The present study provides a detailed overview of the contents of conjugated linoleic acid (CLA) isomers in the most consumed Portuguese CLArich foods (milk, butter, yoghurt, cheese, beef and lamb meat), by using silver ion-HPLC. In addition, the contribution of these ruminant-derived foods to the daily intake of CLA isomers was estimated based on Portuguese consumption habits. The total CLA concentration in milk and dairy products ranged from 4·00 mg/g fat in yoghurt to 7·22 mg/g fat in butter, and, regarding meats, from 4·45 mg/g fat in intensively produced beef to 11·29 mg/g fat in lamb meat. The predominant CLA isomers identified in these products were cis-9,trans-11 (59·89-79·21 %) and trans-7,cis-9 (8·04 -20·20 %). The average estimated total CLA intake for the Portuguese population was 73·70 mg/d. Milk and cheese are probably the two products with the highest contribution to the final CLA intake, as a result of their high fat content and consumption values. The results also suggested that cis-9,trans-11 and trans-7,cis-9 are the isomers most represented, with, respectively, 76·10 and 12·56 % of the total CLA intake. Being the first detailed report on the contents of total and individual CLA isomers in Portuguese commercial ruminant-derived foods, we further discuss the implication of the results for diet characteristics and human health.
Conjugated linoleic acid: Dairy products: Ruminant-derived meats: Silver ion-high-performance liquid chromatographyConjugated linoleic acid (CLA) refers to a heterogeneous group of geometrical and positional isomers of linoleic acid (18: 2n-6) with conjugated double bonds. These double bonds can either be trans or cis configured, and a wide spectrum of isomers with variations in position (from 6,8-to 12,14-) and geometry (trans,trans, trans,cis, cis,trans and cis,cis) has been described 1,2 . Twenty different CLA isomers occur naturally in food, especially in ruminant-derived fat 3 . The major CLA isomer (cis-9,trans-11 (c9,t11), also known as rumenic acid), as well as the usually second most prevalent isomer (trans-7,cis-9; t7,c9), are produced in the rumen during microbial biohydrogenation of dietary 18: 2n-6 and in the tissues through D9-desaturation of the rumen-derived trans-octadecenoate (trans-11-18: 1) 4 . It is now accepted that the major contribution to these CLA isomers in ruminant-derived milk 5 and meat 6 is endogenous synthesis by D9-desaturation. With the exception of these two isomers, the origin of all other CLA isomers is supposed to arise from ruminal biohydrogenation of dietary unsaturated C18 fatty acids, even if the metabolic pathways are not yet elucidated 7 .Many experimental studies, using laboratory animals as well as human and cell-culture systems, suggest that CLA exhibits interesting biological activities: anticarcinogenic, anti-adipogenic, anti-diabetogenic, anti-atherogenic and antiinflammatory 2 . The National Academy of Sciences of the USA recognised CLA as the only fatty acid that unequivocally inhibits carcinogenesis in experimental anim...
