It is a group of naturally occurring fatty acids synthesised as intermediates in the biohydrogenation of linoleic and linolenic acid in the rumen of animals, and thus is predominantly found in dairy products and ruminant meat ( Reference Chin, Liu and Storkson 2 ). ‘Conjugated linoleic acid‘ is a term used to describe a mixture of positional and geometric isomers of linoleic acid containing conjugated double bonds. Naturally CLA-enriched milk is defined for the purpose of the present review as milk obtained from grass-feeding regimens that have proven to result in higher levels of c9, t11-CLA than do conventional feeding regimens (see below). Cows' milk contains predominantly the cis-9, trans-11 isomer of CLA ( c9, t11-CLA). In order to gain approval, the scientific evidence must be based on human studies, with human intervention studies accorded a higher weighting ( 1 ). Health claims on foods in Europe must now be selected from community lists of approved claims or be the subject of a scientific dossier to gain approval ( 1 ). The primary purpose of the present review was to determine if the level of scientific evidence available for potential human health benefits of conjugated linoleic acid (CLA) is sufficient to support health claims on foods based on naturally CLA-enriched milk. In conclusion, further substantiation of the scientific evidence relating to CLA and human health benefits are required before health claims can be confirmed. This may be partly due to the differences in the concentration of CLA administered in animal and human studies. Four published intervention studies using naturally enriched CLA products were identified however, the results were inconclusive. There is no evidence of a consistent benefit of c9, t11-CLA on any health conditions and in fact both synthetic isomers, particularly t10, c12-CLA, have been suspected of having pro-diabetic effects in individuals who are already at risk of developing diabetes. A previous review of the relevant studies found that 3.2 g CLA/d resulted in a modest body fat loss in human subjects of about 0.09 kg/week, but this effect was attributed to the t10, c12-CLA isomer. Of these studies, the only evidence that is broadly consistent is an effect on body fat and weight reduction. Most human intervention studies have utilised synthetic CLA supplements, usually a 50:50 blend of c9, t11-CLA and trans-10, cis-12-CLA ( t10, c12-CLA). A search of the scientific literature was conducted and showed that almost all the promising research results that have emerged in relation to cancer, heart health, obesity, diabetes and bone health have been in animal models or in vitro. The primary purpose of the present review was to determine if the scientific evidence available for potential human health benefits of conjugated linoleic acid (CLA) is sufficient to support health claims on foods based on milk naturally enriched with cis-9, trans-11-CLA ( c9, t11-CLA).
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