Can I use a grain free food instead of the one recommended?

No. The absence of grain plays no part in a food trial, because adverse reactions far more often involve animal protein sources. A food chosen on your own also risks containing proteins the animal has already eaten, which makes the trial worthless.

How long do the eight weeks really last if I slip up once?

It depends on the amount and the timing. An isolated episode in the early weeks usually means restarting the count, while an episode in the final phase may only require an extension. The decision belongs to your vet, who can only make it if the episode is reported.

My cat refuses the new food: what should I do?

Do not push through with fasting. In cats a reduced intake for more than two or three days carries a real risk of liver disease, so refusal must be reported to your vet at once. They may suggest a different food, a different texture or gently warming the meal to raise its smell.

Can I keep anti itch treatment going during the trial?

Only as directed by your vet. Some treatments are kept for the first weeks to make the animal comfortable and then tapered, precisely so that any improvement can be attributed to the food. Stopping or continuing on your own makes the result impossible to interpret.

Practical feeding45 minLevel: AdvancedUpdated on August 4, 2026

Running a food elimination trial without mistakes

The full elimination diet protocol: choosing the food, eight weeks without contamination, weekly scoring and the rechallenge that closes the trial.

Audience: Pet owners
Species: Dog, Cat
Scope: Valid everywhere

What you need before you start

L'alimento di esclusione indicato dal veterinario, idrolizzato oppure a proteina nuova mai assunta prima
L'elenco scritto di tutto ciò che l'animale ha mangiato negli ultimi sei mesi, cibo, premi e integratori compresi
Una bilancia da cucina e un contenitore ermetico dedicato solo al nuovo alimento
Una scheda di punteggio settimanale per prurito, cute, feci e frequenza delle scariche

An elimination diet is the only recognised way to find out whether an animal's skin or gut signs depend on what it eats. It is not a laboratory test: it is a clinical trial that runs for weeks and gives an answer only if it is carried out without exceptions. Nearly every elimination diet that concludes nothing fails not because of the food chosen but because of household contamination, a treat from a neighbour, a flavoured toothpaste, the other pet's bowl. This guide covers the setup, which takes about forty five minutes, and then the trial period itself, which runs eight to twelve weeks, with the scores to record every week.

When an elimination diet makes sense

A food trial is proposed when the signs are compatible with an adverse food reaction and the other frequent causes have been ruled out or treated. Starting one while a flea infestation or a bacterial dermatitis is still active makes the result unreadable, because any improvement could come from the treatment rather than from the change of food.

Itching present all year round, without the seasonal peak typical of environmental allergy.
Recurrent ear infections that come back a few weeks after each treatment.
Skin signs accompanied by gut signs, in particular more than three bowel movements a day.
Onset before the first birthday or after six years of age, when environmental allergies begin less often.
Partial or absent response to anti itch medication, which is more common in adverse food reaction.

Choosing the food with your vet

There are three routes, and the choice depends on the animal's dietary history and on how much rigour you can guarantee at home. Bringing your vet a written list of everything eaten in the last six months is what makes the choice possible: without that list, a protein assumed to be novel may already have appeared in a forgotten treat.

Types of elimination diet compared
Type	Strength	Weakness
Hydrolysed protein food	Proteins broken into very small fragments, less recognisable by the immune system	Palatability sometimes low, especially in cats
Commercial novel protein food	Easy to use and nutritionally complete	Risk of traces of other undeclared proteins
Home cooked diet formulated by a nutritionist	Complete control over ingredients	Requires professional formulation and preparation time
Generic grain free food	None for the purposes of the trial	It is not an elimination diet and must not be used as one

