Omega balance is quoted as one number, and the working range is 5 to 10 parts omega-6 to 1 part omega-3. Most dry foods sit between 15 to 1 and 25 to 1, because chicken fat and seed oils are cheap, stable and heavy on the omega-6 side. A cat on that food is not short of omega-6; it is short of the other end. Closing the gap takes 150 to 300 mg of EPA plus DHA a day, or 2 fish meals a week, and an opened bottle of oil has roughly 6 months of useful life in the fridge.
Why the ratio moves at all
Both families compete for the same desaturase enzymes, so the ratio decides which eicosanoids get built. Linoleic acid is the adult minimum at 0.5 percent of dry matter, and arachidonic acid at 0.02 percent DM is essential because cats cannot make it efficiently. Once those two floors are met, extra omega-6 adds nothing except substrate for inflammatory signalling, while the omega-3 side supplies EPA and DHA for cell membranes, skin barrier and retinal tissue.
Typical kibble carries 2 to 3 g of linoleic acid per 1,000 kcal against 0.05 to 0.1 g of alpha-linolenic acid. That is the arithmetic behind a 20 to 1 or 50 to 1 label, and it is why topping up the omega-3 side does more than cutting omega-6 further.
Flax does not count
Cats convert alpha-linolenic acid to EPA at a few percent of intake, so flaxseed oil at roughly 50 percent ALA is close to useless as an EPA source for a cat. The effective supplements are marine: fish oil, fish, krill, and green-lipped mussel. One gram of a standard 18/12 salmon oil supplies 180 mg of EPA and 120 mg of DHA, 300 mg together, and that single figure is what every dose below is built on.
Three signals of a shortfall
Three findings repeat in practice. A dull coat with fine dandruff along the spine that brushing does not fix within 3 to 4 weeks. Slow healing of small wounds, with skin that tears rather than stretches. Poor coat and growth in kittens under 16 weeks, when DHA is being laid into retinal and neural tissue.
None of these diagnoses anything alone. Flea control, a skin scrape and an 8 week elimination trial come first, because an omega imbalance and an allergic cat look identical from a metre away. What the ratio does is tell you whether the diet is a plausible contributor.
Two fish meals a week, sized
A 4 kg cat at 30 mg of EPA plus DHA per kg needs 120 mg a day, 840 mg a week. Canned sardine in water runs about 1.4 g of EPA plus DHA per 100 g, so a 30 g portion carries 420 mg and two portions a week deliver 840 mg. No bottle, no dropper, no refusal.
Tuna is a poor substitute: skipjack canned in water carries 0.2 to 0.3 g per 100 g, and a tuna-heavy diet adds mercury and thiamine issues. Cooked salmon trimmings at 1.5 g per 100 g hit the weekly target in 55 to 60 g. If the cat will not touch fish, 0.5 ml of 18/12 salmon oil is 150 mg, which covers a day at the low end of the range.
Dose by weight, not by dropper
| Product | EPA + DHA | Dose for a 4 kg cat | mg a day | Ratio shift | Note |
|---|---|---|---|---|---|
| Salmon oil 18/12 | 300 mg per ml | 0.4-0.8 ml a day | 120-240 | 20:1 down to 6:1 | dark glass, fridge, 6 months |
| Sardine, canned in water | 1.4 g per 100 g | 30 g twice a week | 120 average | same shift in food form | drain the brine, count the salt |
| Krill oil | 200 mg per ml | 0.6 ml a day | 120 | modest shift | phospholipid form, costly |
| Cod liver oil | 200 mg per ml | not a daily default | 0-120 | shifts ratio, adds vitamins | 1 tsp is about 4,500 IU vitamin A |
| Flaxseed oil | 50% ALA, no EPA | not useful | 0 | under 5% conversion | counts as omega-3 on paper only |
| Sunflower oil, chicken fat | near zero | none | 0 | pushes ratio up | common in cheap kibble |
Weigh the oil or use a marked syringe; dropper counts vary by 30 to 50 percent between products. Put oil on food, not into the mouth: a cat that gets oil squirted often refuses the next meal.
