A digestive pattern, not a single bad day
Food intolerance means a dog's digestive system handles a particular ingredient poorly, and the signs are mostly gastrointestinal: soft stools or recurring diarrhea, noticeable gas, stomach gurgling, occasional vomiting, and sometimes a dog who approaches the bowl without enthusiasm. The defining feature is the pattern. A stomach bug passes in a day or two and does not return on schedule; intolerance repeats while the ingredient stays in the diet and settles when it is removed. It often surfaces after a food change, a new treat routine, or a well-meaning stretch of rich table scraps, and it builds rather than exploding overnight.
Intolerance is not the same as allergy
The two are often confused, and the distinction helps you describe what you see. An allergy involves the immune system and in dogs it more often shows in the skin, with itchy paws, ears, and belly, though digestion can be involved too. An intolerance is a digestive handling problem: the gut simply does not process that ingredient well, so the signs center on stools, gas, and comfort after meals, and they tend to scale with how much of the ingredient was eaten. A dog may manage a little of the problem food and suffer after a full portion, which is a useful clue to report.
Building the diary that actually solves it
Because memory blurs meals together, a written record is the tool that cracks these cases. Note everything eaten, including treats, chews, dental sticks, medication flavorings, and anything scavenged, alongside stool quality, timing of any vomiting or gas, and appetite. Patterns hide in the extras: many dogs blamed on their main food turn out to be reacting to a daily chew or a training treat given twenty times a session. Keep the diet otherwise steady while you observe, because changing three things at once destroys the evidence. Resist the urge to trial several new foods rapidly at home; each switch muddies the picture and can upset the gut on its own.
When the vet needs to rule out more
See the veterinarian promptly if there is blood in stool or vomit, weight loss, repeated vomiting, dehydration, a puppy or senior is affected, or signs persist despite a consistent diet, because parasites, pancreatic trouble, and other conditions imitate intolerance convincingly. The vet may suggest a structured elimination approach, feeding a limited diet for a set period and reintroducing ingredients one at a time, which only works if every household member follows it strictly, including treat-givers and visiting grandparents. That is a plan to run with professional guidance rather than improvise, both for accuracy and for nutrition.
Feeding forward once you know
Once a trigger is identified, the management is refreshingly dull: stop feeding it, read ingredient labels on everything including treats, and keep the dog's diet boringly consistent. Tell anyone who feeds your dog what must be avoided, in writing, because one kind stranger with the wrong biscuit can restart the cycle. Keeping the diary and the confirmed trigger list in KinsLe, stored with his Pet Passport, means a sitter, a family member, or a new veterinarian can see at a glance what he cannot eat and why.