Guinea pigs are vegetarians and don’t consume much fat, so it’s uncommon for them to become overweight through their normal dry food/fresh veg diet. Weight gains can be indicative of a number of things, from lack of exercise, to a few too many treats (sugary fruits or shop-bought delicacies), to illness or pregnancy. This is why it’s advisable to weigh your guinea pigs a part of a regular health check.
If your guinea pig is very overweight, it may have trouble moving around properly, and can develop sores on its feet. Once these are treated, you might want to put your guinea pig on a diet.
If your guinea pigs are piling on the weight, reduce their dry food intake and cut out all sugary fruits and treats
If your guinea pig has stopped growing but still piling on the pounds, tweak its diet a little. Don’t reduce the hay supply, as they always need plenty of this for digestion. Instead, reduce the dry food and limit their fresh food to one cup of fresh vegetables (no fruit).
Another good idea is to scatter fresh food around your GPs’ enclosure, so they have to run around a bit to get at it. Toys like balls and paper tents are also great ways to encourage your pet to be more active and shed the extra flab.