Treat the Switch as a Dated Experiment
A food change only becomes useful evidence if it has clear dates. Write down the day you bought the new food and the day you served the first mixed bowl, then note the stages as you increase the new food and reduce the old, ending with the date your cat was eating the new food alone. Record both foods precisely — full product name, flavour or recipe, and whether it is wet, dry, or a mix — because saying you changed brands months later, without which recipe you changed to, will not help a vet narrow anything down. Keep the note in the same place as your cat's other health records from the first entry, not on a loose scrap that migrates around the kitchen.
Log the Specific Changes You Are Watching For
Decide in advance which signs you will track, and use the same short list each time you write an entry. Stool consistency and frequency, any vomiting, appetite at each meal, water drinking, energy and playfulness, scratching or ear shaking, and coat feel cover most of what food can plausibly affect. Write brief dated entries a few times a week during the transition and for a few weeks after, even when the entry is simply that everything is normal. A run of normal entries is evidence too: it tells you the wobble in week four probably is not the food settled in week one, or that a problem genuinely began only after the switch was complete.
Record the Other Changes That Could Confuse the Story
Cats rarely change one thing at a time. In the same log, note anything else new in the window around the switch: a different treat, a new water fountain, a house move, a new pet, building work, or a course of treatment from the vet. Without those notes, owners understandably blame the most obvious change — the food — for a reaction actually driven by stress or a new treat given far more often than intended. If several cats share your home, record which cat the note is about and whether the others eat the same food without issue, since a problem in one cat on a shared diet points differently from a problem in all of them.
Weigh and Photograph, Do Not Rely on Impressions
Weight drift is the change owners notice last and remember worst. Weigh your cat at the start of the switch and at intervals afterwards, using the same method each time, and write the figures in the log rather than trusting that she looks about the same. A dated photo in similar light each month makes coat and body condition comparisons honest. If you later discuss the change with a vet, bring the log itself: dates, proportions, weights, and symptom entries turn a vague account of trouble starting around the food change into a timeline a clinician can actually reason from.
Know When the Log Says Stop and Ask
A gradual transition commonly produces a brief, mild tummy upset, but stop and seek your vet's advice if your cat refuses the new food entirely for more than a day, vomits repeatedly, has persistent loose stools, loses weight noticeably, or seems unwell in herself, rather than pushing on to complete the switch. Note that decision and its date in the log too, including which food you returned to. KinsLe's Pet Passport keeps those dated diet notes together with your cat's wider health records in one packet, so if a change surfaces weeks later, the switch, the timeline, and the outcome can be read together at the vet's desk.