Setting up the trial in nine steps

Write the full dietary history
List foods, treats, table scraps, supplements, palatable pastes and chews from the last six months. Ask everyone else in the household too: half of all contamination comes from people who were not part of the decision.
Set the start date and the end date
Mark day one and the day of the eighth week on the calendar. A trial without an end date drifts and loses the moment when it should be interpreted.
Record the starting point
Before changing food, score the itch from 0 to 10, photograph the affected skin areas and note weight, number of daily bowel movements and stool consistency. Without a baseline, improvement cannot be demonstrated.
Clear the house of the previous foods
Take treats out of coat pockets, bags and the car glovebox, and store the old bag out of reach. Whatever stays within reach gets used sooner or later, out of habit.
Switch to the new food over five days
Replace about a quarter of the ration every day and a half, reaching one hundred per cent by the fifth day. In cats the transition may need longer, because prolonged refusal is dangerous and must be reported to the vet immediately.
Dedicate a container and a bowl to the new food
Store the new food in a dedicated airtight container and wash the bowl with hot water and detergent before the first meal. Greasy residue in plastic bowls holds traces of the previous food.
Separate the other animals in the house
In a multi pet home, meals must be served at separate stations and bowls removed when the meal ends. Even a few mouthfuls stolen from the other bowl invalidate the trial.
Tell everyone who has access to the animal
Write a note and pin it where food is prepared: no treats, no scraps, no mouthfuls. Warn neighbours, boarding kennels, pet sitters and anyone who crosses paths with the animal during the day.
Score once a week
Always the same day, always the same observer. A score given by different people on different days does not produce a comparable curve.

The contamination sources that ruin everything

A food trial breaks with very small amounts. This list gathers the sources most often forgotten, almost all of them in good faith and almost all of them outside the bowl.

Training treats, chew sticks and pressed bones of any origin.
Veterinary toothpaste flavoured with chicken or malt, often used every evening without a second thought.
Flavoured tablets and palatable pastes used to give medication: ask your vet for a neutral form.
Oral parasite products with a palatable base, which may only be changed on veterinary advice.
Supplements, oils and coat pastes, which almost always contain flavours of animal origin.
The other pet's bowl, the rabbit's bedding, bird food dropped on the floor.
Scraps picked up on walks, garden compost and open bins.
Mouthfuls offered by children, neighbours and guests who did not know about the trial.

Measuring the response week by week

Improvement in adverse food reaction is gradual and often barely visible day to day. That is why you need a numeric score recorded in the same way every time, which makes visible a trend the eye does not catch.

What to record every week and how to express it
Parameter	How it is measured	What improvement looks like
Itch	Score from 0 to 10 given by the same observer	A drop of at least two points from the starting week
Extent of lesions	Photograph of the same areas in the same light	Visible reduction of the reddened area
Daily bowel movements	Count over twenty four hours	Return to a stable number for the species
Stool consistency	Scale from 1 to 7	Central values stable for at least five days
Ear infections	Number of episodes in the period	No new episode during the trial
Weight	Weekly weighing before a meal	Weight stable or recovering

Gut signs respond first, often within two or three weeks. Skin signs are slower: a large share of dogs improve by the eighth week, but some need up to twelve weeks before the picture can be interpreted. In cats the useful window is generally between six and eight weeks.

The rechallenge, the step almost nobody completes

Improvement during elimination does not by itself prove that the cause is dietary: it could coincide with the end of the pollen season or with the effect of an ongoing treatment. The trial only closes when the previous food is reintroduced and you watch whether the signs come back. It is an uncomfortable step, but it is the one that turns an impression into a diagnosis.

Reintroduce the previous food only after your vet has confirmed the improvement.
Observe for a maximum of fourteen days: in most cases the signs return within that window.
If the signs return, go straight back to the elimination food and consider the adverse food reaction confirmed.
If the signs do not return, the cause was probably something else and the trial should be discussed again with your vet.
To identify the individual ingredient, reintroduce one protein source at a time, each for a maximum of two weeks.

Frequently asked questions

Can I use a grain free food instead of the one recommended?: No. The absence of grain plays no part in a food trial, because adverse reactions far more often involve animal protein sources. A food chosen on your own also risks containing proteins the animal has already eaten, which makes the trial worthless.
How long do the eight weeks really last if I slip up once?: It depends on the amount and the timing. An isolated episode in the early weeks usually means restarting the count, while an episode in the final phase may only require an extension. The decision belongs to your vet, who can only make it if the episode is reported.
My cat refuses the new food: what should I do?: Do not push through with fasting. In cats a reduced intake for more than two or three days carries a real risk of liver disease, so refusal must be reported to your vet at once. They may suggest a different food, a different texture or gently warming the meal to raise its smell.
Can I keep anti itch treatment going during the trial?: Only as directed by your vet. Some treatments are kept for the first weeks to make the animal comfortable and then tapered, precisely so that any improvement can be attributed to the food. Stopping or continuing on your own makes the result impossible to interpret.

What to do next

Write the full dietary history, put the end date on the calendar and record a baseline with an itch score and a few photographs. Remove every source outside the bowl, treats and toothpaste included, score once a week on the same day, and close the trial with a rechallenge: without that step you have a coincidence, not a diagnosis.