CLA is a group of positional and geometric isomers of octadecadienoic (linoleic) acid having a conjugated double-bond system starting at carbon 9, 10, or 11. All configurationscis-trans, trans-cis, cis-cis, and trans-transare possible in each of the three positional systems. The structures below depict the c-9,t-11 and t-10,c-12 isomers, which are the most abundant ones in foods. Conventional linoleic acid, an essential dietary fatty acid found mainly in plant oils, is also illustrated, showing its double bonds in the cis configuration at carbons 9 and 12.

The conjugated diene structure is not usual in fatty acids, although the existence of CLA has been known for many years. Investigators in the 1950s learned that microorganisms in the rumen of ruminant animals such as cows and sheep produce CLA from polyunsaturated fat. Then the c-9,t-11 isomer was shown to be the first intermediate product in the biohydrogenation of linoleic acid by the anaerobic rumen bacterium Butyrivibrio fibrisolvens. The reaction is catalyzed by the enzyme linoleate isomerase, which converts the cis-12 bond of free linoleic acid to a trans-11 bond. The normal intestinal flora of rats can also convert linoleic acid to the c-9,t-11 isomer, but the reaction does not take place in animals lacking the requisite bacteria. One reason why ruminants produce more c-9,t-11-CLA than nonruminants is that hydrolysis of fat within the rumen provides more unesterified linoleic acid than is available to bacteria in nonruminants.

Structures of t-10, c-12-CLA (top), c-9, t-11-CLA (center), and ordinary linoleic acid, c-9, c-12-octadecadienoic acid (bottom). The molecules are aligned at their carboxyl end to show the influence of the double bonds (yellow) on molecular shape.

Foods from plants, too, contain CLA, but the distribution of isomers differs sharply from that in animal foods, where the microbial product predominates. In particular, there is proportionately much less c-9,t-11, which appears to be the biologically active form. A mixture of conjugated linoleic acid isomers is also produced during food processing, by thermal isomerization and by some industrial processes for partial hydrogenation. Energetically, formation of a trans bond is favored (hence the notorious trans monounsaturated fatty acids in margarine); but in contrast to the isomerase-catalyzed reaction, it may form between either pair of carbons in the conjugated system. Because no current analytic technique can separate the c-9,t-11 and t-9,c-11 isomers, differential identification of these two in foods is not possible. Plant foods probably contain both; animal foods too may contain small amounts of t-9,c-11-CLA in addition to the microbial product. It is interesting that oxidative reactions have no important effect on CLA during food processing and storage. Nor does conventional processing alter the total CLA content of foods, although concentrations of c-9,t-11- (t-9,c-11-) CLA may be lower after processing.

So CLA is largely a product of microbial metabolism in the digestive tractprimarily of ruminants, but to a lesser extent in other mammals and some birds. Whether eaten in the diet or synthesized in the digestive tract, it is absorbed from the gut and distributed throughout the body, where the c-9,t-11 (and perhaps the t-9,c-11) isomer is incorporated into blood lipids, cell membranes, and fat tissue. In addition, mammals secrete relatively large amounts into their milk. Little is known about the further metabolism of CLA in animals, but what evidence there is suggests that the slow elongation of the carbon chain and further desaturation of carbon-carbon bonds it undergoes in some tissues leaves the original conjugated diene structure intact.

The exciting part of the CLA story began serendipitously, as such stories frequently do. In the late 1970s, at the University of Wisconsin's Food Research Institute, Michael Pariza was investigating mutagens in cooked beef. To his surprise, he discovered a fraction from grilled and raw beef that consistently modulated mutagenesis in the Ames (Salmonella) test and frequently showed marked antimutagenic activity. The active material was identified as CLA, and subsequent work by Pariza and his associates and others began to reveal its astonishing range of biological effects.

Researcher prepares CLA by heating linoleic acid with ethylene glycol and KOH (J. Food Compos. Anal. 1992, 5, 185).

CLA is an in vitro antioxidant, and in cells it protects membranes from oxidative attack. In relation to other important dietary antioxidants, it quenches singlet oxygen less effectively than b-carotene but more effectively than a-tocopherol. It appears to act as a chain-breaking antioxidant by trapping chain-propagating free radicals. Pariza postulates that its antioxidant activity arises from steric hindrance of reactions involving the carbon-centered radical.

