Chapter 11. Fat Requirements
FAT AS AN ENERGY SOURCE
The dietary fat requirement of dogs and cats depends on the animal’s need for energy (calories) and for essential fatty acids (EFAs). Dietary fat contributes more than twice the amount of metabolizable energy (ME) per unit weight than does protein or carbohydrate and is also a highly digestible nutrient. The apparent digestibility of most fats included in pet foods is typically 90% or higher. 1.2. and 3. Because of its high energy content and digestibility, increasing the level of fat in a pet’s diet appreciably increases energy density. Both dogs and cats are able to maintain health when consuming diets that contain wide ranges of fat content, provided that other nutrients are adjusted to account for the changes in energy density. Because animals normally eat or are fed to meet their energy needs, consumption of a more energy-dense ration will result in decreased consumption of the total volume of food. Therefore, if nutrients are not adjusted in relation to fat, nutrient deficiencies can result.
Dietary fat provides a concentrated source of energy that is highly digestible. Dogs and cats can thrive on foods containing a wide range of fat content, provided that other nutrients are adjusted to account for the changes in energy density.
Periods of high energy demand in dogs and cats occur during growth, gestation, lactation, and prolonged periods of physical exercise. Feeding an energy-dense, high-fat diet during these periods can allow an animal to consume adequate calories without having to ingest excessive amounts of dry matter (DM). In addition, feeding a diet containing a sufficient concentration of dietary fat during strenuous physical work may have metabolic benefits. Fatty acids are the primary source of energy used by the body during prolonged physical exertion. Studies of dogs engaging in endurance events such as long-distance sled races have found that the consumption of a high-fat diet enhances dogs’ ability to use fatty acids for energy, which can ultimately contribute to improved performance 4. and 5. (see Chapter 24, pp. 245-248 for a complete discussion).
However, most adult pets today live relatively sedentary lifestyles and do not need foods containing high concentrations of fat. Although high-fat pet foods are capable of providing good nutrition and supporting optimal health, sedentary animals may be inclined to overconsume these diets because of their high palatability and energy density. If adult pets are fed performance diets, strict portion-controlled feeding should be used to prevent excessive energy consumption and weight gain. Likewise, feeding high-fat, energy-dense foods during periods of rapid growth must be strictly monitored. This is especially important for large and giant breeds of dog because high-fat (energy-dense) foods that are balanced for all essential nutrients are capable of supporting a high rate of growth if they are fed ad libitum. Maximal growth rate has been shown to be incompatible with proper skeletal development in dogs and is a risk factor for the development of several skeletal disorders. Portion-controlled feeding should therefore be used to control a growing pet’s weight gain, rate of growth, and body condition (see Section 4, pp. 231-233 and Section 5, pp. 496-497).
FAT AS A SOURCE OF ESSENTIAL FATTY ACIDS
In addition to providing energy, fat is necessary in the diet of dogs and cats as a source of the EFAs. Two families of fatty acid are essential: the omega-6 (n-6) and the omega-3 (n-3) fatty acids. The n-6 fatty acids have their first carbon double bond located at the sixth position on the carbon chain from the methyl end and the n-3 fatty acids have the first double bond located at the third carbon position (see Chapter 3, p. 19). The parental forms of the n-6 and n-3 families are linoleic acid and alpha-linolenic acid, respectively. Linoleic acid is comprised of an 18-carbon chain and has two double bonds (18:2n-6), and alpha-linolenic acid is the same length, but contains three double bonds (18:3n-3). Both of these fatty acids can be converted in the body to other long-chain polyunsaturated fatty acids (LCPUFA) through elongation and desaturation reactions.
