Health effects of CLA

Observations in animal trials pointed out that differences exist between mammalian species regarding their response to CLA. It can be assumed that not all the impacts that were proved for animals also pertain to humans (Wahle et al., 2004). In order to clarify the effect of dietary CLA intake on the process of human carcinogenesis, more clinical studies are needed. 

The anti-diabetic effect of CLA may depend on both species and types of isomer. Rumenic acid seemed to be inactive (Martin Valeille, 2002), but the role of trans-10,c -12 isomer is controversial some studies verified decreasing glucose levels and increased insulin sensitivity with increased intake of trans-l0,cis-l2 CLA (see the review by Khanal, 2004), while others reported opposite effects - that is, the promotion of insulin resistance (Khanal, 2004 Moloney, Yeow, Mullen, Nolan, Roche, 2004 Wang Jones, 2004). 

Several human studies were conducted in order to assess the effect of dietary CLA on blood parameters for cardiovascular diseases. In most experiments there were no 

The enumeration of possible effects of CLA on human health has not been completed yet. CLA intake may have an influence on bone mass. The mechanism of this action, however, is unambiguous (see the review by Park (2009)). Another possibility is the utilization of the impact of CLA on inflammation and immune function for asthma therapy (MacRedmond Dorscheid, 2011). 

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BENEFICIAL HEALTH EFFECTS OF CLA REPORTED FROM BIOMEDICAL STUDIES WITH... 

Sensory quality can be defined as texture, flavour (taste), aroma and visual aspect. The sensory properties of milk are highly influenced by its fat content (Phillips et al., 1995a). As a result, research has examined the effects of various food additives on sensory quality when used as a substitute for fat in milk (Philips et al., 1995b). Frpst et al. (2001) showed that a combination of thickener, whitener and cream aroma in 0.1% fat milk was successful in mimicking the sensory quality of 1.3% fat milk. With the interest in the production of milk enriched with cis-9, trans-l 1 CLAs, owing to their relevance to human health (Tricon et al., 2004), recent research has examined the effects of CLA on the sensory quality of dairy products and found that it is possible to produce CLA-enriched dairy products with acceptable sensory characteristics (Jones et al., 2005). 

CLA isomers are functional food components when their search for mutagens in cooked meat instead identified CLA as an antimutagen (see Pariza, 1999). As a result of this discovery, research on CLA has increased exponentially over the last decade and a number of potential health benefits of CLA have been reported. The anticarcinogenic activity of CLA has been established clearly, but biomedical studies with animal models have identified an impressive range of additional positive health effects for CLA as summarized in Chapter 17. Particularly noteworthy is the fact that CLA is a potent anticarcinogen when supplied as a natural food component in the form of CLA-enriched butter as discussed later in this review. 

A broad overview of the biological effects of CLA is presented elsewhere in this volume (Chapter 17), so the emphasis in the following section will be two-fold. Firstly, the biology of trans-10, cis-12 CLA in the dairy cow will be summarized because under certain dietary conditions, production of this isomer in the rumen can profoundly affect milk fat synthesis. Secondly, the biological effects of RA when supplied as a natural component of the diet will be reviewed because this CLA isomer represents a functional component of milk fat that has potential health benefits. Although other CLA isomers are present in milk fat, they are present at concentrations much too low to have a significant effect. 

Animal data repeatedly showed beneficial effects of CLA on cardiovascular health. These findings were repeated in humans, although not consistently. Blood lipid levels vary greatly with diet and exercise, which are more easily controlled in animal studies than in free living subjects. In addition, most human studies were designed to measure effects of CLA on body composition. Only the study by Noone et al. (81) was designed specifically to address blood lipids, and this study did show beneficial effects of CLA. 

There is accumulating evidence from animal studies that CLA has potential health benefits for humans (Sebedio et al., 1999 Roche et al., 2001). Potential anti-cancer properties of CLA have been reported using rodent mammary and colon cancer models (see Sebedio et al., 1999). Potential beneficial effects of CLA on body composition (i.e. reduced fat mass) have been reported in rodents, chickens and pigs (see Sebedio et al., 1999 Roche et al., 2001). Effects of CLA on plasma triacylglycerol concentration, on glucose homeostasis, on atherosclerosis, and on inunune function have also been reported in animal studies (see Sebedio et al., 1999 Roche et al., 2001). Many of these animal studies have used a mix of CLA isomers, but predominantly the cis 9, trans 11... 

Claims that CLA is beneficial for health remain as yet unconvincing. Human studies investigating the effects of CLA supplements have tended to use mixtures of isomers and results are inconsistent. Studies attempting to use relatively pure preparations of single isomers suggest that some of the effects of CLA may be isomer-specific and have highlighted a potential detrimental effect of trans- 0,cis- 2 CLA on blood lipids, in particular on HDL-cholesterol levels. There may also be detrimental effects of trans- Q,cis- l CLA on insulin sensitivity in obese subjects with metabolic syndrome, but this remains to be confirmed. 

The reductionist approach of isolating dairy product components including calcium, CLA, and trans-fatty acids in dietary interventions trials often yields inconclusive results. It is therefore highly plausible that dairy products exert maximum health benefits when consumed in their natural form. Because individuals do not generally consume these individual dairy components in isolation, examining the effects of these foods in their whole forms should be encouraged. 

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