Conjugated Linoleic Acid (CLA) and Milk Fat: A Good News Story

Milk fat contains several compounds with known anti-carcinogenic properties. One of these is conjugated linoleic acid (CLA), and dairy products are the major source of CLA in human diets. In addition to its anti-cancer effects, biomedical studies with animal models have demonstrated that CLA has a range of beneficial effects. The processes for the biosynthesis of CLA in dairy cows have been established and involve the absorption of fatty acid biohydrogenation intermediates produced in the rumen and their subsequent conversion to CLA by the mammary enzyme ∆-desaturase. Studies have further identified the substantial effect of dietary factors on milk fat content of CLA and identified a marked range among individual cows in the ability to synthesize CLA. Thus, through diet manipulation and perhaps even genetic selection, the milk fat content of CLA can be enhanced. Overall, the presence of CLA in milk fat is a clear example that milk contains microcomponents that provide benefits beyond those associated with traditional nutrients. INTRODUCTION Milk and dairy products are important dietary sources of nutrients, providing energy, high quality protein, and a variety of vitamins and minerals. Recent research has focused on altering the fat and protein content of milk and other dairy products in order to improve their nutrient content to more aptly reflect current dietary recommendations and trends. However, there is increasing recognition that foods can also be contributing factors in the prevention and development of some disease conditions. For example, it has been estimated that approximately one-third of cancer deaths may be related to diet (Parodi, 1999). As a result, additional focus is been given to designing foods that have beneficial effects on human health. The term “functional foods” is often used as a generic description for the beneficial effects of ingested foods that go beyond their traditional nutritive value (Bauman et al., 2001). Parodi (1997; 1999) has written a number of reviews summarizing the role of dairy products in the prevention of cancer. As listed in Table 1, a number of milk fat components have been identified as having anti-carcinogenic properties, and one that has received recent focus is conjugated linoleic acid (CLA). The discovery of a “functional food” role for CLA occurred over two decades ago when Pariza and coworkers found that ground beef contained an anti-mutagenic factor that consisted of a series of conjugated dienoic isomers of linoleic acid (Pariza et al., 2001). Subsequent work demonstrated that chemically synthesized CLA was able to reduce the incidence of several types of tumors in animal models. Most naturally occurring anti-carcinogens are present only at trace levels and of plant origin. However, CLA is unique among naturally occurring anti-carcinogens in that it is potent at extremely low levels and present in dairy products and meat from ruminant animals. As the biomedical studies with CLA expanded, it became apparent that CLA had a range of positive health effects in experimental animal models (Table 2). These included beneficial effects on reducing body fat accretion, delaying the onset of type II diabetes, retarding the development of atherosclerosis, improving the mineralization of bone and modulating the immune system (Belury, 2002). This has resulted in an exponential increase in CLA-related research over the last few years. The following sections will provide background and review the biology of CLA related to dairy production.