The LCPUFAs of greatest physiological importance are arachidonic acid (AA), synthesized from linoleic acid, and eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), produced from alpha-linolenic acid (see Chapter 31, Figure 31-3, p. 388). During metabolism, the n-6 and n-3 families compete for the same enzymes and metabolic pathways, although their end products differ. Because of the basic difference in the location of the first double bond, interconversion between the two families of fatty acids is not possible. Therefore dietary requirements for linoleic acid (n-6) and alpha-linolenic acid (n-3) and their respective derivative LCPUFAs must be addressed distinctly, and the competitive relationship between the two families of fatty acids must always be considered when examining dietary effects of EFAs.
Two key liver enzymes involved in the production of LCPUFAs are delta-6-desaturase and delta-5-desaturase. In dogs, as in humans, the rate of conversion of the parent fatty acid to other LCPUFAs is regulated by delta-6-desaturase. This enzyme functions at a high enough rate to provide dogs with adequate quantities of AA to meet needs when dietary linoleic acid is present. 6 In addition, adult dogs are capable of converting alpha-linolenic to both EPA and to a precursor of DHA, docosapentaenoic acid (DPA), although at relatively low rates of conversion. 7 There is evidence that the DPA that is produced in the liver of dogs and cats from alpha-linolenic acid is transported to target tissues such as the retina, where it is converted to DHA. 8. and 9. As a result, the adult dog does not have a dietary maintenance requirement for AA and probably does not require EPA or DHA, provided that adequate levels of linoleic acid and alpha-linolenic acid are provided in the diet. Conversely, there is evidence that adequate EPA and DHA may be necessary for healthy reproductive performance in dogs (see below).
In contrast, cats are rather unusual in that the rate of delta-6-desatruase activity in the feline liver is limited, leading to reduced production of LCPUFAs from parent fatty acids. 10. and 11. This imposes a greater dependency on dietary sources of AA (and possibly also upon EPA and DHA) upon the cat. Early studies reported that when linoleic acid but not AA was included in the diet, cats developed impaired platelet aggregation and thrombocytopenia, and queens failed to deliver viable kittens. 12. and 13. However, the male cat’s reproductive performance was not impaired by a lack of dietary AA. This difference was attributed to the testes’ ability to produce adequate AA from linoleic acid for its own use. More recent studies confirmed that adult male cats do not demonstrate a dietary requirement for AA, and further examined the needs of reproducing females. 14 These results suggest that like males, female cats do not typically require a dietary source of AA for initial reproductive success. However, a dietary source of AA may be needed for healthy litters in queens that experience multiple pregnancies. These studies suggest that while adult cats have less capacity for producing AA than do adult dogs, they are still able to synthesize adequate amounts to meet their needs during normal adult maintenance. The degree to which adult cats are capable of converting alpha-linolenic acid to its derivative LCPUFAs has not been studied in detail. However, one study of the effects of dietary fatty acids on skin and plasma fatty acid profiles provided evidence that suggested negligible conversion of alpha-linolenic acid to EPA or DHA in cats. 15
Finally, it is known that EFA status of the body is negatively influenced by the physiological stress of pregnancy and lactation. 16 Recent studies with dogs have shown that reproducing female dogs develop reduced EFA status and these changes are exacerbated with increasing number of litters. 17 Specifically, increased EFAs are needed during gestation and lactation to supply fetal tissues with EFAs through the placenta and after birth through the milk. Of particular importance to reproducing females and their developing fetuses is DHA, which is needed for normal neurological and retinal development in puppies and kittens (see Chapter 22, pp. 228-229 for a complete discussion). 18 Because females have limited ability to enrich their milk with DHA and EPA from dietary alpha-linolenic acid, the best way to supply developing fetuses and neonates with needed LCPUFAs is through dietary enrichment of the mother’s diet with these fatty acids during pregnancy and lactation. 19. and 20. In addition, recent evidence shows that, similar to humans and other species, newborn puppies can convert milk alpha-linolenic acid to DHA early in life, but they lose this ability after the neonatal stage of life. 21. and 22. The minimum amount of alpha-linolenic acid that is needed to supply adequate DHA for the neonate is not known and requires further study. Therefore, while dietary n-6 and n-3 LCPUFAs are probably not required by either dogs or cats during adult maintenance, increased demands occur during early development, growth, gestation, and lactation to make these LCPUFAs conditionally essential. 23
Increased essential fatty acids (EFAs) are needed during gestation and lactation to supply fetal tissues with EFAs through the placenta and after birth through the milk. Of particular importance to reproducing females and fetuses is docosahexaenoic acid, which is needed for normal neurological and retinal development in puppies and kittens. The best way to supply the needed long-chain polyunsaturated fatty acids is through dietary enrichment of the mother’s diet with these fatty acids during pregnancy and lactation.
