Conjugated Linoleic Acid : More Than an Anticancer Factor

Lokenga Badinga and Charles R. Staples Department of Animal Sciences University of Florida Gainesville, Florida Definition and Sources of Conjugated Linoleic Acid Conjugated linoleic acid (CLA) is a collective term describing a mixture of positional and geometrical isomers of linoleic acid (LA, C18:2) involving a double bond at positions 8 and 10, 9 and 11, 10 and 12 or 11 and 13 (Eulitz et al., 1999).Each of these positional conjugated diene isomers can occur in cis-trans, trans-cis, cis-cis or trans-trans geometrical configuration (Eulitz et al., 1999).Conjugated linoleic acid is produced in the rumen as a result of incomplete biohydrogenation of LA as well as during the commercial manufacture of dairy products (Chin et al., 1992; Kepler et al., 1966).In the rumen, dietary lipids are rapidly hydrolyzed and the resulting unsaturated free fatty acids can undergo biohydrogenation by the rumen microorganisms (Figure 1).As a result, ruminant animals absorb mainly saturated fatty acids, and foods made from ruminants contain mainly saturated fatty acids, regardless of the fatty acid composition of the ruminant’s diet.However, when biohydrogenation is not complete, CLA can escape the rumen and be absorbed from the gastrointestinal tract, thereby providing the peripheral tissues with various isomers of CLA (Kelly et al., 1998). Conjugated linoleic acid is present in essentially all foods (Chin et al., 1992), but the principal dietary sources in human diets are dairy products and other foods derived from ruminant animals (Bartlet and Chapman, 1961; Chin et al., 1992).The predominant CLA in ruminant fats is the cis-9, trans-11 isomer which accounts for more than 80% of total CLA isomers in dairy products (Chin et al., 1992).It originates from CLA produced by the rumen bacteria as an intermediate in the biohydrogenation of LA (C18:2) or from tissue synthesis of CLA by )9desaturase conversion of trans-11 fatty acid.Milk fat concentration of CLA varies widely among dairy herds (Kelly et al., 1996; Kelly et al., 1998) and is affected by a number of dietary factors (Table 1) including the type and amount of fatty acid substrate (Noble et al., 1974), the forage to grain ratio (Gerson et al., 1985), and the nitrogen content of the diet (Gerson et al., 1983).Kelly et al. (1998) reported that feeding a high LA oil (sunflower) increased CLA concentration to 24.4 mg/g of milk fat compared with values of 13.3 and 1.7 mg/g of fat for high oleic (C18:1) and high linolenic (C18:3) acids, respectively.Recently, Dhiman and coworkers (1999) found that grazing cows had a 5.7 times higher concentration of CLA in milk than did cows fed diets containing preserved forage.In another study, Jiang et al.(1996) reported that, for a constant supply of LA, CLA content could be affected by the ratio of forage to concentrate.A review of country and seasonal variations in CLA concentrations of milk fat showed a range of 8.6 to 100 :mol/g (Riel, 1963).Seasonal variation was very marked, with values during the summer period often up to three to four times higher than winter values.These observations collectively suggest that, given an adequate dietary intake of LA, dietary constituents which provide substrates for the optimal growth of bacteria producing LA isomerase will maximize CLA output.However, the recent observation (Kelly et al., 1998) that milk CLA concentrations vary substantially among individual cows (9.9 to 51.7 mg CLA/g of fat) that consume the same diet and are subjected to the same management regimen suggests that additional factors such as individual genetic regulation of rumen microflora may also operate in the control of ruminal CLA synthesis (Moore et al., 1993). CLA and Prevention of Cancer Interest in CLA as an anticarcinogen stemmed from the original observation by Pariza and Page 1 of 8 CONJUGATED LINOLEIC ACID : MORE THAN AN ANTICANCER FACTOR