The 6 month oxidation clock
Polyunsaturated fat oxidizes at a rate that roughly doubles with every 10 C rise, so the same bottle lasts 6 months in the fridge door and a few weeks on a shelf above the stove. Unopened oil keeps 18 to 24 months; once opened, write the date on the cap. Peroxide value under 5 meq/kg and TOTOX under 19 are the usual freshness limits on a certificate of analysis.
Signs of a rancid bottle: a sharp paint-like smell, a bottle that hisses when squeezed, or oil that has turned cloudy. Capsules protect better than bottles because each dose is sealed; a 1,000 mg capsule at 18/12 gives 300 mg, so one capsule covers 2 days at 150 mg a day. Do not store oil next to the food bin in a warm cupboard, and do not top up a half-empty bottle with fresh oil.
Upper limits and what oil is not
Above 100 mg per kg a day some cats produce loose stool, and above 200 mg per kg there is a bleeding-time effect worth stopping 10 to 14 days before surgery. Fat is 9 kcal per gram, so 1 ml of oil a day is 9 kcal, near 4 percent of a 250 kcal cat's intake and enough to matter in a weight-loss plan.
Fish oil is not a treatment for kidney disease, arthritis or skin allergy on its own. It is a nutrient correction, and a cat with vomiting, weight loss or a non-healing sore needs a vet visit rather than a bigger dose. Whether a cat needs supplementation, and at what dose, is the vet's call once the diet and the bloodwork are on the table.
Five steps that hold up
1. Work out the ratio your food probably runs at from its fat sources, since most labels do not print it. 2. Add 2 fish meals of 30 g a week before buying any bottle. 3. If using oil, measure 0.4 to 0.8 ml a day and recheck coat and stool at 6 weeks. 4. Date the bottle and discard it at 6 months. 5. Stop fish oil 10 to 14 days before any planned surgery.
Wet food makes the fish step easier, because a 78 percent moisture product leaves room in the calorie budget for a 30 g portion; the dry matter comparison is in wet versus dry food, and the energy split when both are fed is in wet food ratio. Check anything unfamiliar against the toxic list before it goes in the bowl.
Terms mentioned in this article
Frequently asked
What omega-6 to omega-3 ratio should cat food have?
A working range of 5 to 10 parts omega-6 to 1 part omega-3, against the 15 to 1 to 25 to 1 that most dry foods deliver from chicken fat and seed oils. The floors are 0.5 percent linoleic acid and 0.02 percent arachidonic acid in dry matter, so the job is raising the omega-3 side rather than cutting omega-6 below those minimums.
Can I use flaxseed oil instead of fish oil?
Not as an EPA source. Cats convert alpha-linolenic acid at only a few percent, and flaxseed oil is roughly half ALA with no EPA or DHA at all, so a label that counts it as omega-3 overstates what the cat can use. Marine sources, meaning fish, fish oil, krill or green-lipped mussel, are the ones that move the ratio in a cat.
How long does an opened bottle of fish oil keep?
About 6 months in the fridge door, against 18 to 24 months unopened. Oxidation doubles for every 10 C rise, so a bottle kept near the stove is spent in weeks. Write the opening date on the cap, keep it in dark glass, and discard it early if it smells sharp or pours cloudy. Capsules avoid the problem because each dose stays sealed.
Related guides
- How Much Wet Food a Cat Needs: Ratios, Water Math, Cost
- Cat Grass: 7-Day Wheatgrass and 10 cm Grazing
- How to Switch Cat Food: The 7-Day Schedule, With Grams
- Omega-3 Dose for Cats: mg/kg to Millilitres
- Switching an 11-Year-Old Cat's Food: 7-Day Gram Plan
- Taurine in Cat Food: 250-500 mg, and Who Falls Short