Hydrolyzed protein is not a medicine and not a cure for itching. It is a diagnostic tool with a number attached: peptides small enough that the immune system stops recognizing them, usually under 10 kDa, fed as the only food for 8 weeks. If the itching, vomiting or diarrhea improves in that window and returns within 7-14 days of reintroducing the old food, you have an answer. If nothing changes after 8 weeks of a strict trial, you have a different answer, and both are useful.
What hydrolysis actually does
Hydrolysis breaks peptide bonds with heat, acid or enzymes until a long protein chain becomes short fragments. A native food allergen such as casein (24 kDa) or fish parvalbumin (12 kDa) arrives with 5-10 IgE-binding epitopes spread along the chain; the immune system needs 2 adjacent epitopes to cross-link a mast cell and trigger a reaction. Cut the chain into pieces under 3 kDa and most of those pairs disappear. Commercial ultra-hydrolyzed diets aim well below the label claim: a formula marketed as under 10 kDa usually has 95 percent or more of its peptides under 1 kDa, with the largest fragments at 3-5 kDa.
Two families of formula, two jobs
| Type | Median peptide size | Largest peptides | Protein sources |
|---|---|---|---|
| Ultra-hydrolyzed (casein, feather, soy) | Under 1 kDa | 3-5 kDa | 1-2 |
| Partially hydrolyzed | 3-10 kDa | 10-20 kDa | 1 |
| Single novel protein, intact | 10-40 kDa | 40-60 kDa | 1 |
| Grocery or regular diet | 20-60 kDa | Over 60 kDa | 3-8 |
The first row is for diagnosis: a cat with a suspected food allergy, inflammatory bowel disease or eosinophilic disease needs peptides small enough that a previous exposure does not matter. The second row is for maintenance after a diagnosis, and for prevention in a cat with a known sensitive gut; it is cheaper per kilogram and more palatable, but it will not carry a strict elimination trial on its own. Row 3 is the fallback when a cat refuses hydrolyzed food: a protein the cat has never eaten, at 8 weeks, with the same rules.
Why 10 kDa is the working cutoff
Three numbers explain the choice. Epitope spacing on a linear allergen is roughly 10-20 amino acids, which is 1-2 kDa of chain, so fragments need to be short enough that a single epitope cannot pair with a second. Most reported feline food allergens — beef, dairy, chicken, fish — sit in the 10-40 kDa band, and a diet that keeps 90 percent of its protein under 3 kDa removes the majority of those targets. The 10 kDa label number is a floor for the whole batch, not an average, which is why two diets both claiming under 10 kDa can behave differently: one has a median of 0.8 kDa and the other of 6 kDa. Ask the manufacturer for the median, not the ceiling.
The 8-week trial, week by week
Eight weeks is the minimum because skin turnover in a cat runs 21-28 days and the inflammatory cascade in the gut takes 3-4 weeks to settle. Stopping at 4 weeks is the single most common reason a real food allergy gets called a failure.
| Week | What to record | Decision point |
|---|---|---|
| 0-2 | Body weight, stool score 1-7, itch score 0-10, food grams per day | Weight loss over 5 percent means the diet is not being eaten |
| 2-4 | Same 4 numbers, plus vomiting episodes per week | No change is normal; do not switch yet |
| 4-6 | Same 4 numbers, plus ear and chin lesions | First improvement usually appears here |
| 6-8 | Same 4 numbers, photo of the same 2 body sites | Judge at day 56, not day 30 |
| 8-10 | Reintroduce the old food for 7-14 days | Relapse confirms the diagnosis |
Weigh the cat weekly on a kitchen scale and keep the grams in the same notebook as the itch score. A 4.5 kg cat that drops to 4.2 kg in 3 weeks is losing 6.7 percent of body weight, which is a bigger problem than the itch you were chasing. Expect to feed 55-70 kcal per kilogram per day and to transition over 5-7 days, mixing 25 percent new food on days 1-2, 50 percent on days 3-4, 75 percent on days 5-6, and 100 percent from day 7.
Six things that break a trial
Most failed trials fail on compliance, not biology. Table scraps and a lick of tuna count: 10 percent of daily calories from another source keeps a reaction going. A 4-cat household needs all 4 on the same food, or separate rooms with 2 doors, because 1 bowl of old kibble is a 30-second experiment. Flavored liquids, chewable wormers and toothpaste carry protein, as do some capsules and 1-2 hairball gels. Treats must be the same kibble, weighed into the daily total. A cat that hunts mice is on a diet you cannot control.
Cost, calories and palatability
Hydrolyzed dry food is expensive per bag and cheap per calorie, which is the number that matters. A 2 kg bag in the 350-600 CNY band feeds a 4.5 kg cat for 20-26 days at 60 g per day; a 1.5 kg bag of the same line at 40 g per day lasts 37 days. Compare cost per 100 kcal, not cost per kilogram, and check the kcal/kg on the label: 3,600-3,900 kcal/kg is typical, and a diet at 3,200 kcal/kg needs 10-15 percent more grams for the same energy. Palatability is the real risk: 20-30 percent of cats refuse hydrolyzed food for the first 3-5 days, which is why the transition should start before the old bag runs out. For cats over 11 years, senior protein levels covers how much protein is enough at 30-40 percent of calories.
When 8 weeks changes nothing
A strict 8-week trial with no drop in the itch score makes food allergy unlikely, and the next steps change: skin cytology plus a flea plan for allergic dermatitis, a serum or elimination workup for environmental allergy, ultrasound and B12 for chronic enteropathy. If the trial worked but the cat will not eat the diet, a single novel protein for 8 weeks is the standard fallback, and a home-cooked diet with a veterinary nutritionist is the last option. Never start a second hydrolyzed brand without a 7-14 day washout: overlapping trials produce an answer you cannot read.
How to start this week
- Ask for the median peptide size and the percent under 1 kDa, in writing.
- Buy 8 weeks of food before you start; running out on day 40 restarts the clock.
- Weigh the cat today, then weekly, and write the grams down.
- Move every cat in the house onto the same food, or feed in separate rooms.
- Book the day-56 recheck now, and plan the 7-14 day rechallenge with the old food. For the label-by-label comparison between hydrolyzed products, see hydrolyzed protein diets.
Frequently asked
Is hydrolyzed food the same as hypoallergenic food?
Not exactly. Hypoallergenic is a marketing word; hydrolyzed describes a process and a peptide size. A diet with a median peptide of 0.8 kDa and 95 percent of fragments under 1 kDa can support an elimination trial, while a partially hydrolyzed diet at 3-10 kDa is better for maintenance than for diagnosis. Ask for the median and the percentage under 1 kDa rather than trusting the front of the bag, because both products may print the same 10 kDa claim.
Can I mix hydrolyzed food with the old food to get the cat to eat?
Only during the 5-7 day transition, at 25, 50 and 75 percent steps. After day 7 the bowl has to be 100 percent hydrolyzed food, because 10 percent of daily calories from another source keeps the reaction alive and makes the 8 weeks unreadable. If the cat refuses, warm the food to body temperature, feed 4 small meals instead of 2, and try a shallow plate; do not add broth, tuna water or treats.
How long before I see a difference in itching?
Skin takes longer than gut. Vomiting and stool quality often shift in 2-4 weeks, while itch scores usually move between weeks 4 and 6 and peak around week 8, because epidermal turnover runs 21-28 days and the inflammatory cascade needs 3-4 weeks to settle. Photograph the same 2 body sites weekly under the same light, and judge at day 56. Stopping at day 30 is the most common way a real food allergy gets labelled a failure.