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E. C. NABER Commentary Animal products provide a wide variety of nutrients important in the human diet. They are the sole food sources of some nutrients and pro- vide one half or more of other nutrients. However, considerable latitude exists in diet composition, so that adequate nutrition can be maintained with lower intake levels of animal products. Data are available on the apparent consumption of foods by U.S. consumers. These data permit an evaluation of the adequacy of the U.S. diet and of the contributions of animal food products. However, considerably more and better data are required in order to more pre- cisely establish the level of consumption of animal food products and of the nutrient contribution of these foods to the diets of various population segments. Some evidence exists that the reduction of dietary saturated fat and cholesterol with replacement by dietary polyunsaturated fat reduces blood levels of cholesterol and may reduce certain forms of cardiovascu- lar disease. It is not clear whether the reduction in atherosclerotic disease from such diets is due to diet composition or to a reduction in caloric intake. While total dietary fat intake in the United States has increased during the past 50 years, all of the increase has been in the consumption of vegetable fats and oils. Consumption of animal fats has declined during this same period. While animal products supply an abundance of high-quality protein for the human diet, data suggest that the typical American consumes an excess of protein, much of which is derived from animal products. 240

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Commentary 241 On the other hand, the calcium, iron, and zinc provided by animal food products are not in adequate supply in some population groups. An analysis of the desires and wants of consumers shows that a de- mand for reduced fatness of animal products has existed for many years. Most consumers prefer less fat, not only in red meats, but also in poultry and dairy products. However, certain fat levels are necessary for the palatability and hence acceptability of both meats and ground or com- minuted meat products. Recent changes in the economics of animal feeding have resulted from sharp increases in cost of feed grains. These changes have in- creased costs of producing animal products and have forced further consideration of production practices that might reduce carcass fat and, as a result, feeding costs. Conservation of grain for direct use as human food may become more important in the future. A comparison of the efficiency of protein production to digestible energy input shows that the best yields are obtained through milk, broiler meat, and egg production, while pork and beef production give lower yields. However, ruminants are capable of utilizing low-quality roughages and nonprotein nitrogen for production of animal foods. Eco- nomically sound feeding systems must be designed to maximize protein recovery when related to inputs of energy. Some genetic variation is available and can be used by the breeder to change fat composition of animal products. The heritability of fat depo- sition in cattle and pigs is high and is related to the rate of maturity in young animals to be used for food. On the other hand, heritability of fat secretion in milk and eggs is moderate to low, and a strong positive correlation exists between the amounts of fat and other constituents in these products. Economic incentives could lead to breeding programs that would alter fat content of many of the meat-producing animals. Carcass composition changes in the ruminant animal are primarily a function of maturity or body weight. The principal effect of plane of nutrition on fat content of the ruminant body is by virtue of its effect on growth rate and hence upon body weight. As a result, the best manage- ment tool available to reduce fat content of the ruminant carcass is to feed the ration that produces the most economical weight gains but to market the animals at reduced body weights. Extremes in environ- mental temperature tend to promote fat deposition when a high plane of nutrition is provided. Changes in the marketing standards and system, as well as changes in the expectations of the meat industry, will be needed to provide incentive for the production of meats with reduced fat content. Systems of nutrition and management to reduce fat content of milk

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242 E. C. NABER are available. They consist mostly of the use of high-grain diets and the feeding of finely ground roughages. It is not known whether or not total milk production would be adversely affected by such diets. Body fat content in pigs and poultry is also a function of age or maturity. Hence marketing at younger ages will have an important effect on reducing fat content in the body of the nonruminant animal. In addition, it is known that the use of well-balanced rations minimizes fat deposition, because many nutrient imbalances stimulate overcon- sumption of feed and hence of energy that appears primarily as depot fat. A principal nutritional consideration is the maintenance of suitable protein-to-energy ratios in feed formulation. For the pig, limited feeding during the finishing period to reduce energy consumption ap- pears to reduce carcass fat content without adverse effects on gain in body weight. It has generally been believed that the eating quality of meat animal products is related to their fat content. Flavor, juiciness, and tender- ness of cooked meats may be influenced by their fat content. A review of research in this area shows that there is a low or moderate relation- ship between fat content and the flavor and aroma of meat. There also appears to be a low or moderate association between fat content and the juiciness of cooked meat. A similar type of association exists between fat content and tenderness. Much of the variation in eating quality is not, however, attributable to the fat in meat, and thus the importance of fat content of meat in eating quality has been overemphasized. Among the external factors interacting with fat content on eating quality are chilling rate of the carcass following slaughter and cooking methods. Federal grading standards for certain red meats have been widely used in the meat industry. These grades permit market identification of meat products by the food industry and the consumer. Some consideration is currently being given to changes in the grade standards that would re- duce the amount of carcass and intramuscular fat required for several grade categories. However, major changes in fat content of meat will not be brought about by changing grade standards. These changes will more likely come about for economic and nutritional considerations. The fat content of many dairy products has been reduced in recent years. Sales of low-fat dairy products have increased markedly, while sales of traditional dairy products, including butter, have declined. Hence it seems desirable that the butterfat standard for milk pricing be re- placed by a pricing system that gives suitable emphasis to other com- ponents of milk. Such a pricing system would give recognition to the increasing value of nonfat milk components and stimulate interest in production of milk for its protein content rather than for fat content.

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Commentary 243 Limited progress toward component pricing of milk is being made. While the major emphasis of this symposium was on reduction of fat content of animal products for economic and nutritional considerations, some interest has also been shown in the alteration of fatty acid and sterol composition of the lipid component in meat, milk, and eggs. It has been clearly demonstrated that the fatty-acid composition of tissue lipids in monogastric animals can be altered to resemble the composi- tion of dietary fat. Thus, feeding of unsaturated fat will increase the unsaturated fatty-acid content of the pig and poultry carcass. As might be expected, these increases in unsaturated fatty acids in tissue lipids reduce their stability and lower their melting points and result in storage and handling problems. Tissue lipid changes due to diet can be produced in the ruminant animal and hence in milk only if the dietary lipid is coated to prevent utilization in the rumen. Such coated lipids bypass the rumen and are digested and absorbed, resulting in tissue and milk lipid alteration. It is unlikely that the feeding of protected, coated vegetable oils to the dairy cow will compete economically with the direct addition of vegetable oils to skim milk. Dietary and drug-induced changes in the fatty acid and sterol content of egg yolk lipids have been demonstrated. Reduction of the cholesterol content of egg yolk is frequently accompanied by increases in other sterols. The nutritional consequences of this substitution is currently unknown. The utility of animal products with altered lipid composi- tion in the human diet remains to be demonstrated.

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