Capture everything eaten, not just the meals
Most food diaries fail at the edges, because the upset stomach rarely comes from the measured breakfast. It comes from the dental chew, the crust a toddler dropped, the rich training treats at class, the cat food raided at midnight, or the dead thing found on a walk. So the diary's first rule is total honesty: every item, with roughly how much and when, goes in the same day. Enlist the whole household, since a diary kept by one person while another hands out unrecorded extras will chase the wrong suspect for weeks. Keep the notebook or note where food is prepared, so logging becomes part of feeding rather than a separate evening chore.
Pair each day's food with the same stomach signs
Food alone is only half the evidence. Beside each day's intake, record stool firmness in a simple consistent scale of your own, how many times your dog went, any vomiting and when, audible wind or a gurgling belly, and whether appetite and energy looked normal. Use the same words every day so weeks can be compared at a glance, and note the timing carefully: many dogs react the following day rather than the same evening, so a sign on Tuesday may belong to Monday's new treat. Add anything else that can unsettle a stomach, such as a stressful journey, a worming date, or a stay away from home, so you do not blame a food for a reaction travel caused.
Change one variable at a time, then wait
A diary becomes an investigation only when you test it. Once a week or two of complete logging shows a suspect, perhaps a particular protein, a rich chew, or an abrupt portion increase, remove or change just that one thing and hold everything else steady for several days, longer if your vet advises. If the signs settle and stay settled, you have a strong lead; if nothing changes, restore nothing yet and test the next suspect from your log instead. Resist the temptation to overhaul the whole diet at once, because a dog who improves after five simultaneous changes leaves you feeding a complicated diet without knowing which part of it actually helped him.
Know when the diary hands over to the vet
Home detective work suits a dog who is bright, eating, and having intermittent loose stools, but it has clear limits. See your vet promptly if there is blood in stool or vomit, repeated vomiting, no interest in food, obvious pain, weight loss, or a puppy, senior, or already unwell dog going downhill, and take the diary with you. Even for milder cases, bring the log to a routine visit after a few weeks rather than guessing indefinitely, because ongoing signs can have causes no diet sheet will reveal. Date every page, even through a run of good days, because an unbroken stretch of normal entries is itself strong evidence when you look back over the month. Filed in KinsLe's Pet Passport, your diary sits alongside his vaccination and health records as one packet, so the vet judging his stomach sees the full history rather than your summary of it.