Fish oil supplements are commonly used in human medicine to prevent and manage disease, as well as being a part of a healthy, balanced diet. Their use in canine and feline medicine is relatively limited, but expanding knowledge and research into these supplements have increased their use.
Definitions, nomenclature and requirements in healthy animals
Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are long-chain polyunsaturated omega-3 fatty acids derived from marine animals (fish) and plants (algae). Fatty acids are components of a triglyceride molecule but also exist and function in the body independently.
Classification of fatty acids is based on:
- Length (ie short-, medium- and long-chain)
- Presence or absence of double bonds (ie saturated without and unsaturated with double bonds)
- Number of double bonds (ie monounsaturated with one double bond or polyunsaturated with two or more double bonds)
- Location of the first double bond on the fatty acid chain (ie omega-3 with the first double bond at the third carbon, omega-6 at the sixth carbon or omega-9 at the ninth carbon)
These differences in fatty acids can have a number of effects, including on their physical properties (eg melting point) and their functions.
EPA and DHA are considered fish oil-derived omega-3 fatty acids. EPA is 20 carbons long and has five double bonds. DHA is 22 carbons long and has six double bonds. Alpha-linolenic acid (ALA) is also considered an omega-3 fatty acid (18 carbons long with three double bonds) but is derived from non-marine sources, such as flaxseed and other oils. The beneficial effects of omega-3 fatty acids are only seen with DHA and EPA, not ALA. For this reason, ALA has to be first converted to DHA to have a therapeutic effect. However, the efficiency of conversion is very poor in dogs and cats.
The beneficial effects of omega-3 fatty acids are only seen with DHA and EPA; ALA has to be first converted to DHA to have a therapeutic effect
DHA and EPA are not essential nutrients in adult dogs and cats, compared to the essential omega-6 fatty acids linoleic acid (in dogs and cats) and arachidonic acid (in cats but not dogs). However, in immature animals, DHA has vital functions in the normal development of the brain and retina. Fish oils may also help with the decline in cognitive function in elderly dogs and cats.
Mechanism of action of fish oil supplements
The principal mechanism of action of fish oil supplements is anti-inflammatory. This action is achieved when DHA undergoes the inflammatory cascade to produce mediators which cause vasodilation, anti-thrombotic effects and reduced chemotaxis. This is compared to when the omega-6 fatty acid arachidonic acid undergoes the inflammatory cascade to produce pro-inflammatory and pro-thrombotic mediators.
In addition to these primary anti-inflammatory effects, EPA and DHA are also thought to have several additional beneficial effects, including through the production of protectins (which have protective functions in neural and retinal tissue) and resolvins (which regulate leukocyte infiltration), and by influencing gene transcription.
Use of fish oil supplements to manage disease
Fish oil supplements are used in human medicine to manage several conditions, from critical illness to chronic disease. Use stems from epidemiological studies into the diet of the Inuit people, which is naturally high in oily fish containing EPA and DHA and suspected to be the reason for their relatively low rate of cardiac and vascular disease.
Fish oil supplements are used in human medicine to manage several conditions, from critical illness to chronic disease
Kidney disease
Fish oil supplements can help mitigate proteinuria in dogs with chronic kidney disease. This is supported by an experimental study (Brown et al., 1998), which demonstrated the reno-protective effects of omega-3 fatty acid fish oil supplements compared to omega-6 fatty acid supplements (safflower oil) and saturated fat (beef tallow). This protective effect included reduced proteinuria and a decline in renal function. On the basis of this study, fish oil supplementation is recommended for proteinuric kidney disease in dogs. Similar studies have not been reported in cats.
Heart disease
Fish oil supplementation is commonly used in people to manage cardiovascular disease and is reported to reduce mortality and morbidity associated with this condition. A study by Smith et al. (2007) into Boxer dogs with arrhythmogenic right ventricular cardiomyopathy demonstrated a reduced number of arrhythmias (ventricular premature contractions) after six weeks of fish oil supplementation compared to dogs that received flaxseed oil (a source of ALA).
In addition to this, fish oil supplementation is thought to help with cardiac cachexia syndrome. This phenomenon occurs due to increased inflammatory mediators, such as tumour necrosis factor and interleukin 1, which cause appetite suppression, increased energy expenditure and muscle catabolism. One study (Freeman et al., 1998) in dogs with heart failure demonstrated reduced inflammatory markers, decreased muscle wasting and improved survival when fish oil supplementation was provided.
