Metabolic basis for the essential nature of fatty acids and the unique dietary fatty acid requirements of cats.

C are unique among mammals with respect to metabolism of fatty acids. Except for some ruminants, all other mammals, including cats, are able to synthesize nonessential saturated and monounsaturated fatty acids de novo from glucose or amino acids via a common precursor, acetyl CoA. The products of this synthesis are 16and 18-carbon saturated fatty acids that can subsequently be desaturated to monounsaturated fatty acids of the n-7 and n-9 fatty acid families (eg, 16:1n-7 and 18:1n-9). These acids are the result of the introduction of a single double bond between carbons ∆-9 and ∆-10 of the respective saturated acids. Enzymes regulating these reactions are active when high-carbohydrate, low-fat diets are fed but have low activity when high-fat diets are fed. Consequently, animals fed relatively high-fat diets will only synthesize limited amounts of the needed fatty acids. Instead, they directly use the dietary supply of fatty acids. When low-fat diets that are devoid of essential omega-6 and omega-3 fatty acids are fed, mammals will further elongate and desaturate 18:1n-9, which results in the production of 20:3n-9 (ie, 5,8,11-eicosatrienoic acid). Measurable quantities of the latter fatty acid are a hallmark of EFA deficiency (Figure 1), which leads to classical signs of this disorder, such as poor growth, scaly dermatoses, and scruffy hair as well as numerous other metabolic consequences. Of necessity, mammalian diets must include EFAs (both n-6 and n-3 fatty acids) that animals are incapable of synthesizing. By contrast, plants can manufacture all of the aforementioned fatty acids, including saturated, monounsaturated, and polyunsaturated omega-6 and omega-3 fatty acids. Synthesis of polyunsaturated fatty acids is accomplished via enzymes not found in mammalian tissues that result in the insertion of additional carbon-carbon double bonds in distinct locations into a less unsaturated precursor. This process typically happens at a carbon atom located some distance away from the carboxyl carbon and is a unique characteristic of the enzyme involved. For example, insertion of a double bond between carbon atoms 9 and 10 is performed by stearoyl-Co-A desaturase (a ∆-9 desaturase), whereas insertion between carbon atoms 6 and 7 is performed by a ∆-6 desaturase. Mammals have both stearoyl-Co-A and ∆-6 desaturases. Plants have additional desaturases, including a ∆-12 desaturase that generates linoleic acid (18:2n-6) from the monounsaturated precursor, oleic acid (18:1n-9), and a ∆-15 desaturase that inserts a double bond into 18:2n-6 to generate α-linolenic acid (18:3n-3). In this way, terrestrial plants synthesize n-6 and n-3 fatty acids. Furthermore, marine plants are able to insert additional double bonds into 18:3n-3 and subsequently elongate the carbon chain, which accounts for the formation of the omega-3 LCPUFAs and their abundance in marine plants. Terrestrial plants, however, do not analogously insert additional double bonds into 18:2n-6. Thus, arachidonic acid (20:4n-6) is not found in plant tissues. The inability of animals to synthesize n-6 and n-3 fatty acids via ∆-12 and ∆-15 desaturases is the fundamental basis of the essential nature of these fatty acids and the need for a dietary supply. Once ingested in most mammals, except for felids, these dietary essenFrom the Comparative Nutrition Laboratory, Department of Small Animal Clinical Sciences, College of Veterinary Medicine, Texas A&M University, College Station, TX 77843-4474. Metabolic basis for the essential nature of fatty acids and the unique dietary fatty acid requirements of cats