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FUNCTIONS OF ESSENTIAL FATTY ACIDS FOR DOGS AND CATS
Most mammals, including dogs and cats, are capable of synthesizing the nonessential 16- and 18-carbon fatty acids from glucose or amino acids. These fatty acids belong to two families, the n-7 and n-9 fatty acids, and are produced in highest quantities when an animal is fed a low-fat diet. Alternatively, when fed a diet containing adequate fat, animals preferentially use the fatty acids provided by the diet to meet their needs. The two 18-carbon EFAs, linoleic acid (n-6) and alpha-linolenic acid (n-3) are synthesized by terrestrial plants, while marine plants are capable of inserting further double bonds and elongating these carbon chains to produce the n-3 LCPUFAs. However, no plants are capable of producing AA from linoleic acid. As a result, AA is found only in animal tissues. Each of the EFAs and those that are classified as conditionally essential have important roles in cell membranes, the immune system, and the circulatory system. Because of the competition between linoleic acid and alpha-linolenic acid for the same metabolic pathways, the effects of these fatty acids will be influenced by both their absolute quantities in the diet and by their relative levels to each other.
Linoleic Acid and Arachidonic Acid
Linoleic acid has essential functions in maintaining the epidermal water barrier of the skin. It is incorporated into a fraction of cellular phospholipids in the epidermal keratinocytes called ceramides. Ceramides are extruded from keratinocytes into the intercellular spaces as lamellar granules and function to enhance cell cohesion and create an effective water barrier. 24 Linoleic acid’s important role in skin health is reflected in the changes to skin and coat that are seen during EFA deficiency. Linoleic acid is also important as a precursor of the LCPUFAs that are incorporated into cell membranes in the form of phospholipids and other lipid components. These function to maintain normal membrane fluidity, structure, and function. The type of fatty acids that predominate in a cell membrane differs among cell types and is also influenced by the fatty acid composition of the diet. The n-6 fatty acids that are derived from linoleic acid are found as storage fatty acids in adipose tissue, and in liver, kidney, and muscle cells. Linoleic acid–derived fatty acids found in cell membranes also have interactive roles with regulatory proteins that are important for cell metabolism and signaling.
The most important LCPUFA produced from linoleic acid is AA, which is a major cell membrane fatty acid and a precursor of certain types of eicosanoids. The eicosanoids are a diverse group of 20-carbon substances that are produced and released from cell membranes in response to physical or chemical trauma and that have local effects upon immune and inflammatory responses. The four primary types of eicosanoids are the prostaglandins, prostacyclins, thromboxanes, and leukotrienes. AA, which generally comprises more than 20% of the total fatty acids in cell membrane phospholipids in the skin, is converted to prostaglandins of the 2 series (prostaglandin E 2) and leukotrienes of the 4 series (leukotriene B 4). These eicosanoids are proinflammatory and are important mediators of inflammatory and allergic responses (see Chapter 31, pp. 386-395 for a complete discussion).
Arachidonic acid, produced from linoleic acid, is a major cell membrane fatty acid and a precursor of certain types of eicosanoids. The eicosanoids are 20-carbon substances that are released from cell membranes in response to physical or chemical trauma and have local effects on immune and inflammatory responses. The four primary types of eicosanoids are the prostaglandins, prostacyclins, thromboxanes, and leukotrienes.< div class='tao-gold-member'>
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