Conjugated linoleic acid (CLA) is a collective term for numerous and often unidentified isomers of octadecadienoic acid with a conjugated diene structure. CLA, particularly the cis-9, trans-11 18:2 isomer, has been the subject of increasingly intense research since it was shown to be a powerful natural anticarcinogen in foods. Subsequently, many other important biological effects of this and other CLA isomers have been documented (see Yurawecz, et al, Advances in Conjugated Linoleic Acid Research, Volume I, AOCS Press, 1999). The most important dietary source of CLA is foods of ruminant origin—it has been known since the classic work of Tove and associates (Kepler, et al, J. Biol. Chem. 241:1350-1354, 1966) that cis-9, trans-11 18:2 is an intermediate in the ruminal biohydrogenation of linoleic acid. In fact, this isomer has been given the trivial name rumenic acid, because of this origin (Kramer, et al Lipids 33:835, 1998). It was assumed that ruminal bioydrogenation and synthesis of rumenic acid is the source of CLA in ruminant fats.

In 1994, Dr. Peter Parodi, a research scientist at the Dairy Research and Development Corporation of Australia wrote a review of the anticarcinogenic nature of CLA in milk fat (Austral.J.Dairy Tech. 49:93-97,1994). Dr. Parodi had previously identified rumenic acid as the principle component of the long-known conjugated dienes in milk fat ( J.Dairy Sci. 60:1550-1553, 1977). In his review, Dr. Parodi, quoting Pollard, et al (Lipids 15:306-314, 1980) and Holman and Mahfouz (Prog. Lipid Res.20:151-156, 1981), pointed out that vaccenic acid (trans-11 18:1), could be desaturated to rumenic acid in mammalian tissues by microsomal delta-9 desaturase. Vaccenic acid is formed by ruminal biohydrogenation of rumenic acid, and occurs in ruminant fats at concentrations up to five times that of CLA. Having researched ruminant fat metabolism and milk fat synthesis and composition since 1965, I immediately saw the potential importance to quantify the extent of desaturation of vaccenic acid in tissues ("endogenous" synthesis of CLA) and its contribution to the total dietary supply of CLA. I devised experiments to quantify the desaturation and wrote proposals for funding, but was delayed until a student was recruited to carry out the studies. We finally undertook the studies in 1998 with collaboration of my colleague Dr. Karla Roehrig advising us in conduct of experimentation with laboratory mice.

I believe that the study is highly cited for three reasons: 1) the intense research activity in all aspects of CLA (production in ruminant animals, nutritional and biological effects, anticarcinogenic effects); 2) it is the first publication to quantify desaturation of vaccenic acid (we showed that 50% of the vaccenic acid that remained in the tissues of the mouse was desaturated, with rumenic acid as the product); 3) it demonstrated the importance of endogenous synthesis to the total supply of CLA in the tissues. Subsequently, it has been estimated that up to 64% of the CLA in milk fat is synthesized endogenously (Griinari, et al, J. Nutr. 130:2285-2291,2000), and we will soon report (Turpeinien, et al, Am. J. Clin. Nutr., accepted for publication) that 19% of vaccenic acid is desaturated to CLA in humans.

The latter study shows that because vaccenic acid occurs in the diet at up to five times that of CLA, endogenous synthesis doubles the supply of CLA to tissues. This should prove to be a very significant discovery for human nutrition. Further, it is a significant discovery for animal fats, and the dairy industry in particular, because it sheds light on the beneficial components of these fats for human nutrition and health. It also shows that not all trans fatty acids are "bad" fatty acids, as promoted by some. In fact, researchers in milk fat synthesis and composition have officially opposed mandatory labeling of foods for trans fatty acid content (Western Regional Research Committee W-181 "Modifying Milk Fat Composition for Enhanced Manufacturing Qualities and Consumer Acceptability").