The term conjugated linoleic acid (CLA) refers to a mixture of positional and
geometric isomers of linoleic acid with a conjugated double bond system; milk
fat can contain over 20 diVerent isomers of CLA. CLA isomers are produced
as transient intermediates in the rumen biohydrogenation of unsaturated
fatty acids consumed in the diet. However, cis-9, trans-11 CLA, known as
rumenic acid (RA), is the predominant isomer (up to 90% of total) because it
is produced mainly by endogenous synthesis from vaccenic acid (VA). VA is
typically the major biohydrogenation intermediate produced in the rumen and
it is converted toRAby D9-desaturase in the mammary gland and other tissues.