The term CLA refers to several positional and geometric conjugated dienoic isomers of linoleic acid (LA), of which the trans-10,cis-12 isomer has been reported to reduce adiposity and increase lean mass in mice and other animals when included at 1% of the diet. However, most dietary CLA in humans is obtained from dairy products, with the cis-9,trans-11 CLA isomer, also known as rumenic acid, accounting for >90% of the total CLA intake. Commercial CLA preparations containing trans-10,cis-12 and cis-9,trans-11 CLA isomers in diverse proportions, are attracting consumer interest because of the purported body fat-lowering effects of CLA, coupled to perception of such products as natural compounds, devoid of harmful effects. Nevertheless, despite numerous studies on CLA effects on body composition for nearly a decade, the mechanisms by which CLA isomers elicit their effects remain largely unknown. The purpose of this paper is to provide an updated literature review of the studies performed on animals and humans, as well as to describe the potential mechanisms involved in CLA effects on body wt. and composition and metabolism. Experiments in humans have not been able to show a significant effect on body wt., body composition or wt. regain related to either of the CLA isomers. In fact, some studies suggest a tendency towards a decrease in body fat mass and an increase in body lean mass, while some others raise concern about the possibility of deleterious effects of trans-10,cis-12 CLA on lipid profiles, glucose metabolism and insulin sensitivity. It is concluded that the evidence currently available regarding the effectiveness of CLA in humans is unconvincing.
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