REVIEWS: Conjugated Linoleic Acid: Historical Context and Implications

Conjugated linoleic acids (CLA) are implicated as anti-carcinogenic, anti-atherosclerosis, and anti-inflammatory agents in a variety of experimental model systems. However, evidence of dietary CLA protection against human mammary cancer risk is mixed and comes from European countries where the death rate from mammary cancer is relatively scarce. Unfortunately, epidemiological data are inconclusive, especially from retrospective studies. Prospective human study evidence will take more time. European values as great as 1.9% CLA in the fat of milk products from grass-fed ruminants has been reported; ordinary monogastric meat and egg products contain substantially less CLA in fat (0.3%). It is now recognized that the principle CLA in ruminant meat and milk is the natural diene, cis (c)-9, trans (t)-11 isomer (rumenic acid) of C18:2 (octadecadienoic acid). Another isomer, C18:2 t-10, c-12, also contributes to the unique biological activity of CLA, but does not readily accumulate in ruminant lipids and is found only in commercial preparations of mixed CLA isomers. Evidence in humans suggested that the role of the dietary mixed isomer CLA in the loss of body fat mass (BFM) was only modest compared with the results from animal model studies, and urine metabolites of prostaglandin F_2α (PGF_2α), indicative of lipid oxidation stress, have been elevated during supplementation. In addition, the fatty acid C18:1 t-11 (vaccenic) is now believed to be the principle precursor of endogenous c-9, t-11 CLA in both ruminants and monogastrics. This finding helps explain the discrepancy between measured c-9, t-11 CLA originating from the rumen and that secreted in cow’s milk. Manipulation of ruminant meat and milk by feeding marine or vegetable oils is clearly associated with increases in vaccenic acid as well. This relationship requires a re-examination of human foods for vaccenic acid content and quantitative measures of CLA endogenous synthesis in humans as well to formulate dietary strategies to capture CLA’s potential protective health benefits.