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progressively decreased in the higher-fat diets. This may have adversely affected performance of bitches fed the diet containing 11.7 percent fat. Ontko and Phillips (1958) fed diets containing 8 or 16 percent fat and observed satisfactory reproductive performance. Provided appropriate adjustments are made to maintain an adequate nutrient-to-energy ratio, it would appear that diets with rather widely varying concentrations of fat will also permit satisfactory reproductive performance.

Sedentary adult dogs have a greater tendency to become obese when fed high-fat diets ad libitum rather than high-carbohydrate diets. When adult female Beagles were fed ad libitum a diet containing 51 percent of energy from fat for 25 weeks, they gained twice as much body fat as dogs fed a diet containing 23 percent of energy from fat (Romsos et al., 1978). Other animals also tend to gain more body fat when fed a high-fat diet than when fed a low-fat diet (Sclafani, 1980). A slight restriction in food intake, however, will prevent development of obesity even if high-fat diets are fed.

The concentration of fat in the diet may affect work performance of dogs. Fatty acids are the primary source of energy for skeletal muscle during exhaustive exercise (Therriault et al., 1973). When adult Beagles that had been maintained in a high state of physical conditioning and had been fed a cereal-based diet with 7 percent fat were fasted for 5 days, their endurance capacity increased by 74 percent relative to their endurance in the fed state (Young, 1959). It was concluded that this improvement in work performance was mediated by an enhanced ability to mobilize body fat. Consumption of a high-fat diet rather than a high-carbohydrate diet has been shown to lengthen the time to exhaustion of Beagles on a treadmill by approximately 30 percent (Downey et al., 1980) and to cause a greater elevation in plasma-free fatty acid concentration during exercise of sled dogs (Hammel et al., 1977).

Essential Fatty Acids

Dogs, like other animals, have a dietary requirement for certain polyunsaturated fatty acids. These fatty acids have been shown to stimulate growth and cure the dermatitis characteristic of dogs fed a diet very low in fat or a diet in which the fat is completely saturated (Hansen et al., 1948, 1954; Hansen and Wiese, 1951; Wiese et al., 1965, 1966).

Members of the linoleic acid n-6 (denotes the position of the first double bond from the terminal end of the chain) family including linoleic acid, C18:2 (n-6); -linolenic acid, C18:3 (n-6); and arachidonic acid, C20:4 (n-6) all exhibit essential fatty acid activity. The shorthand terminology used denotes the number of carbon atoms in the fatty acid (followed by a colon) and the number of double bonds. Because C18:2 (n-6) can be desaturated and elongated to form C18:3 (n-6) and C20:4 (n-6) (Mead, 1980), and because the latter two fatty acids are only minor components of most natural fats, the requirement for fatty acids of the n-6 family is usually expressed as linoleic acid. Linoleic acid concentrations of a variety of ingredients used in dog foods are shown in Table 6 (see p. 46).

The minimum amount of linoleic acid (or other members of the n-6 family of fatty acids) required by the dog has not been precisely determined. The pathological and biochemical changes in the skin produced by an essential fatty acid deficiency can be reversed when 2 to 6 percent of the ME requirement is provided by linoleic acid or arachidonic acid (Hansen and Wiese, 1951; Wiese et al., 1966). One percent of the ME requirement as linoleic acid does not appear to be adequate for growing puppies (Wiese et al., 1966).

Several factors including rate of growth and concentrations of saturated fatty acids, trans fatty acids, and monounsaturated fatty acids in the diet influence the requirement for essential fatty acids (Mead, 1980; Holman, 1981). Beagle puppies fed a low-fat diet at 200 kcal ME per kilogram of body weight per day exhibited skin lesions within 2 to 3 months (Wiese et al., 1962). When the level of intake was reduced to 150 kcal ME per kilogram body weight per day, lesions appeared in 3 to 4 months. Puppies fed 100 kcal ME per kilogram of body weight per day did not grow, nor did they exhibit gross or histological evidence of fat deficiency during the 5-month study. Based on data obtained with rodents (Mead, 1980; Holman, 1981), one would predict that high intakes of saturated fatty acids, trans fatty acids, or oleic acid [C18:1 (n-9)] by dogs would compete with the metabolism of essential fatty acids and thereby increase the requirement for essential fatty acids. However, data are unavailable for estimating the extent to which these fatty acids might increase the requirement for essential fatty acids in dogs.

There is speculation that fatty acids of the -linolenate [C18:3 (n-3)] family also exhibit essential fatty acid activity in animals (Mead, 1980; Budowski, 1981). In rats C18:3 (n-3) promotes normal growth but does not prevent the skin lesions associated with lack of n-6 fatty acids (Mead, 1980). Fatty acids of the n-3 family have been suggested to serve some special function in the nervous system (Mead, 1980; Holman et al., 1982). They also are precursors of prostaglandins that may play an important role in control of blood clotting (Budowski, 1981). Data are unavailable to indicate if dogs have a requirement for fatty acids of the n-3 family.

Recommendation

It is recommended that a dog food contain at least 5 percent fat on a dry basis, including 1 percent of the diet