A dog’s gut adapts to a diet. The bacterial population, the enzymes, the transit time — all of it calibrates to what has been going in. Change the input abruptly and the system takes several days to catch up, and the catching up looks like diarrhoea.
The fix is simply to move slowly enough that the gut can keep pace.
The standard transition
Seven days, though ten is better for a sensitive dog:
- Days 1–2: 75% old food, 25% new
- Days 3–4: 50% old, 50% new
- Days 5–6: 25% old, 75% new
- Day 7 onwards: 100% new
Mix the two thoroughly in the same bowl rather than feeding them as separate meals — a dog who can separate them will eat the one they prefer.
When to go slower
Take two to three weeks, moving in smaller increments, if your dog:
- Has a history of digestive sensitivity or chronic soft stool
- Is a puppy under four months
- Is elderly
- Is recovering from illness, surgery or a course of antibiotics
- Is moving between very different food types — dry to raw, or a conventional diet to a novel protein
- Has inflammatory bowel disease or another diagnosed gut condition
When you cannot go slowly
Sometimes a food is recalled, discontinued, or the dog is put on a prescription diet immediately. In that case:
- Feed smaller, more frequent meals for the first few days — a quarter portion, four times a day
- Expect softer stool for up to a week
- Consider a probiotic for the transition period
- Watch hydration, and contact your vet if there is blood, vomiting, or lethargy
Some links on this page are affiliate links. If you buy through them we may earn a small commission at no extra cost to you — it helps keep this site free. We only recommend products we would use with our own dogs.
Purina FortiFlora Canine Probiotic Supplement
Single-serve sachets of a well-studied probiotic strain, widely used around diet changes, travel and antibiotic courses. Very palatable, which also helps with a dog who is unsure about the new food.
What normal looks like during a switch
- Slightly softer stool for two or three days
- A change in stool colour and volume — a food with different fibre content produces visibly different output
- Slightly increased wind
- Mild fussiness for a day or two
What is not normal
- Watery diarrhoea, or diarrhoea lasting beyond four or five days
- Blood or mucus
- Vomiting more than once
- Lethargy, or refusing food entirely
- Signs of abdominal pain
Any of those, stop the transition, revert to the old food, and ring your vet.
If your dog refuses the new food
Fussiness is often learned: the dog holds out, the owner produces something better, and the dog learns that waiting is profitable.
What to do:
- Put the food down for fifteen minutes. If it is not eaten, lift it and offer nothing until the next meal
- Repeat. A healthy adult dog will not starve themselves; most give in by the second or third meal
- Do not top it with something delicious every time. That teaches the dog that refusing produces an upgrade
- Warm it slightly, or add a little warm water. This releases aroma and helps genuinely reluctant dogs
- Check the food is fresh. Rancid fat is the most common reason a dog refuses a food they previously liked
Exceptions: puppies, toy breeds, diabetic dogs and any dog who is unwell should not go without food. And a dog who suddenly refuses a food they have always eaten may be ill rather than fussy — sudden appetite loss deserves a call, especially with any other symptom.
Choosing what to switch to
Reasonable criteria:
- Complete and balanced for the correct life stage, meeting AAFCO or FEDIAF profiles
- Made by a company that employs a qualified nutritionist and runs feeding trials rather than only formulating on paper
- Appropriate calorie density for your dog’s activity level
- Something you can reliably buy. The best food in the world is no use if it is out of stock every other month
Reasons that are less good: an ingredient list that looks appealing to a human, an absence of a currently unfashionable ingredient, and marketing terms like “ancestral” or “human-grade”, which have no regulatory meaning in pet food.
How long to give a new food
Six to eight weeks before judging it, unless something goes obviously wrong.
Coat quality, energy and stool consistency all take weeks to reflect a dietary change, and the first fortnight tells you very little. People frequently switch again after ten days and never give any food a fair trial.
Keep a note
Write down what you fed, when you switched, and what the stool looked like. If your dog turns out to have a food sensitivity later, that record is the most useful thing you can bring to a vet — and it is the thing almost nobody has.
