Fish is not the problem; raw fish is. Herring, smelt, carp, anchovy and the whole carp family carry thiaminase, an enzyme that splits thiamine (vitamin B1) apart in the gut before the cat can absorb it, and a cat eating raw fish daily can be deficient in 2-4 weeks. Cooked fish is a different food, because 60 C destroys the enzyme.
The practical rule most vets give: cooked fish no more than 2 meals a week, raw fish never as a routine food, and no more than 10-20% of daily calories from any fish at all if the rest of the diet is a complete kibble or wet food.
What thiaminase actually does
Thiamine is the coenzyme for carbohydrate metabolism, so a shortage hits the tissues that burn glucose fastest: the brainstem, the peripheral nerves, the heart. The enzyme in raw fish is not heat-stable and not species-specific, which is why it also destroys the thiamine in the cat's own food if both are in the bowl at the same time.
There is a second route. Raw fish carries thiamine-splitting bacteria, and spoiled fish carries more of them, so a fish that has sat at 20 C for 4 hours is worse than a fresh one on both counts. Freezing at -18 C for 30 days does not fix either problem.
The 2-4 week window, and what it looks like
Clinical signs usually appear 2-4 weeks into a thiamine-poor diet, and they start vague: a cat that stops finishing meals, drops 5-10% of body weight, vomits once or twice a week, and sits with its head lower than usual. The textbook sign is ventroflexion, the neck folded down toward the chest, and it can appear within 24-48 hours of the first vague signs.
Later signs are neurological and urgent: wide dilated pupils that do not react to light, a wobbly gait, tremors, seizures, and a slow heart rate under 150 beats a minute. A cat at that stage needs a vet the same day, not a diet change, because thiamine replacement works fast when it is started early and badly when it is late.
Cooking: 60 C is the number
Thiaminase is denatured at about 60 C, so any real cooking defeats it: poaching at 75-80 C for 10 minutes, steaming for 8-10 minutes, baking a fillet to 63 C internal, or a pressure cooker at 115 C for 5 minutes. The centre of the fish has to reach the temperature, which is why a thick fillet needs 12-15 minutes and a thin one 5-6.
Boiling does the job but leaches B vitamins into the water, so poach in a shallow pan and keep the liquid, or steam instead. Canned and pouched fish is already cooked under pressure, so the enzyme is gone; the things to watch there are salt (some brands add 1-2% sodium) and oil.
Which fish, and how much
| Form | Centre temp (C) | Grams per week | Max meals |
|---|---|---|---|
| Raw herring, smelt, carp | 0 | 0 | 0 |
| Poached fillet, 10 min | 75-80 | 60-90 | 2 |
| Steamed fillet, 8-10 min | 75-80 | 60-90 | 2 |
| Baked fillet | 63 | 60-90 | 2 |
| Canned in water | 121 | 40-60 | 2-3 |
| Freeze-dried raw treat | 0 | 5-10 | 2-3 |
| Complete fish-based food | 121 | 200-250 | 7 |
A zero in the heat column means the fish was never cooked. A complete fish-based food is a different category: it is formulated with added thiamine and tested against a nutrient profile, so a fish-first kibble or wet food can be the whole diet. The 2-meal cap applies to fish fed as a topper, not to a complete diet.
Why owners feed raw fish at all
Usually for the coat, the omega-3s, or because the cat loves it. The omega-3 argument is real but does not need raw fish: 30-40 mg per kg of body weight of combined EPA and DHA a day comes from a measured fish oil, and a 4 kg cat needs about 120-160 mg, which is 1-2 soft capsules or 2-3 ml of a pet fish oil.
If the cat loves fish, feed it cooked, unseasoned and boneless as a topper: 10-20% of calories, so 20-40 kcal, which is 25-45 g of white fish. Never feed cooked bones, and never feed a fish with visible parasites, however fresh it looks.
Building a week that stays under the cap
A simple pattern: 2 fish meals, 3 poultry or rabbit meals, 2 red meat or complete wet meals, with fish never on consecutive days. If the cat is on a complete food, the fish meal replaces no more than 1 of 3 daily meals, and the rest of the day stays balanced.
Water-packed sardines are a useful topper at 20-30 g, but they carry 300-400 mg of sodium per 100 g, so 2 servings a week is the ceiling for a cat with any heart or kidney issue. Anchovies in oil are 2-3 times the calories of fish in water, which matters in a 4.5 kg cat with 250 kcal a day.
Signs that need a vet this week
Weight loss of 5-10% over 4-6 weeks, a cat that eats but stays thin, vomiting 2-3 times a week, or a change in the walk. Ask specifically for a thiamine check and a blood panel, and mention the raw fish, because thiamine is not on every routine panel and the history is what prompts it.
Same day for a cat that cannot hold its head up, has seizures, or breathes with an open mouth. Thiamine deficiency is treatable, usually with an injection of 10-25 mg followed by oral thiamine for 2-4 weeks, and most cats improve within 24-72 hours of the first dose; a cat that does not improve needs the diagnosis revisited.
Four mistakes with fish
- Feeding raw fish as the main protein and calling it a natural diet.
- Freezing raw fish for a month and assuming the enzyme is gone.
- Counting canned tuna as a complete meal; it is 1-2% fat and no taurine guarantee.
- Adding fish oil to a complete diet without checking the total, then double-dosing.
What to do this week
- Count the fish meals in the last 7 days; anything above 2 becomes a cooking session.
- Poach the next batch at 75-80 C for 10 minutes, portion 60-90 g, freeze in 30-day lots.
- Swap any raw fish topper for cooked fish or a measured fish oil at 120-160 mg EPA and DHA.
- Weigh the cat now and again in 4 weeks; a 5% drop is a vet visit, not a bigger bowl.
- Ask for a thiamine check if the cat has been on raw fish for 2 weeks or more.
For how fish fits into the wet and dry split, see wet versus dry food and the wet food ratio. Anything the cat should not eat at all is in the toxic exposure tool.
Frequently asked
Is cooked fish safe for a cat every day?
As a topper, cooked fish is fine, but keep it to 2 meals a week and 10-20% of daily calories, which is 25-45 g of white fish for a 4 kg cat. A complete fish-based food is a different category, because it is formulated with added thiamine and tested against a nutrient profile; a fish topper is not a balanced meal.
Does freezing raw fish destroy thiaminase?
No. The enzyme is destroyed by heat at about 60 C, not by cold, so -18 C for 30 days leaves it active. Raw herring, smelt, carp and anchovy should not be a routine food at all; if you cook them, poach at 75-80 C for 10 minutes so the centre reaches temperature.
What do thiamine deficiency signs look like?
They start vague 2-4 weeks into a deficient diet: poor appetite, 5-10% weight loss, vomiting once or twice a week, and a head carried lower than usual. Then come dilated pupils, a wobbly gait, tremors and seizures, which need a vet the same day. Treatment is usually an injection of 10-25 mg plus oral thiamine for 2-4 weeks.