Osteoarthritis
Many nutritional supplements are commonly used to manage canine and feline osteoarthritis. However, a systematic review by Vandeweerd et al. (2012) reported that only fish oil supplementation was associated with any proven benefit. This has been supported by a more recent meta-analysis (Barbeau-Gregoire et al., 2022) into canine and feline osteoarthritis, which confirmed an evidence-based beneficial effect of fish oil supplements and fish oil-enriched diets.
Obesity
Obesity can be considered a pro-inflammatory state that leads to many detrimental metabolic effects associated with excessive body weight. The anti-inflammatory effects of fish oil supplements can, therefore, be used to manage this condition. Fish oil supplementation has been shown to increase serum adiponectin concentrations in healthy dogs. This can be helpful in obesity because decreased adiponectin has been associated with insulin resistance in overweight dogs.
Fish oil supplementation has been shown to increase serum adiponectin concentrations in healthy dogs. This can be helpful in obesity because decreased adiponectin has been associated with insulin resistance in overweight dog
Hypertriglyceridaemia
Evidence from human medicine and in healthy dogs demonstrate a triglyceride-lowering effect of fish oil supplements. This effect is thought to be via influences on gene transcription.
Cancer
Fish oil supplements have been shown to reduce the risk and mortality associated with several cancers in humans, including colorectal, brain and breast cancer. In addition to this, these supplements have been shown to improve body condition and immunity in people with gastrointestinal and pancreatic cancer. However, the role of fish oil supplementation in canine and feline cancer has only been investigated in small studies thus far.
Gastrointestinal disease
Fish oil supplementation has a potential role in chronic enteropathies via several different mechanisms. The use of these supplements in human forms of inflammatory bowel disease (ie Crohn’s disease and ulcerative colitis) has yielded mixed results. Studies into the benefit of fish oil supplements in canine and feline chronic inflammatory enteropathies are needed.
Skin disease
Fatty acids serve important functions in the maintenance of the normal structure and function of the skin. Fish oils have been shown to improve pruritus, self-trauma and coat condition in dogs with dermatitis.
Formulations and doses of fish oil supplements
The recommended dose of fish oil supplements can be expressed as a milligram dose per kilogram body weight, a milligram dose per energy requirement/provision and as a ratio of omega-3 to omega-6 fatty acids.
Variations in the dosing protocol may also be noted depending on the disease being treated. Generally, a DHA dose of 25-30mg/kg and an EPA dose of 40mg/kg is recommended. An omega-6 to omega-3 fatty acid ratio of less than 5:1 can be effective for pruritic skin disease, whereas proportionally larger amounts of fish oils giving ratios of 3.5:1 or lower may be needed for other conditions.
Salmon and krill oil can be a suitable alternative to purified fish oil products, but cod liver oil is not suitable as it contains insufficient EPA and DHA and relatively high amounts of vitamin D and A
Fish oil supplementation is best provided as a purified product containing EPA and DHA. There are several veterinary formulations for this. Alternatively, fish oil supplements made for people can also be used; however, these tend to be in the form of large capsules which can be challenging to administer to some dogs and cats. Salmon and krill oil can be a suitable alternative to purified fish oil products, but cod liver oil is not suitable as it contains insufficient EPA and DHA and relatively high amounts of vitamin D and A, which can be toxic. Finally, several commercial diets have now been formulated with adequate amounts of fish oils to manage disease (eg diets for osteoarthritis).
Adverse effects of fish oil supplements
Despite their availability and potentially widespread use in health and disease, adverse reactions to fish oil supplements are possible (Lenox and Bauer, 2013). The most commonly reported complications are gastrointestinal in origin, including diarrhoea and vomiting. An altered platelet function may also be seen with these supplements, but this does not usually lead to clinical bleeding. Fish oil supplements may also have detrimental effects on wound healing, so avoiding these supplements in the immediate post-operative period is generally advised. Other potential adverse effects include altered immune function, weight gain and nutrient–drug interactions.
Fish oil supplements may have detrimental effects on wound healing, so avoiding these supplements in the immediate post-operative period is generally advised
Summary
Omega-3 fatty acid fish oil supplements exert their beneficial effects via anti-inflammatory and other mechanisms. The use of EPA- and DHA-containing supplements is vital because there is poor conversion of other (non-marine) omega-3 fatty acids (such as ALA) to the active form in dogs and cats. There is currently good evidence of the benefit of fish oil supplementation in proteinuric kidney disease, cardiac disease (such as arrhythmias), skin disease and osteoarthritis.
