Fish oil omega‐3 fatty acids, mainly eicosapentaenoic acid and docosahexaenoic acid, are used in the management of several diseases in companion animal medicine, many of which are inflammatory in nature. This review describes metabolic differences among omega‐3 fatty acids and outlines potential adverse effects that may occur with their supplementation in dogs and cats with a special focus on omega‐3 fatty acids from fish oil. Important potential adverse effects of omega‐3 fatty acid supplementation include altered platelet function, gastrointestinal adverse effects, detrimental effects on wound healing, lipid peroxidation, potential for nutrient excess and toxin exposure, weight gain, altered immune function, effects on glycemic control and insulin sensitivity, and nutrient‐drug interactions.
Adverse effects, if observed, are likely to be dose‐dependent. It is necessary to understand dosages of omega‐3 fatty acids to understand how much fish oil to supplement, or what dietary concentration to aim for when recommending omega‐3 supplementation. Provision of omega‐3 fatty acids can be expressed as milligrams of total omega‐3 fatty acids per kilogram body weight; as milligrams of EPA and DHA per kilogram body weight or metabolic body weight; as a dietary amount on a per energy basis (grams or milligrams per 100 or per 1,000 kcal); or as a dietary amount on a per weight basis (grams or milligrams per 100 grams of diet as fed or on dry matter basis). The amount of omega‐3 fatty acids also can be expressed as a ratio of n‐6:n‐3 fatty acids, or as a ratio of “functional” fatty acids (LA + AA: EPA + DHA). The same enzymes are involved in the metabolism of omega‐6 and omega‐3 fatty acids, resulting in competition between these fatty acids for incorporation into cell membranes and other biological properties. Therefore, dietary excess or deficiency of LA versus ALA may influence conversion rates to downstream products. Dietary amounts of omega‐6 versus omega‐3 fatty acids are frequently expressed as a dietary n‐6:n‐3 ratio in addition to absolute amounts for this reason. However, ALA is not equivalent to EPA and DHA and the total n‐6:n‐3 ratio by itself does not accurately describe the fatty acid composition of the diet. A product with a high total omega‐3 fatty acid concentration could contain high concentrations of ALA, high concentrations of EPA and DHA, or a combination of these fatty acids. Because diets with ALA have different effects when compared with diets enriched in EPA and DHA, the type of omega‐3 fatty acids is crucial information and the lack of distinction between these fatty acids may contribute to the equivocal nature of results of earlier studies.