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THE ROLE OF CONJUGATED LINOLEIC ACID IN HUMAN HEALTH |
The need to catalogue the unique biological properties of conjugated linoleic acid (CLA) and its potential effectiveness as a functional food and nutriceutical is pressing and requires first-rate evidence-based research. The present supplement represents a compilation of research manuscripts presented at a national workshop in Winnipeg, Canada, in March 2003 on the role of CLA in human health. The workshop was organized to determine the current state of knowledge and gaps in experimental evidence related to the physiologic role and mechanisms of action of CLA.
Conjugated linoleic acid is a term that describes positional and geometric isomers of linoleic acid, a dienoic n–6 aliphatic fatty acid. The conjugation of linoleic acid signifies the separation of the 2 double bonds by a single (methylene) bond. The most common dietary sources of CLA are ruminant milk, most dairy products, beef, lamb, and pork, and more than 90% of CLA is in the c9-t11 configuration. Dietary supplements of chemically prepared CLA are widely available in health food stores and over the Internet. These supplements contain a mixture of CLA isomers of varied composition and purity.
Interest in CLA was kindled by the serendipitous observation some 20 y ago that CLA obtained from lipids extracted from barbecued beef patties displayed anticarcinogenic properties (1). Since then, tantalizing observations have been reported of CLA’s near-magical properties that ameliorate a wide assortment of disease processes. In various animal models, CLA can reduce or prevent adiposity, improve insulin resistance, inhibit adipogenesis, inhibit tumor development, and modulate inflammatory reactions (2). These observations have led to much speculation about the potential role of CLA in human health. The significance of these findings is enhanced substantially because they often reflect the action of a specific CLA isomer—ie, t10-c12, the most potent modifier of biochemical processes and regulatory systems.
It is reassuring to see the growing number of laboratories now involved in studies of the molecular, biological, and gene-regulatory targets of CLA in attempts to clarify the mechanism of action of CLA effects (3). This focus augurs well for timely answers to the many questions raised.
Despite all the promising findings, definitive conclusions about the usefulness of CLA in humans as a nutriceutical in the treatment or prevention of chronic diseases are limited. This point is exemplified by significant and unanticipated species variations in the antiadipogenic properties of CLA. Whereas mice are particularly sensitive in a dose-responsive fashion to CLA supplements, recent results of human studies are variable and therefore unconvincing (4). Whether this variability is due to selection bias, cohort size, or dosing is not known, but it implies design flaws. Thus, the importance of appropriately powered clinical trials cannot be overstated, and they are essential to validation of CLA’s efficacy. Until that validation is achieved, we must reserve judgment, exercise caution in promoting CLA as a therapeutic product in the management of patients, and encourage continuing research in this nascent area of inquiry.
Finally, the workshop and this supplement were sponsored by a variety of institutions, organizations, and industrial partners interested in the potential nutriceutical value of CLA in human health. The organizers of the workshop wish to express appreciation for their support and encouragement in this endeavor.
REFERENCES
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