Transitioning pet food safely: a one-week mixing schedule

Mixing the new food into the old one in increasing amounts over at least a week is the standard way to change a dog's or cat's diet. Some animals can switch from one food at breakfast to another at dinner without trouble, while others need a transition of four to six weeks, according to the veterinary nutritionists at Tufts University. A week is a sensible default because it is hard to know in advance which kind of animal you have.

What a sudden change does

The bacteria in the gut respond quickly to what the animal eats. When healthy Beagles were switched abruptly to a higher-fiber diet or a protein-rich canned diet, their fecal microbiome and its metabolites shifted within days, then settled again within about six days. That time scale fits the usual advice to spread a change over about a week.

The shift is not always harmless. In another study, Beagles moved abruptly from dry kibble to a canned diet produced wetter, less-formed stools, with falls in some bacterial groups and rises in others, including E. coli and Clostridium perfringens. The canned diet had far more protein and fat and far less carbohydrate than the kibble. Changes that big in nutrient balance deserve extra care.

A standard one-week schedule

This is the general schedule Tufts suggests for a healthy adult dog or cat:

The percentages refer to calories, not to scoops. Foods differ a lot in energy density, so a cup of the new food may carry noticeably more or fewer calories than a cup of the old one. Work out the daily amount of the new food from its label or with the vet, then split the day's ration according to the table. Exact calories matter less than a gradual change for these few days. A kitchen scale makes the measuring easier.

When to go more slowly

Stretch each step to several days for an animal that has had digestive problems before, for a large change in the type of food (dry to wet, or a very different fat or fiber level), and for picky cats. A slower change costs little. Speed only matters when the old food has run out or been recalled.

Puppies and kittens deserve particular caution. Diarrhea dehydrates a small young animal much faster than an adult, so any upset during a change of food needs watching closely.

Signs the change is going too fast

Some soft stool during a diet change is common. More than that suggests the new food is being added too quickly, or that it does not suit the animal. Watch for:

• Diarrhea or repeatedly loose stools
• Vomiting
• Excess gas or noisy gut sounds
• Constipation
• Reluctance to eat

If the signs are mild and the animal is otherwise bright, stay at the last ratio that caused no problems for another day or two until they pass, then continue. For example, a dog with loose stools on reaching half and half goes back to 75% old food until things settle.

Continued vomiting, watery diarrhea, pain or lethargy are reasons to call the vet rather than adjust the schedule. The animal may be intolerant of something in the food, such as its fat or fiber level, or the problem may have nothing to do with the diet. Call sooner for a puppy, kitten or elderly animal.

Special situations

Dry to wet food, or wet to dry

Wet and dry foods differ in water content, texture and usually in the balance of protein, fat and carbohydrate. Use the one-week schedule as a minimum and take longer if stools soften. Calculate portions by calories: a can and a cup are not equivalent.

Prescription diets

Ask the vet how to start a therapeutic diet. For some conditions the vet will want a gradual change; for others the new diet may need to start straight away. Follow the specific instructions rather than the general schedule.

Cats that refuse new food

Some cats avoid unfamiliar food. Tufts suggests that for some cats it works better to offer the new food in a separate dish beside the old one and let the cat choose, rather than mixing. If the cat ignores the new food, remove it after an hour and offer fresh food at the next meal. Once the cat eats the new food readily, reduce the old food in steps of about 25%.

Never try to make a cat accept a new food by withholding everything else. An overweight cat that stops eating for days can develop hepatic lipidosis, a serious liver disease. A cat that is refusing both foods needs to see a vet.

Picky dogs

• Slow the schedule down, starting with a smaller share of new food.
• Warm wet food slightly to bring out its smell.
• Moisten dry food with a little warm water so the two foods blend.
• If using broth, check that it contains no onion, garlic or chives. The ASPCA warns that these can damage red blood cells in dogs and, even more readily, in cats.
• Offer meals at set times and pick up what is left, so the dog comes to the next meal hungry.

More than one pet

If only one animal is changing food, feed it separately until the change is complete. Otherwise it is impossible to know how much of each food it actually ate, and another pet may end up eating the new food with no transition at all.

Sources

1. How do I switch my pet's food? Petfoodology, Cummings School of Veterinary Medicine at Tufts University, 2019.
2. Longitudinal fecal microbiome and metabolite data demonstrate rapid shifts and subsequent stabilization after an abrupt dietary change in healthy adult dogs. Animal Microbiome, 2022.
3. Impact of an abrupt change from dry to canned diet on digestive function and gut microbiota in dogs. Journal of Animal Science, 2025.
4. Hepatic lipidosis: clinical review drawn from collective effort. Journal of Feline Medicine and Surgery, 2018.
5. People foods to avoid feeding your pets. ASPCA Animal Poison Control Center.