AbstractBackgroundThe Omega‐3 Index is a test that measures the amount of the long‐chain omega‐3 polyunsaturated fatty acids (n‐3 PUFAs), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in red blood cell membranes, which is expressed as a percentage of all fatty acids. However, alpha‐linolenic acid (ALA) from flaxseed oil, which is a short‐chain n‐3 PUFA, is often promoted in pet feed as a n‐3 source, implicitly assuming it is an effective precursor of EPA and DHA.ObjectiveThis study was aimed to compare the effect of supplementation with a plant‐based short‐chain n‐3 PUFA source (flaxseed oil, FSO) with a marine long‐chain n‐3 PUFA source (astaxanthin krill oil, AKO) to increase the Omega‐3 Index in dogs.MethodsTen adult Alaskan Huskies of both genders were supplemented daily with 1,155 mg of EPA/DHA from AKO, whereas another 10 dogs received 1,068 mg ALA from flaxseed oil for 6 weeks. Fatty acid and Omega‐3 Index measurements of the two groups were taken after 0, 3 and 6 weeks for comparison.ResultsThe EPA and DHA concentrations increased significantly only in the dogs fed with AKO resulting in a significant increase in mean Omega‐3 Index, from 1.68% at baseline to 2.7% after 6 weeks of supplementation (p < .0001). On the contrary, both EPA and DHA concentrations decreased significantly in the dogs fed with FSO, which led to a significant decrease in mean Omega‐3 Index from 1.6% at baseline to 0.96% at study end (p < .0001).ConclusionsThe results showed that supplementation of AKO from Antarctic krill led to a significant increase in the Omega‐3 Index in comparison to FSO in dogs. This suggests that preformed marine EPA and DHA sources are needed in dog feeds, as the dietary requirements proposed by feed industry organizations are not met with conversion from short‐chain n‐3 fatty acids.