How to switch your pet's food without stomach trouble

A pet's gut microbiome adapts to its current diet. Change the diet overnight and the microbiome lags behind — the usual result is soft stool, diarrhea, gas or a hunger strike. The fix is boring and reliable: transition gradually over 7–10 days, mixing the new food into the old in increasing proportions.

The standard 7-day schedule

Keep the total daily calories constant throughout — the transition changes the recipe, not the amount. If the new food has a different caloric density (very common when moving between dry and wet), recompute portions with the calculator before day 1, using the new food's kcal from its label.

When to slow down to 10–14 days

• Puppies and kittens, seniors, and pets with a history of digestive trouble.
• Big recipe jumps: chicken → fish, kibble → raw, standard → prescription diets.
• Cats in general — they are both more sensitive and more stubborn than dogs.

If things go wrong mid-switch

Soft stool for a day is common: hold the current ratio for 2–3 extra days before increasing again. Outright diarrhea: drop back to the previous ratio that worked, wait for normal stool, then advance more slowly. Vomiting, blood, lethargy or refusal to drink are not transition symptoms — call your vet. And if your cat simply refuses the new mix for more than a day, don't wait it out: cats that stop eating risk hepatic lipidosis, a genuine emergency, within days.

Three mistakes that undo the schedule

1. Free-feeding during the switch — you lose track of the actual ratio eaten. Use measured meals for the whole transition.
2. Judging the food in week one — mild stool changes early on say little about long-term fit; give a new diet 4–6 weeks before evaluating coat, energy and weight.
3. Changing more than one thing — new food + new treats + new schedule at once makes any reaction impossible to attribute. One variable at a time.