Omega-3 for dogs and cats. Why only EPA and DHA count
The bottle says oil, the arithmetic runs on EPA and DHA. How much per kilogram makes sense, why plant-based linseed oil delivers something different from fish oil, and how to tell when an oil has gone off.
Alongside green-lipped mussel, omega-3 fatty acids have the broadest evidence base in small animal nutrition. They are also the ingredient where the label and the active substance sit furthest apart.
EPA and DHA are the figure that counts
"Omega-3" is an umbrella term. What matters for dogs and cats are the long-chain fatty acids EPA and DHA from fish, krill or algae. The plant-based alpha-linolenic acid in linseed or rapeseed oil is also an omega-3, but dogs convert only a small share of it into EPA and DHA, and cats essentially none. A linseed oil is therefore not a substitute for a fish oil, however high the omega-3 number on the bottle.
As a reference we use 50 to 100 mg of EPA plus DHA per kilogram of body weight per day for dogs and 30 to 60 mg per kilogram for cats. That puts a 20 kg dog at 1000 to 2000 mg a day, and a 4 kg cat at 120 to 240 mg.
What that means for a bottle of salmon oil
Salmon oil contains roughly 15 to 30 percent EPA and DHA depending on origin and processing. A millilitre of oil weighs just under a gram. So a 5 ml teaspoon delivers somewhere around 750 to 1500 mg of EPA and DHA, if the share is declared and sits at the top of that range.
If the bottle says only "pure salmon oil" with no fatty acid profile, you cannot work out the serving. That is not a bad oil, but it is not a figure you can calculate with. Farmed salmon oil is often lower in EPA and DHA than oil from wild catch, because the fatty acid profile follows what the fish were fed.
Freshness is not a side issue here
Long-chain fatty acids oxidise. An oil that has gone off smells sharp rather than mildly of fish, and it no longer delivers what the label says. In practice:
- Dark bottle, into the fridge once opened, used up within a few weeks.
- Buy small bottles even when the price per litre is worse.
- Added vitamin E is common and sensible, because it keeps the oil stable in the bottle.
Where care is warranted
Oil brings energy with it. 5 ml is around 45 kilocalories, which for a 4 kg cat is a noticeable share of the daily requirement. If you add oil, keep an eye on the food. Very high amounts over long periods can affect blood clotting, which is worth raising with your vet before planned surgery.
Supplements are not a medicine. They cannot treat any condition, and a change you notice in your animal should be looked at first.