How do I change my pet's food without upsetting their stomach?

The gut is not just a tube — it carries a population of bacteria adapted to whatever the animal has been eating, and it takes days for that population to shift to a new food. Change everything at once and the mismatch shows up as wind, loose stools and a miserable few days. The animal is usually perfectly happy to eat the new food immediately; it is the gut that needs the transition, which is why doing it slowly is not fussiness.

Safety

What you'll need

• The old food, enough for a week
• The new food
• Kitchen scales (optional)

Step by step

1. Buy the new food before the old runs out.The most common reason for a rushed change is discovering the bag is empty. A transition needs a week of overlap, so buy the new food while there is still a week of the old left.
2. Work out the correct daily amount of each food first.The two foods almost certainly have different energy densities, so a straight swap by weight over- or under-feeds. Calculate the full daily amount for each food, then mix by proportion of those amounts rather than by scoops.
3. Days one to three: about a quarter new food.Three quarters of the old food's daily amount, plus a quarter of the new food's daily amount, mixed in the same bowl. Watch the stools, which are the only feedback you get.
4. Days four and five: about half and half.Half of each food's daily amount. This is where problems usually show if they are going to, so if things are still firm here the rest generally goes smoothly.
5. Days six and seven: about three quarters new.A quarter of the old, three quarters of the new. At this point the gut has mostly adapted.
6. Day eight onwards: all new food.Recheck the portion against the new food's chart, because you are now feeding a different food entirely and the correct weight will not be the one you were feeding a fortnight ago.
7. Stretch it to two or three weeks for a sensitive animal.Any pet with a history of digestive upsets, an elderly animal, a puppy or kitten, or a rabbit gets a slower change. There is no cost to going slowly and a real cost to rushing.
8. Change only one thing at a time.New food, new treats and a new chew all in the same week means you cannot tell what caused the problem. Hold everything else steady until the transition is finished.
9. For rabbits and other small herbivores, go slower still.Rabbit guts are particularly intolerant of sudden change, and a rabbit that stops eating or stops producing droppings for twelve hours is an emergency. New greens are introduced one at a time over weeks, not days.

Tips

• Keep about a week of the old food in a sealed tub after the change is finished. If the new food turns out not to suit, you have a soft landing.
• A single soft stool on day four is not a failure. Blood, straining, repeated vomiting or a flat, quiet animal is a different thing entirely and means a vet.
• If you are changing food because of an itch or a suspected food sensitivity, talk to the vet first. Diagnosing that properly needs a structured elimination diet, and switching brands at random usually just delays the answer.

Common mistakes

• Swapping straight over on the day the old bag runs out — The gut bacteria need days to adapt. An abrupt change reliably produces wind and loose stools, and people then blame the new food and switch again.
• Mixing by scoops rather than by proportion of the correct daily amount — The two foods have different energy densities, so mixing scoop for scoop usually changes the calories as well as the food and confuses two variables at once.
• Pushing on through diarrhoea to finish the schedule — The schedule is a guide, not a deadline. Going back a step and holding costs nothing; carrying on turns a mild upset into a week of it.

If it doesn't work

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