The bag ran out on Tuesday. The shop had a different brand on offer, it looked much the same, and you thought nothing of it.
By Thursday you’re up at two in the morning and the kitchen floor has taken a beating.
Changing a dog’s food is one of the most common causes of diarrhoea there is, and almost all of it is avoidable. The mistake isn’t usually the new food. It’s the speed.
Why sudden changes cause trouble
Two systems have to keep up, and neither of them does it quickly.
The first is the microbiome. The hundred trillion bacteria in a dog’s gut are adapted to whatever they’ve been fed. Different foods mean different proteins, fats and fibres arriving in the colon, and the bacterial populations best suited to digesting them are not the same. Change the food overnight and you’ve changed the fuel supply faster than the population can shift, so some groups starve and others bloom.
The second is the dog’s own digestive machinery. Enzyme production adjusts to the diet over days rather than minutes, and the gut lining takes time to adapt to a different fat level or fibre content.
Do it gradually and both systems keep pace. Do it in one meal and you get fermentation happening in the wrong places by the wrong organisms, which is diarrhoea and wind.
This is also why a dog can be perfectly fine on a food for years and still react badly when you switch to something objectively better. The quality of the new food is largely irrelevant to what happens in the first week – the speed of the change is what does the damage.
The transition, properly
Seven to ten days is the standard, and it works for most dogs.
| Days | Old food | New food |
| 1-3 | 75% | 25% |
| 4-6 | 50% | 50% |
| 7-9 | 25% | 75% |
| 10 | 0% | 100% |
Mix the two together in the same bowl rather than feeding them as separate meals. The point is that the gut meets a gradually changing mixture, not alternating extremes.
Support through the changeover helps, and daily Probiotics for Dogs started a few days before you begin rather than on day one give the population a head start. This is the single easiest thing to get right and the one most people only think of after the diarrhoea has already started.
Some dogs need three or four weeks
Ten days is a guideline, not a rule, and there’s a decent list of dogs who need considerably longer.
Dogs with any history of sensitive digestion. Dogs recovering from illness or a recent course of antibiotics. Puppies. Elderly dogs. Dogs who have eaten the same food for years, because a population that has had no variation at all is less adaptable than one that’s seen a few changes.
For those, stretch each stage to five or six days and go in smaller increments – 90/10 before 75/25, and so on. It’s slower and it’s a great deal quicker than dealing with a fortnight of loose stools.
The harder switches
Dry to wet, or wet to dry. Big change in moisture and often in fat. Go slowly.
Kibble to raw, or kibble to fresh cooked. The biggest microbiome shift of the lot. Raw-fed dogs have measurably different gut populations from kibble-fed dogs, and getting from one to the other takes time. Two to three weeks minimum, and handle raw food with proper kitchen hygiene – separate boards, hot washing, and be careful in households with young children, elderly people or anyone immunosuppressed.
Changing protein source. Chicken to fish, or beef to lamb. Usually easier than changing format, but worth the same care in a sensitive dog.
Changing between very different fat levels. A working dog food and a light formula can be twenty percentage points apart. That’s a substantial change for a pancreas.
What to expect while you’re doing it
Slightly softer stools around days two to four are common and not a reason to abandon the plan.
Volume may change. Higher-fibre foods produce more; highly digestible ones produce less. Colour usually changes too, depending on the protein and any colourings.
A bit more wind for the first week is normal.
Slow down if stools go properly loose but the dog is otherwise well and eating. Drop back to the previous ratio for three or four days, then go forward more gradually.
Stop and ring the vet if there’s blood, repeated vomiting, lethargy, or a dog who won’t eat at all. That’s not a transition problem.
The portion trap
The thing almost everyone gets wrong, and it causes as many upsets as the transition itself.
Foods differ substantially in calorie density. One kibble might be 340 kcal per hundred grams, another 420. Scoop the same volume of the new food and you may have increased his intake by a fifth without realising, and overfeeding is a very common cause of soft stools.
Read the feeding guide on the new bag, work it out for your dog’s weight, and weigh it on kitchen scales rather than using a scoop. Then check his body condition after a month and adjust.
When you have to change suddenly
Sometimes there’s no choice. The food is discontinued, there’s a recall, the shop is out of stock, or you’re away and forgot to pack it.
Do what you can. If you can get a bag of anything, a couple of days of a mildly wrong food is usually better than two days of nothing or two days of chicken and rice.
Where possible, pick something close to what he’s on – same format, similar protein, similar fat percentage. Read the analytical constituents on both bags and match them roughly rather than grabbing whatever’s on the end of the aisle.
Feed slightly less than usual for the first two days, split across more meals. Add fibre – a spoonful of plain tinned pumpkin does a lot of work here. Keep the probiotic going.
Then expect a few soft days and don’t panic about them. An unavoidable fast change with support behind it usually passes in under a week.
Reading the label on the new food
Three things tell you most of what you need to know about whether a change will be easy or difficult.
The first few ingredients. They’re listed by weight, so the first three or four are effectively what the food is. If they’re broadly similar to the old food, the transition will usually be smoother.
Protein and fat percentages, from the analytical constituents panel. A jump of five or more percentage points in fat is the single most likely thing to cause digestive upset, and it’s the number worth checking before anything else.
Fibre. Very low fibre foods produce small, sometimes loose stools; higher fibre produces more volume and often firmer results. If your dog has had persistently soft stools, a food with a bit more fibre is a reasonable direction to try.
Calorie content is on there too, usually as kcal per 100g, and that’s the number you need for the portion maths above.
Why people change food, and when not to
Perfectly good reasons: moving from puppy to adult food, a vet recommendation, a diagnosed allergy or intolerance, weight management, or a food being discontinued.
Less good reason: fussiness. Changing food because a dog turned his nose up teaches him that refusing a meal produces something more interesting, and owners who change food repeatedly for this reason usually end up with a fussier dog and worse digestion. If he’s healthy and at a good weight, a missed meal is not a crisis.
Also worth resisting: changing during any other upheaval. A house move, a boarding stay, fireworks week or a recent illness are all bad moments to introduce a new food. Do one thing at a time.
Give the new food a fair trial
Digestion usually settles within two to three weeks. Everything else takes longer.
Coat condition, skin, energy levels and body composition need six to eight weeks before you can judge them fairly, because that’s how long it takes for a change in diet to show in the coat at all.
Owners regularly abandon a food at three weeks because the coat hasn’t improved. That’s not enough time to know.
Elimination diets are different
If you’re doing a proper food trial for a suspected allergy, the rules change. Your vet will guide the transition, but the critical part isn’t the mixing schedule – it’s absolute strictness. No treats, no chews, no table scraps, no flavoured medication, nothing at all except the trial food, for the full eight to twelve weeks.
One dropped biscuit from a child’s hand invalidates the entire trial, which is why so many of them fail to give a clear answer. Tell everyone in the house, and be honest with your vet about any slips rather than quietly hoping they didn’t matter.
If it goes wrong anyway
Some dogs simply don’t get on with a particular food, and that becomes clear once the transition is complete and the stools still aren’t right after three or four weeks.
Go back to the old food – transitioning back gradually, same as before – and let things settle for a fortnight before trying anything else. Then change one variable rather than several: same protein, different brand, or same brand, different protein.
Keep a note of what you’ve tried and how he responded. After three or four attempts nobody remembers accurately, and a short written list is the most useful thing you can take to a vet appointment about chronic digestive trouble.
