A food trial is 8 weeks of one protein, no exceptions, with 4 numbers written down every day. There are only 2 real choices at the start, hydrolyzed or novel whole protein, and the rest of the work is bookkeeping: a cat that eats a single mouthful of the old food on day 40 goes back to day 1. Most failed trials fail on contamination, not on the wrong diet.
Why 56 days, not 14
The skin barrier renews roughly every 21 days, so a diet change needs 3 turnover cycles before the surface looks different. Itch driven by food usually starts falling between day 14 and day 28 and keeps falling to day 56, which is why a 2 week test tells you almost nothing. The practical structure is a checkpoint at day 28, a decision at day 56, and an extension to 12 weeks only if the curve is still going down at day 56. Cats that improve and then plateau at week 9 are the ones worth extending.
2 choices: hydrolyzed or novel whole protein
Hydrolyzed diets break the protein into fragments small enough that the immune system is unlikely to recognize them, typically under 10 kilodaltons, and they are built on a protein the cat may have eaten before. Novel whole protein diets use 1 meat the cat has never met, such as kangaroo, rabbit, venison or insect, with 1 carbohydrate source, and they depend on a true dietary history. If the cat has eaten rabbit-flavored treats at any point, rabbit is not novel, and the trial is dead before it starts.
The trade-off is taste and cost. Hydrolyzed diets taste flat to many cats and cost more per month, while novel whole protein diets are more palatable but collapse if the owner cannot remember every treat, table scrap and flavored toothpaste from the past 2 years. For a household with children, guests and 2 cats, hydrolyzed is usually the safer design.
Option, protein size, cost, and trial length
| Option | Protein size | Trial length (weeks) | Cost per month (USD) |
|---|---|---|---|
| Hydrolyzed dry | Under 10 kDa | 8 | 90 to 140 |
| Hydrolyzed canned | Under 10 kDa | 8 | 150 to 220 |
| Novel: kangaroo | Whole, 1 source | 8 | 70 to 120 |
| Novel: rabbit | Whole, 1 source | 8 | 65 to 110 |
| Novel: insect | Whole, 1 source | 8 to 10 | 60 to 100 |
Prices are US retail for a 4 kg cat eating 200 to 250 kcal a day. Canned hydrolyzed costs the most because the water content means 2 to 3 times the volume for the same calories.
4 records, 56 rows, one page
Column 1 is intake: grams eaten, or a 0 to 3 score if you do not weigh food, and the point is to catch a cat that quietly cuts back by 30 percent in week 3. Column 2 is stool score 1 to 7, where 4 is a soft log that holds shape and 7 is water. Column 3 is the itch score, taken at the same 10 minute window each day, ideally after the cat wakes and before anybody plays with it. Column 4 is vomit count in 24 hours, with hairballs counted separately. Add weight once a week, same scale, before the first meal. That is 5 numbers a day for 56 days, and it takes 90 seconds.
Scoring itch so 2 people get the same number
Use 4 levels and write the behavior, not the impression. Score 0 is no scratching, grooming or head shaking in the 10 minute window. Score 1 is 1 to 2 events, or one brief ear flick. Score 2 is 3 to 10 events, or any licking that lasts more than 20 seconds at one site. Score 3 is more than 10 events, broken skin, or scratching that interrupts eating and sleeping. Two people scoring the same cat should land within 1 point; if they differ by 2, watch the cat together for 10 minutes and agree on the anchors.
5 rules that stop cross-contamination
One bowl, used by that cat only, washed in hot water for at least 60 seconds after every meal. One scoop, stored inside the food bag, never in the drawer with the old kibble. Zero treats, zero table scraps, zero dental chews, and zero flavored toothpaste or chewable supplements for 56 days, since a single 2 g treat is enough to keep a reaction going. Dry food kept sealed at 22 degrees Celsius or below and used within 6 weeks of opening, because fats oxidize and a stale bag gets refused. Other cats in the house fed in a separate room, with the door closed for the 15 minutes the trial cat eats.
Reading the result at day 28 and day 56
At day 28, compare the average itch score of week 4 with week 1. A fall of 50 percent or more means the diet is probably working, so continue unchanged to day 56. A fall of 20 to 50 percent means keep going and look for a hidden source before you change anything. No change at all at day 28 means the design needs a review: check the records for a 30 percent intake drop, ask again about treats, then discuss switching category with your vet rather than switching mid-trial. At day 56, a cat with a normal itch score and a normal stool score moves to a provocation step, which is a planned reintroduction of the old food over 7 to 14 days, and only then do you know the answer.
What quietly breaks a trial
Four leaks account for most failures. A second cat's bowl left down, which the trial cat cleans at 03:00. Flavored flea products and toothpaste, both of which contain animal protein. Hunting and scavenging outdoors, which cannot be controlled and means the cat stays indoors for 56 days. And the family member who gives 1 piece of chicken at the table because the cat looks hungry. Write the rules on the fridge, in 5 lines, and give every person in the house the same 5 lines.
5 steps to start this week
- Write the last 2 years of food, treats, toothpaste and supplements on one page; that list decides which protein is still novel.
- Pick 1 category with your vet, then buy 8 weeks of the same diet in one order so the formula cannot change mid-trial.
- Set the 4 columns plus weight on a single sheet, 56 rows, and fill day 1 today.
- Remove every other food from the house, or store it sealed in a room the cat cannot enter.
- Book the day 28 and day 56 checks now, so the decision dates cannot slide.
If the choice between the 2 categories is still open, the side-by-side comparison sits in hydrolyzed versus novel protein. For how single-protein diets are built and labeled, read novel protein diets. Before you accept any new protein source, check the plant and food safety list at the toxic lookup.
Frequently asked
Can the trial be shorter if the itching stops in 2 weeks?
No. Itch can fall for reasons other than the diet, including season, flea control and stress, and the skin barrier needs about 3 cycles of 21 days before it is rebuilt. Stopping at day 14 leaves you unable to tell a real food response from a seasonal dip, and you will be back at day 1 within a month.
My cat refuses the hydrolyzed food. Can I mix in a little of the old food?
Not during the trial. Mixing 10 percent of the old food keeps the exposure going and invalidates all 56 days. Transition over 7 days by mixing the new diet with the previous trial diet, not with the regular food, and if refusal lasts more than 48 hours, ask your vet about the other category instead.
Does one treat really reset the clock?
In practice, yes. A 2 g piece of chicken or a flavored dental chew is enough protein to sustain a reaction, and the record sheet can no longer separate diet effect from that exposure. Treat it as day 1 again, note the date and the cause on the sheet, and keep the rest of the routine identical so the next 56 days are clean.