Food Allergy or Food Intolerance? How to Tell, and How to Test

“Allergy” gets used for any adverse reaction to food, but the distinction matters because it changes what you look for.

The difference

Food allergy is an immune response to a protein. The immune system misidentifies a food protein as a threat and mounts a reaction. Signs are typically skin-based: itching, recurrent ear infections, inflamed paws. A minority also have digestive signs.

Food intolerance is not immune-mediated. It is a digestive problem — the dog cannot process something properly. Signs are gut-based: loose stool, wind, vomiting, discomfort. Lactose intolerance is the familiar human example.

Food poisoning or dietary indiscretion is neither. It is a one-off response to something spoiled or inappropriate.

What dogs react to

Contrary to marketing, the common culprits are the proteins dogs eat most, not exotic ingredients:

• Beef
• Dairy
• Chicken
• Wheat
• Lamb
• Egg
• Soy

Grain allergy specifically is rare. The grain-free market was built on a human dietary trend rather than canine evidence.

A dog cannot become allergic to something they have never eaten. Allergy develops through exposure over time, which is why it usually appears to a food the dog has been eating happily for months or years.

Signs pointing to food allergy

• Non-seasonal itching — year-round, unaffected by the weather
• Itchy ears and recurrent ear infections, often the first and sometimes the only sign
• Itchy feet, with licking and chewing between the toes
• Face, armpit, groin and perineum involvement
• Recurrent skin infections
• Onset at any age, including under one year and over six, which distinguishes it somewhat from environmental allergy
• Digestive signs in around a quarter of cases

Signs pointing to intolerance

• Loose stool or diarrhoea
• Excessive wind
• Vomiting, often shortly after eating
• Gurgling, discomfort
• Stool volume out of proportion to the food eaten
• No itching

The tests that do not work

Blood allergy tests for food. Serum IgE testing for food allergy is not reliable in dogs. It produces high rates of false positives and does not correlate well with clinical reactions.

Saliva and hair tests sold to owners. These have been tested by researchers who submitted samples of tap water, synthetic fur and samples from non-existent animals. They came back with positive results. They are not diagnostic.

Skin patch and intradermal testing. Useful for environmental allergy. Not for food.

There is currently no reliable laboratory test for food allergy in dogs. If someone sells you one, that is the relevant fact.

The test that does work

An elimination diet trial, done strictly.

1. Choose the diet. Two options, both from your vet:

• A novel protein diet — a protein and carbohydrate source your dog has genuinely never eaten. This is harder than it sounds, because many “novel” commercial foods have been contaminated with common proteins in testing
• A hydrolysed diet, where proteins are broken into fragments too small for the immune system to recognise. Usually the more reliable option

Home-prepared novel protein diets are possible but must be formulated by a qualified nutritionist, particularly for long trials.

2. Feed it, and nothing else, for eight to twelve weeks.

Nothing else means nothing else:

• No treats, except pieces of the trial food
• No chews, dental sticks or rawhide
• No flavoured medications or flavoured heartworm preventives — ask your vet for unflavoured alternatives
• No table scraps
• No toothpaste with flavour
• No licking other pets’ bowls
• No scavenging on walks. Use a basket muzzle if necessary
• Tell every family member, and every visitor

A single lapse a week invalidates the whole trial. This is the reason most elimination diets fail, and it is worth being blunt about before you start.

3. Assess. Skin takes time to respond. Do not judge at four weeks. Score the itching weekly — a simple 0 to 10 — and keep a record.

4. Challenge. This is the step almost everyone skips, and it is what turns a suspicion into a diagnosis. If the signs resolve, reintroduce the original food. If they return within two weeks, the diagnosis is confirmed.

5. Identify the trigger. Reintroduce single ingredients one at a time, two weeks apart, to find which one is responsible. Then you can choose commercial foods knowing what to avoid.

Practical support during a trial

Secondary skin and ear infections usually need treating alongside, or the dog stays itchy for reasons unrelated to the diet — which makes the trial impossible to read. Your vet will usually deal with these first.

A probiotic can help settle the gut during a diet change.

Bathing helps a great deal with itch while you wait for the diet to work.

Living with a diagnosed food allergy

• Read every label, every time. Formulations change without notice
• Watch for “animal derivatives” and similar vague terms
• Find treats made from the same protein as the main food
• Be careful with dental chews and flavoured medications
• Warn anyone who might feed your dog
• Expect occasional flare-ups from accidental exposure — and do not conclude the diagnosis was wrong because of one

And the most common outcome

Many dogs who are investigated for food allergy turn out to have environmental allergy instead, which is far more common. The elimination trial is how you find that out — and a negative result is still a useful result, because it directs attention where it belongs.