Approximately 64% of the fatty acids in milk are saturated (Palmquist et al., 1993). Recent research has shown, however, that not all saturated fatty acids increase blood cholesterol in humans. Fatty acids of less than 12 carbon atoms are neutral or actually may decrease cholesterol. Stearic acid (C18:0) acts similarly to oleic acid (cis-C18:1) to decrease cholesterol (Ney, 1991). Only three saturated fatty acids (lauric, C12:0; myristic, C14:0; and palmitic, C16:0) now are considered to be hypercholesterolemic. These three fatty acids constitute about 44% of total milk fatty acids. According to a group of nutritionists from industry and academia (O’Donnell, 1989), the “ideal” milk fat for human health would contain < 10% polyunsaturated fatty acids, < 8% saturated fatty acids, and ³ 82% monounsaturated fatty acids. Fatty acids with less than 12 carbons are not included in this total. Concerns about the negative effects of trans-isomers of unsaturated fatty acids (Ney, 1991) indicate that any increases in poly- or monounsaturated fatty acids in milk should be primarily in the cis configuration. However, recent research has shown that the predominant trans isomer in milk fat (t-11 octadecenoic) is an important precursor of conjugated linoleic acid (CLA) (Santora et al., 1998).

     The role of CLA (active isomer believed to be cis-9, trans-11 octadecadienoic acid) in diet and health has become an important issue recently upon recognition of its role as a potent anticancer agent, and perhaps many other physiological effects (Chin et al., 1992; Clement et al., 1994; Ip et al., 1994; Jiang et al., 1996; Parodi, 1997). Recently, it has been shown that trans-11 octadecenoic acid, an important fatty acid in milk fat, is efficiently converted to CLA in the body (Corl et al., 1998; Griinari et al., 1998; Santora et al., 1998). Thus, the trans-11 monoene content enhances the value of milk fat as a source of CLA.

      Real or perceived concerns about the effects of milk fat on health and well-being not only decrease the economic value of dairy products (and thus producer incomes), but more importantly may compromise consumption of highly nutritious foods. Dairy products contribute the following percentages of total intakes for adults: Ca, 42-46; P, 18-23; K, 13-15; Mg, 10-13, and Zn, 10-12. For adolescents and children, dairy foods contributed much greater percentages of these nutrients (O’Donnell, 1993). The importance of full fat dairy products in the diet is heightened further by the discovery of the role of CLA in health. Thus, it is important for public health and well-being that consumption of dairy products be maintained or increased so that intake of important nutrients is not compromised.

     Modification of the fatty acid profile of milk should be beneficial to human health and improve the image of dairy products to health-conscious consumers (Noakes et al., 1996). As a consequence, sales of dairy products should increase, which would be of direct benefit to dairy producers and processors. Research to this end has been encouraged in several forums on research priorities, including the NRC Board on Agriculture Committee on Technological Options to Improve the Nutritional Attributes of Animal Products (“Designing Foods”) (1988), the FAIR 95 agenda (1993), ESCOP (1994), and a round table discussion by prominent nutrition researchers (Berner, 1993b). Although it is not likely that the “ideal” milk fat composition could be achieved, manipulation of the composition of milk fat is possible through feeding practices for dairy cows (Grummer, 1991; Palmquist et al., 1993). For example, feeding supplemental fats may increase contents of C18:0 and C18:1 while decreasing contents of C14:0 and C16:0; at the same time, however, content of more desirable short-chain fatty acids also may be decreased. Karijord et al. (1982) reported the composition and its variance of milk fat from Norwegian dairy herds. The coefficients of variation for individual fatty acids of milk fat ranged from 9 - 22%. Stage of lactation and month of season accounted for 10 - 25% of the variance, with the remainder being attributed to individual animal variation. Stage of lactation was more important than season with regard to variance. Nutritional inputs were not considered in the model. Karijord et al. (1982) concluded that genetic approaches might be used to alter milk fat composition. Although Gibson (1991) concluded that practical possibilities are limited to make changes through traditional breeding approaches or transgenic technologies, more recent research has documented differences among breeds in extent of unsaturation of dietary fatty acids (Beaulieu and Palmquist, 1995; DePeters et al., 1995). Further, progress in inducing transgenic animals with unique capability to secrete specific agents in milk is developing rapidly (Proceedings, Lactation Workshop, ADSA/ASAS annual meeting, 1998). Increased knowledge of the control and regulation of milk fat composition by mammary tissue is needed in order to develop, through rational scientific approaches, new dietary strategies for alteration of milk fat. Quantifying changes produced by defined nutritional and environmental manipulations in carefully designed and coordinated scientific experimentation will allow prediction of changes in milk fat composition that could be expected by feeding specialized diets to dairy cows.

     Milk fat composition also can be altered by manufacturing processes, such as fractionation, blending, or interesterification. These practices, however, may compromise flavor, mouthfeel, or other physical properties of the modified dairy products (Berner, 1993a). Increased unsaturation of milk fat may cause problems with oxidative stability (Charmley and Nicholson, 1994; Granelli et al., 1998) which may or may not be alleviated by supplementing with vitamin E (Focant et al., 1988) and may cause rejection of milk by consumers (Palmquist, 1997). Also, the variation in milk fat composition which now exists in commercial milk causes difficulty to produce consistent milk fat fractions. Consistent, high quality milk fat fractions are required to develop some new dairy foods. Technologies also exist or are being developed to remove cholesterol from milk fat; these technologies may increase consumer acceptance but have limited nutritional impact because dairy products are a minor source (5%) of dietary cholesterol (Berner, 1993a).

      A coordinated effort to study nutritional regulation and manipulation of milk fat composition offers the best opportunity for successfully producing milk of altered fat composition. Such an ambitious goal likely will not be achieved by a single investigator or institution. Cooperative research through the regional research system is a rational approach to focus attention and progress on this important topic. Usefulness of the data generated by this approach will be extended by incorporation into models of feeding and metabolism of dairy cattle. Specialized technologies to evaluate the composition and functionality of milk, such as determination of positional isomers and manufacturing characteristics, would best be shared through cooperative research to avoid unnecessary duplication of expensive equipment or specialized labor. Furthermore, it is essential that any changes in milk fat composition be evaluated for resultant effects on flavor, texture, and processing characteristics of milk and dairy products. Because few experiment stations possess dairy products research centers with such capabilities, a cooperative approach will be necessary to properly evaluate milk with altered fat composition. Inclusion of the Cooperative Extension Service in the activities will facilitate and enhance communication of progress to the public.