Conjugated linoleic acids (CLAs) and white adipose tissue: how both in vitro and in vivo studies tell the story of a relationship.

The distribution of adipose tissue in mammals is dependent on genetic and environmental factors, and in health the fundamental role of adipocytes is to store triacylglycerol during energetic excess and to mobilize this reserve during energy expenditure or reduced food intake. This requires an accurate balance, which is maintained through the interactions of several regulatory factors, as well as dietary manipulations. Dietary supplementation with CLAs (conjugated linoleic acids) is regarded as promising in many mammalian species for obtaining good body mass repartition and diminution of fat depots. CLAs are a group of positional and geometric isomers of conjugated dienoic derivatives of linoleic acid, naturally present in foods originating from ruminant species, and normally present in human adipose tissue. CLAs can, however, also be obtained as commercial supplements, usually containing synthetically prepared isomeric mixtures, and as dietary supplements CLAs are widely used by obese people, above all in the USA and Europe. CLAs are claimed to have protective effects against human degenerative pathologies, such as cancer, atherosclerosis, and diabetes, as well as showing beneficial effects on immune functions and food and energy intakes. The mechanisms of action of CLAs are not fully clarified at present, because in vitro and in vivo studies are not always in agreement, and possibly because CLAs act in different ways and with different consequences when administered in the diet to different species. The present review summarizes the ascertained mechanisms of action of CLAs, the mammalian species of major interest in which important studies have been conducted, and the future prospects for the use of CLAs in both humans and food animal species. The following topics will be discussed, taking evidence from both in vitro and in vivo studies, to provide a possible rationale for the therapeutic or dietary utilization of CLAs: decreased energy/food intake, increased energy expenditure, decreased pre-adipocyte differentiation and proliferation, and increased apoptosis of adipocytes. All of these parameters, in turn, affect decreased lipogenesis and increased lipolysis. For the future, interactions with individual hormonal substrates, changes in gene expression of proteins involved in lipid metabolism, and anti-tumorigenic effects will possibly constitute areas for scientific development and deepening of knowledge of dietary CLAs.