Introducing a new food to pets
The most important recommendation for introducing a new food to your pet's diet is to do so gradually. Your pet's digestive system is fragile, and your pet needs time to adjust to the new product. This is especially true for cats.
To introduce a new food, first add 25% of the new food to 75% of the old food for 2 to 3 days. If your pet is still eating and you see no signs of discomfort or changes in bowl movements you can proceed to the next step. If your pet refuses to eat the new food you may need to mix in less of the new food to start with.
For the next few days give your pet a mix of half the old food plus half the new food. As your pet becomes more used to the new food you can gradually decrease the old food until you have entirely switched to the new food. This process could take a week or more.
You can also follow this process when adding a new vitamin/supplement to your pet's diet: add the vitamin in small amounts, slowly, so that the familiar food is not overpowered with the smell and taste of the vitamins.
The most common issue with a change in food is that the pet may have loose or runny stools. Cut back on the amount of new food in the mix for a few days and see if this helps, and then increase the percentage of new food again more gradually. If these changes in stool persist, or if your pet is vomiting, consult your veterinary consultant.
See a discussion of foods that may be toxic to your dog or cat.