Variations in polyunsaturated fatty acid formation in the cat and dog
Although dogs and cats are both mammals, they differ in many aspects of intermediary metabolism including specific pathways for fatty acid metabolism. Dogs have considerable capacity (relative to cats) to convert linoleic acid (LA), an essential omega-6 polyunsaturated fatty acid (PUFA), to its longer chain and more unsaturated derivative arachidonic acid (AA or 20:4n-6) via the pathway shown on the left side in Figure 1. Based on the dog's capacity to synthesize AA from LA, it does not have a requirement for AA 1. The rate limiting step for the synthesis of AA from LA is the delta-6 desaturase. This enzyme desaturates LA to yield gamma-linolenic acid (GLA) which is elongated (by the elongase enzyme) to form dihomo-gamma-linolenic acid (DGLA) and that is further desaturated by a delta-5 desaturase and elongated again to yield AA.
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As an omnivore, the dog does not require AA directly but can utilize LA for its essential omega-6 fatty acid requirements for growth and general health 1,2. The delta-6 desaturase is also the rate limiting step that converts α-linolenic acid (ALA, an omega-3 essential fatty acid) to its longer chain and more unsaturated derivatives: eicosapentaenoic acid (EPA) and docosapentaenoic acid (DPA). EPA and DPA can be further elongated and desaturated to form the neuro-bioactive fatty acid docosahexaenoic acid (DHA), although the process is not considered to be efficient. This is why DHA should be supplemented during gestation and lactation in the diets for the proper development of puppies and kittens. Hence, the DHA supports optimal growth and proper neural and retinal development.
Cats are strict carnivores, and they rely on several preformed nutrients in their diet for optimal nutrition and health. In regard to PUFA formation from essential fatty acids, cats lack the ability to make their own 20-carbon omega-6 and omega-3 PUFA due to the very low level or lack of delta-6 desaturase activity in liver 3,2. Therefore, these 20-carbon fatty acids (AA and EPA) are essential to cats and need to be provided in the diet. The delta-6 desaturase is needed to convert EPA to DHA via the Sprecher's pathway [22:5n-3 (DPA) -> 24:5n-3 -> 24:6n-3 -> 22:6n-3 (DHA)] 4. This makes DHA more of an essential nutrient in cats than in dogs and many other species of mammalian carnivores.
In summary, dogs need the essential fatty acids LA and LNA in the diet for growth, development, maintenance, gestation and lactation, and general health, although the addition of preformed long-chain omega-3 PUFA (EPA and DHA) will boost their nutritional status. Cats need not only LA, but also AA, EPA, and DHA for proper nutrition and health throughout the life cycle.
References
- Fat and fatty acids. In: National Research Council, Editors. Nutrient requirements of dogs and cats. Washington, DC: The National Academies Press; 2006. p. 81-110.
- Nutrient requirements and dietary nutrient concentrations. In: National Research Council, Editors. Nutrient requirements of dogs and cats. Washington, DC: The National Academies Press; 2006. p. 354-370.
- Bauer JE. Fatty acid metabolism in domestic cats and cheetahs. Proc Nutr Soc 1997;56:1013-1024.
- Sprecher H. Metabolism of highly unsaturated n-3 and n-6 fatty acids. Biochim Biophys Acta 2000;1486:219-231.
Key Points
- Dogs can convert LA and ALA to their respective 20 carbon long-chain PUFA; and therefore, dogs can utilize dietary sources of LA and ALA to satisfy their needs for the essential fatty acids and their 20 carbon derivatives.
- Cats lack the ability to make their own 20 carbon omega-6 and omega-3 PUFA due to the very low level of delta-6 desaturase activity in liver; hence, cats need preformed AA, EPA, and DHA.
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