Because many antimutagens and antioxidants are also anticarcinogens, Pariza began to study the effects of CLA in animal models of cancer. In mice, CLA inhibited cancers of the skin and forestomach caused by exposure to carcinogens. In rats, it inhibited mammary and colon cancer. Researchers at Washington State University found that physiologic concentrations of CLA kill or inhibit cultured cells of human malignant melanoma, colorectal cancer, and breast cancer. These effects may be partly attributable to CLA's antioxidant activity. However, CLA also modulates the activity of cytochromes P450 and suppresses the activity of ornithine decarboxylase and protein kinase C, enzymes involved in carcinogenesis. It may suppress protein and nucleic acid synthesis in cancer cells, as well.

Rabbits and hamsters are frequently used to study diet-induced atherosclerosis. When rabbits and hamsters were fed cholesterol-supplemented diets, animals who also received CLA had lower levels of total and LDL ("bad") cholesterol in their blood and developed less atherosclerosis in their aortas.

The tissue breakdown (catabolism) that normally follows stimulation of the immune system partitions energy away from important processes such as growth. Although the effects of CLA on the immune system and inflammatory response resemble those of fish oil and may be mediated by similar mechanisms, Pariza and his colleague Mark Cook found that CLA is better able than fish oil to prevent anorexia and growth suppression in mice injected with endotoxin. Similar favorable actions of CLA on the immune system were seen in rats and chickens.

Because the immune system is under constant assault from "outside forces" such as bacterial endotoxin, one consequence of CLA's energy-partitioning effects might be to promote growth. In fact, Pariza and his co-workers did observe that rat pups whose mothers' diet was supplemented with CLA during gestation and lactation gained weight faster than pups whose mothers were fed only normal chow. Continued supplementation with CLA after weaning maintained the growth advantage of these pups, who utilized their feed more efficiently, gaining more weight and more lean body mass per unit of feed eaten. Since CLA is found in milk, Pariza postulates that it is a growth factor for rats and possibly for other mammals as well. These experimental results suggest uses for CLA in animal agriculture.

What foods contain the most CLA? Lamb. Beef. Surprisingly, turkey (but not chicken). Above all, dairy products (not fat-free ones, because they wouldn't contain much conjugated fatty acid). The CLA content of these foods ranges from 2.5 to 11.0 mg per gram of fat, and 75% or more is c-9,t-11 (t-9,c-11). Common plant oils, in contrast, contain only 0.1-0.7 mg/g of CLA, of which less than half is c-9,t-11 (t-11,c-9). Seafood is a poor source. Unfortunately, a CLA-rich diet is a high-fat diet. However, large-scale production of CLA utilizing the action of the bacterial isomerase on linoleic acid may be feasible, permitting future fortification of foods, should this appear desirable, and other commercial uses.

Pariza, who enjoys a grilled cheeseburger as much as anyone, grins as he reflects on the negative image of animal fats in contrast to the positive image of plant oils. He points out that conventional linoleic acid, the major component of corn oil, is so far the only fatty acid proven to enhance cancer in experimental animals. So it is ironic that in its conjugated form, which is found mainly in animal fats and foods of animal origin, it is the most powerful naturally occurring fatty acid with proven ability to protect against cancer! In fact, it is the only known antioxidant/anticarcinogen primarily associated with animal foods. Moreover, amounts close to those in a normal mixed human diet are effective, not only in cancer protection but also in all the other biological activities of this compound. This well illustrates the fallacy of the "good food, bad food" approach to diet and nutrition. CLA is no panacea. But it is very interesting, and you will hear more about it.

The chemistry and biology of CLA was the topic of a technical session during the 1996 annual meeting of the American Oil Chemists Society. For a review of CLA and its mechanisms of action, see Belury, Nutr. Rev. 1995, 5(4-I), 83. A lively discussion is in INFORM 1996, 7, 152. Parodi reviewed the activities and dietary sources of CLA (Aust. J. Dairy Technol. 1994, 49, 93). Werner et al. reported a method for HPLC separation and GC analysis of CLA isomers in cheese (J. Agric. Food Chem. 1992, 40, 1817). Chin et al. developed an extensive data base on dietary CLA (J. Food Compos. Anal. 1992, 5, 185). Mechanistic studies of CLA and cancer are reported in Liew et al., Carcinogenesis 1995, 16, 3037.

I thank Mike Pariza for helpful tips on the manuscript and Paul Schatz for the illustration of CLA structures. Photo of CLA preparation taken by Wolfgang Hoffmann, courtesy of FRI, University of Wisconsin-Madison.