In certain products, marketed as complete diets, there was no EPA detectable. These diets are nutritionally inadequate and, depending where they are sold, violate regulations and laws.
Unlike other mammals, felines require the omega-3 polyunsaturated fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in their diets. In addition, there is evidence that the ideal intake of EPA is higher than that necessary to prevent deficiency symptoms. High intakes of EPA may have beneficial effects on kidney function and on inflammatory skin disorders. In this article, a range is proposed for the ideal levels of EPA and DHA in cat foods.
Vital polyunsaturated fatty acids
Polyunsaturated fatty acids (PUFAs), in particular linoleic acid (LA),alpha-linolenic acid (ALA) and DHA, are incorporated into membranes. DHA is abundant in nervous tissue. LA, ALA and DHA are required for the physical properties of membranes such as fluidity and barrier characteristics. Furthermore, LA and ALA may be converted into arachidonic acid (AA) and EPA, which in turn are substrates for the formation of eicosanoids—a wide range of biologically active compounds.
The initial steps in LA and ALA conversion towards eicosanoids involve enzymes that desaturate and elongate PUFA. As to PUFA metabolism in relation to eicosanoid synthesis, the cat differs from other mammals. The cat lacks desaturase enzymes so that it also requires AA and EPA rather than the parent fatty acids only. As a consequence, the cat is unique in that the fatty acids AA, EPA and DHA are essential nutrients in addition to LA and ALA. A dietary deficiency of EPA and/or DHA leads to abnormalities in cats.
Eicosanoids: A new frontier
The parent PUFAs of the omega-6 and omega-3 series are LA and ALA, respectively. Unlike plants, mammals cannot synthesize LA and ALA. However, they can, except for cats, desaturate and elongate these fatty acids. AA can be synthesized from LA and EPA and DHA from ALA. Cats, and possibly other felines as well, lack delta (D)-6, D-5 and D-4 desaturase enzymes, and thus must obtain not only LA and ALA with their diet, but also AA, EPA and DHA. AA only occurs in foodstuffs of animal origin, rendering the cat an obligate carnivore.
AA and EPA are precursors for the eisosanoids, which include the prostaglandins, thromboxanes and leukotrienes. More than 250 eicosanoids have been described and they have been shown to be biologically active in different test systems.
As a rule of thumb, the eicosanoids derived from AA and EPA are considered to have opposite effects. For example, eicosanoids produced from AA tend to have an inflammatory activity; eicosanoids derived from EPA tend to have an anti-inflammatory activity. The ratio of AA to EPA may determine the net biological effect. The production of the two types of eicosanoids appears to be determined by the intake of their precursors. Consequently, it is possible to influence processes such as platelet aggregation and inflammation by the intake of PUFA.
Sources of AA, EPA and DHA
LA is abundant in various plant oils, including soybean, sunflower and corn oil. ALA also is present in those oils, but at much lower concentrations. LA and ALA occur in animal products, but only after their ingestion and incorporation into the animal fat. Only small amounts of AA, EPA and DHA usually occur in products of animal origin. Fish oils are particularly rich in EPA and DHA. It should be stressed that fish oils vary widely in their composition. By feeding fish oil instead of other oils to cats, the levels of EPA and DHA in their adipose tissue will increase markedly (Van Niel and Beynen, 1997), indicating that the EPA and DHA status of cats can be modulated.
AA, EPA and DHA requirements
The requirements of AA, EPA and DHA by cats are not well defined. The National Research Council 2003 draft of the Nutrient Requirements for Dogs and Cats lumps EPA and DHA together, but mentions that the DHA requirement may be higher than that of EPA.
LA is an indispensable membrane component. LA deficiency in cats produces low growth rates, poor skin and coat condition, fatty liver and high rates of transepidermal water loss (Frankel and Rivers, 1978). For the cat AA also is an essential nutrient because the animal cannot convert LA into AA. Cats require AA in their diet for normal platelet aggregation (MacDonald, et al., 1984a). In female cats, sufficient AA is necessary for adequate reproduction (MacDonald, et al., 1984b). The intake of omega-3 PUFA is essential for normal development of nervous tissue and the retina. A deficiency of omega-3 PUFA has been shown to reduce visual acuity in monkeys and to diminish learning behavior in rats. It is reasonable to suggest that in cats ALA, EPA and/or DHA deficiency will produce similar abnormalities.
Apart from preventing signs of deficiency, omega-3 PUFAs have beneficial effects when ingested at high doses. In cats with allergic skin disease, the associated inflammation, pruritis and dermatitis are ameliorated after the administration of fish oil (Harvey, 1993). The eicosanoids resulting from EPA metabolism have anti-inflammatory activity, which explains the positive effect of fish oil on inflammatory skin disease.
A recent retrospective study indicates that a high intake of EPA is necessary for enhanced survival of cats with chronic renal failure (Plantinga et al., 2005). Although there is no direct evidence, it is reasonable to suggest that a high intake of EPA may contribute to the prevention of renal failure in cats (Beynen, 2004). This would be relevant as chronic renal failure is a common clinical condition in cats, the prevalence reaching 30% in animals over 15 years old. The protective effect of EPA, if any, could be mediated by the production of eicosanoids with anti-inflammatory, anti-aggregatory and/or hypotensive activity. In dogs, supplemental fish oil has been shown to reduce the signs of osteoarthritis (Schoenherr, 2005) and to increase survival in patients with cancer (Ogilvie et al., 2000) or chronic heart failure (Freeman et al., 1998).
Cats must not only ingest sufficient EPA to prevent signs of deficiency. It is also evident that high intakes of EPA have additional, beneficial effects on cat health. Because the eicosanoids synthesized from AA and those from EPA generally have opposite effects, it would appear that low, but sufficient intakes of AA are to be advised. An arbitrary proposal for the ideal AA, EPA and DHA contents of diets for adult cats is given. In order to attain an EPA level of 0.1 g/MJ, a dry cat diet should contain about 1% of a type of fish oil containing 15% of EPA.
EPA and DHA in commercial petfoods
In a recent study, 15 commercially-available, complete senior foods for cats were analyzed (Plantinga and Beynen, 2003). It showed that, except for diet 15, all diets contained less EPA and DHA than is considered optimal by this author. Even more striking is the observation that five diets did not contain EPA, one of these diets also lacking DHA. When an EPA-or DHA-free diet is given to adult cats as sole source of nutrition, EPA and DHA may be released from storage sources, but in the long run an EPA or DHA deficiency will occur. As indicated above, abnormalities of the retina and nervous tissue may develop.
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