Conjugated linoleic acid: Biosynthesis and nutritional significance

Abstract

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 different 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 to RA by Δ;9-desaturase in the mammary gland and other tissues. Biomedical studies with animal models have shown that RA as well as VA have anticarcinogenic and antiatherogenic properties, with the effects of VA being related to its conversion to RA. The anticarcinogenic effects have been observed for a wide range of cancer types, but the most impressive results have been reported in relation to mammary cancer. Of special importance, RA and VA are potent anticarcinogens when supplied as natural food components in the form of VA/RA-enriched butter. The functional food considerations of CLA isomers in dairy products realistically relate only to RA as the major isomer, although this should include VA because in humans it serves for the endogenous synthesis of RA. The RA and VA content in milk fat are directly related and they can be markedly enhanced through the use of diet formulation and nutritional management of dairy cows. Trans-10, cis-12 CLA is another CLA isomer in milk fat which can affect lipid metabolism. It is generally present at low concentrations in milk fat (typically <0.2% of CLA); under some dietary conditions, a portion of the rumen biohydrogenation shifts to produce more of this isomer, although it is still only a minor portion of total CLA. These dietary conditions are associated with milk fat depression and as little as 2 g/d of trans-10, cis-12 leaving the rumen will reduce milk fat synthesis by 20%. Because of the potency and specificity of this CLA isomer, it is being developed as a dairy management tool to allow for a controlled reduction in milk fat output. CLA isomers in milk fat and how they relate to both animal agriculture and human health are rapidly expanding fields. Milk and dairy products offer exciting opportunities in the area of functional foods, and the functional properties of VA and RA in milk further serve to illustrate the value of dairy products in the human diet. © 2006 Springer Science+Business Media, Inc.