Feeding & Nutrition

The bag runs out on a Tuesday. The shop is out of stock, so you grab a different brand, fill the bowl that evening, and by Thursday morning you are cleaning the hallway carpet at six a.m.

Almost every case of diet-change diarrhoea is avoidable, and the fix costs nothing but a week of patience.

Why a sudden switch causes trouble

The gut is not a passive tube. It hosts a complex microbial population — billions of bacteria adapted to the specific protein sources, fat levels, and fibre types the animal has been eating. Digestive enzyme production is tuned to that diet too.

Change the input overnight and the existing microbial population is suddenly working with the wrong substrate. Some bacterial groups starve, others bloom, fermentation patterns shift, and the balance tips. The visible result is loose stool, gas, sometimes vomiting, and a dog or cat who seems generally off.

Cats add a second layer of difficulty. They form strong food preferences early in life and can be genuinely resistant to novelty, a trait called neophobia. A cat presented with unfamiliar food may simply refuse it, and a cat that stops eating entirely is a medical problem rather than a stubborn one.

The standard transition

Seven to ten days is the usual recommendation and works for most healthy animals.

Days one and two: 75 percent old food, 25 percent new. Days three and four: half and half. Days five and six: 25 percent old, 75 percent new. Day seven onward: all new food.

Mix the two thoroughly rather than putting them side by side in the bowl, or a selective eater will simply pick out the familiar pieces.

Watch the stool at each stage. If it stays firm, proceed on schedule. If it loosens, hold at the current ratio for a few extra days until things settle, then continue. There is no prize for finishing in seven days.

When to go slower

Some animals need two to three weeks, sometimes longer.

Pets with a history of sensitive digestion. Animals with inflammatory bowel disease or food-responsive enteropathy. Very young puppies and kittens. Senior animals. Any pet recovering from illness, surgery, or a course of antibiotics. Cats generally, since they tend to be both more food-conservative and more prone to protest.

For these, extending each stage to four or five days and moving in smaller increments — 90/10 before 75/25 — makes the process much smoother.

Bigger changes also need more time. Moving between two similar chicken-based kibbles is a small shift. Moving from dry to raw, or from a standard diet to a novel protein, is a large one and deserves a longer runway.

Situations where you cannot go slowly

Occasionally an immediate switch is necessary — a diagnosed allergy, an acute condition requiring a therapeutic diet, or a recall on the current food.

In those cases, tell the vet you are switching cold and ask about support. Feeding smaller portions more frequently for the first few days reduces the load on the gut. A probiotic designed for pets, ideally one with veterinary evidence behind it, is often recommended around abrupt changes. Some vets suggest a short bland-diet bridge — plain cooked chicken and rice for dogs, for a couple of days — before introducing the new food.

Getting a reluctant cat on board

Cats need a different toolkit.

Start by putting a tiny amount of the new food in a separate dish next to the usual meal. Not mixed in — separate. Many cats will investigate on their own terms when there is no pressure and their normal food is still available.

Warm the new food slightly to release aroma. Cats rely heavily on smell, and a cold refrigerated portion smells of almost nothing.

Texture matters as much as flavour. A cat raised on pâté may reject chunks in gravy regardless of what the protein is. If a cat refuses a new food, try a different texture of the same food before writing off the whole product.

Sprinkling a small amount of a favourite topper — freeze-dried meat crumbled fine, or a little juice from a tin of tuna in spring water — over the new food can bridge the gap. Fade it out over a couple of weeks.

Never starve a cat into accepting food. Beyond being unpleasant, a cat that goes without eating for more than about 24 to 48 hours is at risk of hepatic lipidosis, a serious liver condition. Overweight cats are at highest risk. If a cat is refusing all food, stop the transition and call your vet.

Normal adjustment versus a real problem

Expect some softening of stool, a bit more gas, and possibly a slightly reduced appetite in the first few days. Those are ordinary.

Stop and consult a vet if there is vomiting more than once or twice, diarrhoea containing blood or mucus, diarrhoea lasting beyond 48 hours, lethargy, complete refusal to eat, signs of abdominal pain, or any itching, ear inflammation, or skin reaction suggesting a food sensitivity.

If a transition fails clearly, go back to the previous food entirely, let things settle for a week or two, and start again more slowly — or reconsider whether that particular food suits this particular animal.

A few practical habits

Buy the new food before the old one runs out. Almost every emergency switch happens because someone waited until the bag was empty. Keeping a few days' buffer removes the pressure entirely.

Write down what you feed and when you changed it. If a problem develops three weeks later, that note is genuinely useful diagnostic information.

Change one thing at a time. Switching food, adding a supplement, and starting a new treat in the same week makes it impossible to identify the culprit if something goes wrong.

And once you find a food that works — the animal eats it willingly, the coat looks good, the stool is firm, weight is stable — there is no particular need to keep looking. Stability has its own value.