Omega fatty acids for pets: fish, krill, algae or flaxseed

For dogs and cats, the omega-3 fatty acids that do the useful work are EPA and DHA, and they come from marine sources: fish, krill and algae. Plant omega-3 from flaxseed is not a practical substitute. In one feeding trial, dogs given fish or krill had higher levels of EPA and DHA in their red blood cells after four weeks, while dogs given flaxseed showed no significant change. Omega-3s have reasonable evidence in a few conditions, joint disease and atopic skin disease in particular, but they do not fix everything, and they can have side effects.

The fatty acids that matter

Fatty acids are grouped by chemical structure into omega-6 and omega-3 families. The main ones in pet nutrition are:

• Linoleic acid (omega-6): required in the diet by all dogs and cats. Vegetable oils such as corn and soy oil are rich sources.
• Arachidonic acid (omega-6): an extra requirement for cats, which cannot readily make it from linoleic acid. It is found in fat from meat, poultry and eggs and is absent from vegetable oils.
• Alpha-linolenic acid, ALA (omega-3): the plant omega-3, with flaxseed oil the richest dietary source.
• EPA and DHA (omega-3): the long-chain omega-3s, best supplied by oily fish, krill oil and algae oil.

The Merck Veterinary Manual lists omega-3s as essential during growth and reproduction. It also notes that essential fatty acid deficiency is extremely rare in animals eating a properly preserved, complete and balanced diet. When deficiency does happen, the signs include a dry, scaly, dull coat. Fatty acid supplements are often suggested for a dog with flaky skin, but the vet should rule out underlying metabolic conditions before assuming the diet is short of fat.

Plant and marine sources are not interchangeable

Dogs can convert some ALA into EPA and DHA, but not enough to rely on. In a study of 45 dogs, raw diets with krill meal or fish meal and oil raised the omega-3 index, a measure of EPA and DHA in red blood cell membranes, within four weeks. A diet with flaxseed cake, which supplied more total ALA, did not change it. Krill produced the largest increase in that trial.

Cats cannot readily convert linoleic acid into arachidonic acid, and the source comparison above was done in dogs. For a cat, marine EPA and DHA are the safer choice.

Where the evidence is strongest

Joints

Joint disease is where the trial evidence in dogs is most consistent. Controlled trials in dogs found that foods enriched with fish oil improved weight bearing, and a 2022 systematic review rated omega-3 diets and supplements among the few joint products with clear evidence of pain relief. The joint health guide covers the trials and how omega-3s fit alongside weight control and pain medication.

Itchy skin

Omega-3s are used as an add-on for dogs with atopic dermatitis rather than as a treatment on their own. In a placebo-controlled trial of 22 atopic dogs taking the anti-itch drug oclacitinib, dogs given a fish oil product needed a lower daily oclacitinib dose over 16 weeks and lower itch scores than dogs given placebo. Coat and quality-of-life scores improved only in the fish oil group. Earlier trials reported similar dose-sparing effects with prednisolone and ciclosporin. It is a long-term add-on, not a fix for an acute flare.

Kidney disease

In an experimental study, dogs with surgically reduced kidney function were fed diets with added fish oil, safflower oil (omega-6) or beef tallow for 20 months. The fish oil group had less protein in the urine and less scarring. The omega-6 group did worst, with the lowest filtration rate. It was a laboratory model rather than a trial in pets with naturally occurring kidney disease, and an animal with kidney disease should have its diet set by a vet rather than supplemented at home.

Brain and eye development

DHA is a structural fat in the brain and retina. In a year-long study, Beagle puppies fed a food fortified with DHA-rich fish oil did better on learning tasks and visual contrast tests than puppies on lower-DHA foods. They also had higher antibody levels after rabies vaccination. The high-DHA food also had more vitamin E, taurine, choline and carnitine, so the effect cannot be put down to DHA alone. For older pets, evidence that DHA slows cognitive decline is limited.

The omega-6 to omega-3 ratio

Pet food marketing often quotes an omega-6 to omega-3 ratio. The ratio says something about balance, but it does not say how much EPA and DHA the animal actually eats, and a food can have a favorable ratio while getting most of its omega-3 from flaxseed. The trials above tested amounts of EPA and DHA, so that is the number worth asking a manufacturer for.

Choosing a source

• Fish oil: the most studied source of EPA and DHA. Choose a product that states the EPA and DHA content per dose and has been tested for contaminants.
• Krill oil: supplies EPA and DHA and gave the largest omega-3 index rise in the dog study above.
• Algae oil: a marine source of DHA, and of EPA in some products, that does not rely on fish.
• Flaxseed oil: supplies ALA, which did not raise EPA and DHA in dogs. Not a substitute.
• Coconut oil: mostly saturated fat, with essentially no omega-3.
• Cod liver oil: contains EPA and DHA but also vitamins A and D. Both vitamins are toxic in excess, so it is a poor choice for long-term dosing at the amounts needed for omega-3 effects.

Dose and side effects

Fish oil is not risk-free. A review of potential adverse effects in dogs and cats lists altered platelet function, vomiting and diarrhea, slower wound healing, oxidation of the fats, weight gain from the extra calories, effects on immune function and blood sugar control, contaminant exposure and interactions with medications. They matter most for an animal facing surgery or with a clotting problem, and for diabetic pets.

The dose should be worked out with a vet from the product's EPA plus DHA content and the animal's weight, and adjusted for the reason it is being given. Check the current food first: a therapeutic joint, skin or kidney diet may already supply a meaningful amount. Start with a partial dose and build up over a week or two to limit stomach upset, and count the oil's calories in the daily total.

Sources

1. Nutritional requirements of small animals. Merck Veterinary Manual.
2. Comparison of fish, krill and flaxseed as omega-3 sources to increase the omega-3 index in dogs. Veterinary Sciences, 2023.
3. A placebo-controlled, double-blind study evaluating the effect of orally administered polyunsaturated fatty acids on the oclacitinib dose for atopic dogs. Veterinary Dermatology, 2024.
4. Beneficial effects of chronic administration of dietary omega-3 polyunsaturated fatty acids in dogs with renal insufficiency. Journal of Laboratory and Clinical Medicine, 1998.
5. Evaluation of cognitive learning, memory, psychomotor, immunologic, and retinal functions in healthy puppies fed foods fortified with docosahexaenoic acid-rich fish oil from 8 to 52 weeks of age. Journal of the American Veterinary Medical Association, 2012.
6. Potential adverse effects of omega-3 fatty acids in dogs and cats. Journal of Veterinary Internal Medicine, 2013.