straight away. It is a slow process, but worth being careful. Ideally you wait 24-48 hours until the poos have normalised again before you gradually reintruce veg again after a tummy upset. Yours is very borderline, though.