Pet food supplements: omega-3s, glucosamine and probiotics

A healthy dog or cat eating a complete and balanced commercial food usually needs no supplements. Cornell's Feline Health Center makes the point for cats, and it applies to dogs too: with a complete diet, extra vitamins and minerals are usually unnecessary, more is not better, and supplements can do harm if given without veterinary advice. Supplements earn their place for specific conditions, and the evidence behind the popular ones varies a great deal.

Omega-3 fatty acids have the best support

The long-chain omega-3 fatty acids EPA and DHA, found in fish and algal oils, have better evidence than most popular supplements, and it is strongest for arthritis. A 2022 systematic review and meta-analysis of dog and cat trials found clear pain-relieving effects from omega-3-enriched diets and omega-3 supplements in osteoarthritis.

There is also experimental support for kidney health. In dogs with surgically reduced kidney function, a diet with added fish oil kept filtration rates higher than a safflower oil diet, and caused less protein loss and less kidney scarring over 20 months than diets with safflower oil or beef tallow. That was a laboratory model rather than naturally occurring disease, so it shows promise rather than proof for pets with kidney disease.

A few practical points:

• Source matters. Flaxseed and other plant oils supply ALA, a different omega-3 from the EPA and DHA used in these studies.
• Avoid cod liver oil for routine use. It is rich in vitamins A and D, which build up in the body at high intakes.
• Dose with the vet. The right amount depends on the product, the animal and what its food already contains. Fish oil adds calories and can cause loose stools.
• Freshness matters. Fish oil oxidizes. Keep it sealed and cool, and discard it if it smells rancid.

Glucosamine and chondroitin showed no effect on arthritis pain

These are the best-selling joint supplements, and many large-breed and senior foods include them. The trial evidence does not support them. The same 2022 meta-analysis found a very marked lack of effect of glucosamine-chondroitin products on osteoarthritis pain, and its authors recommended that they should no longer be advised for pain management in dogs and cats. Collagen showed only a weak effect.

A dog that seems to do well on one does not need to stop it in a hurry, but these products should not replace weight control, exercise management, omega-3s or pain medication a vet has prescribed.

Probiotics have limited evidence

Probiotics are widely recommended for diarrhea, but the benefit in dogs appears small. A systematic review of 17 studies found that the data point toward a very limited and possibly clinically unimportant effect in acute gastrointestinal disease, and no significant added benefit in chronic disease, where diet remains the main treatment. The studies were small and often at risk of bias, so better trials could change the picture.

Cats may respond differently. In a placebo-controlled study in an animal shelter, cats given Enterococcus faecium SF68 had fewer episodes of diarrhea lasting two days or more than cats given placebo. No difference was detected in the dogs, although diarrhea was uncommon in both dog groups.

If using a probiotic, choose a veterinary product that names the strain and states the number of organisms. Diarrhea with blood, vomiting, lethargy or loss of appetite, or diarrhea lasting more than a day or two, needs a vet rather than a supplement.

Liver support and vitamin E

S-adenosylmethionine (SAMe), often combined with milk thistle extract, is used by vets alongside treatment for liver disease and with some medications that stress the liver. Controlled evidence in pets is limited, and it belongs in a treatment plan rather than in a healthy animal's bowl.

Vitamin E is an antioxidant that protects fats from oxidation. Complete commercial foods contain it, and diets rich in polyunsaturated fats need more of it. A supplement makes sense mainly when a vet or veterinary nutritionist recommends one, for example as part of a home-made diet.

Supplements that are usually unnecessary

Multivitamins

A food labeled complete and balanced for the animal's life stage already supplies the vitamins and minerals it needs. Adding a multivitamin raises the risk of excess, particularly of vitamins A and D, which are stored in the body rather than excreted. Too much vitamin D raises blood calcium and can damage the kidneys.

Home-made diets are the exception, and a much bigger problem than most owners expect. When 100 published home-made recipes for dogs and cats were prepared and analyzed, none met recommended levels of every nutrient measured. A home-made diet needs a recipe and supplement plan from a board-certified veterinary nutritionist.

Coconut oil

Coconut oil is popular, but there is little evidence of health benefits in dogs or cats. It contains no EPA or DHA, it is high in calories, and the ASPCA Animal Poison Control Center notes that the oils in coconut flesh and milk can cause stomach upset, loose stools and diarrhea.

Turmeric and curcumin

Curcumin has anti-inflammatory effects in laboratory studies, but it is poorly absorbed when swallowed, and good clinical trials in dogs and cats are lacking. Turmeric pastes also add fat and calories when made with oil. Evidence is too limited to recommend them over treatments that have been tested.

When a supplement makes sense

• Osteoarthritis: omega-3 fatty acids, dosed with the vet, alongside weight control and any prescribed pain relief.
• Kidney disease: omega-3s, with the vet checking what a prescribed kidney diet already contains before adding more.
• Home-made diets: the vitamin and mineral supplements specified by a veterinary nutritionist for that recipe.
• Diagnosed conditions: targeted products such as SAMe for liver disease, prescribed and monitored by the vet.

Before buying any supplement, tell the vet what the pet already eats and takes. Supplements can interact with medications, and several products taken together can double up on the same nutrient.

Sources

1. Feeding your cat. Cornell Feline Health Center.
2. A 2022 systematic review and meta-analysis of enriched therapeutic diets and nutraceuticals in canine and feline osteoarthritis. International Journal of Molecular Sciences, 2022.
3. Beneficial effects of chronic administration of dietary omega-3 polyunsaturated fatty acids in dogs with renal insufficiency. Journal of Laboratory and Clinical Medicine, 1998.
4. Clinical effect of probiotics in prevention or treatment of gastrointestinal disease in dogs: a systematic review. Journal of Veterinary Internal Medicine, 2019.
5. Effect of the probiotic Enterococcus faecium SF68 on presence of diarrhea in cats and dogs housed in an animal shelter. Journal of Veterinary Internal Medicine, 2011.
6. Concentrations of macronutrients, minerals and heavy metals in home-prepared diets for adult dogs and cats. Scientific Reports, 2019.
7. People foods to avoid feeding your pets. ASPCA Animal Poison Control Center.