Conjugated linoleic acid (CLA) is a collective term for a group of isomers of linoleic acid. There are eight major CLA isomers, but the cis-9, trans-11 isomer appears to be the most biologically active or relevant form. Conjugated linoleic acid is a fatty acid that is found primarily in meat and dairy products.
Initial research on CLA indicated that it had potential anticarcinogenic properties. In addition to its anticarcinogenic properties, CLA has been reported to protect against atherosclerosis in rabbits and overcome the catabolic response of endotoxin injection. This anticatabolic action of CLA led to additional studies investigating its effect on body composition.
In Vitro Studies
The way a fatty acid can alter body composition remains to be determined. However, some preliminary research may provide some insight as to the mechanism of CLA. Park and coworkers reported that CLA added to the culture medium of differentiated 3T3-L1 adipocytes significantly reduced intracellular triacylglycerol and glycerol concentrations while significantly increasing glycerol in the culture medium. This indicates that CLA somehow stimulates lipolysis. What is lacking from the study is the measurement of FFA in the incubation medium. If triacylglycerol breakdown is occurring, not only will extracellular glycerol increase, but extracellular FFA would also increase. The reason why FFA was not measured is unclear.
In the same study by Park et al CLA supplementation was also investigated in mice. The investigators reported CLA-fed mice exhibited an enhanced norepinephrineinduced lipolysis and HSL activity in isolated epididymal adipocytes. The authors concluded that CLA decreases fat deposition, while increasing lipolysis that could lead to an increase in fatty acid oxidation in muscle cells. Subsequent studies have confirmed that CLA reduces body fat in mice.
West and colleagues investigated the effect of CLA supplementation in mice on either a low-fat (15% of total kcals) or high-fat diet (45% of total kcals). In both the high- and low-fat diet groups, CLA supplementation resulted in a reduction in energy intake, which was associated with a reduction in fat stores. However, a negative consequence of CLA supplementation was a reduction in growth rate and total body protein.
On the other hand, Delany and coworkers reported that CLA supplementation in mice resulted in a marked reduction in fat accumulation and an increase in protein accumulation. In both studies, the mice were of the same breed line (AKR/J) and CLA was provided at a dose of 1.0% to 1.2% body weight. Therefore, the only other factor to consider for the differences would be the type of diet provided. Further research needs to address the effects of CLA supplementation on growth retardation and protein accretion.
Whether CLA has any effect in humans is difficult to determine. A few studies have been conducted investigating the effects of CLA on body composition. However, these studies are not published in peer-reviewed journals, and only two have been presented in abstract form. There are preliminary studies and some potential flaws exist in the methodology, analysis, or interpretation of the data by the authors, which will be pointed out. However, they will help in aiding other researchers to develop more sound protocols for CLA research in humans.
Supplements of CLA was investigated in collegeaged males with resistance-training experience. The researchers tested the subjects for changes in strength and body composition by dual-energy x-ray absorptiometry (DEXA) following 28 days of CLA supplementation. The researchers reported the group consuming CLA had slightly greater gains in strength, but there was no change in fat-free mass or fat mass.
Because FFM did not increase, where did the increase in strength come from? In general, when resistance-trained individuals are used as subjects, it is unlikely that any changes in strength will be due to neural adaptations, as seen in untrained individuals. Therefore, the increase in strength observed in this study is more than likely due to testing procedures or the subjects not putting forth their best effort. In conclusion, the results of this preliminary study show body fat did not change when subjects consumed CLA.
Interestingly, one researcher did report that the CLA supplemented group did experience an increase in bone mineral content in only 28 days although not significant, it did approach a level of significance . An increase in bone minerals content of only 1.1% is within the coefficient of variation of measurement of DEXA analysis for bone mineral content, which ranges from 0.8% to 1.5%.
Furthermore, a control group was not included in the study design to rule out the possibility of such an error occurring. Finally, how quickly can bone density change Most studies that have investigated changes in bone density during resistance training have been conducted over months (4 to 18 months) to detect changes in bone density.
Even then, there is some conflicting evidence on whether resistance training can increase bone density. Therefore, it seems unlikely that bone mineral content or density would change in that short period of time. Although bone density is not part of fat reduction, the results of the study indicate that testing or interpretation errors can occur when testing subjects and analyzing data.
A 6-week study investigating CLA supplementation was performed in novice bodybuilders. The treatment group received 7.2 g of CLA, and the placebo group received vegetable oil. The authors reported that skinfoldcorrected arm girth and body mass increased to a greater extent in the CLA group. The methodology used to measure arm girth is questionable. It is subject to error and the authors did not report the reproducibility of the procedure. A more sensitive method, like MRI or CT scan, for assessing muscle growth, may have been a more appropriate method of measurement.
An unpublished study used in press releases reports that “CLA preferentially produces deposition of lean body mass rather than fat mass” (Pharmanutrients press release). This study investigated the effects of CLA supplementation during a 6-month treatment program with 80 obese subjects. The study was poorly controlled. The subjects were asked to reduce their energy intake by about 500 kcal/day, and were asked to exercise at least three times per week for a minimum of 30 minutes. Of the 71 subjects who finished, there were no differences in body composition, following the treatment period, both the CLA group and the placebo group lost the same amount of weight.
The weight loss was a combination of fat mass and fat-free mass. On average, the CLA group lost 3.0 Ibs of FFM, while the placebo group lost 2.5 Ibs of FFM. Of the 71 subjects, the author finds that only 15 gained FFM, and 10 of these subjects were in the CLA group. Based on this observation, the authors conclude that CLA is able to preferentially increase FFM rather than fat mass during an anabolic state. The interpretation of the data in this manner is not appropriate.
In a sense, the authors are digging for something that is not there. The appropriate conclusion that can be drawn from the data presented in the abstract is that during an unsupervised diet and exercise program, CLA does not result in an increase in FFM. Furthermore, CLA was unable to prevent the loss in FFM that occurred during the treatment and exercise program.
Though the CLA research in mice seems promising, more well-controlled research is needed in humans to determine whether CLA works in preserving or enhancing FFM while promoting fat loss during periods of dieting.
Safety and Toxicity
In the animal studies that have been performed, and the few human studies, there have been no reports of adverse health incidences or toxicity. It is unlikely that CLA would result in any problems when consumed in the recommended amount. The usual dose is about 2 to 3 g. A person normally consumes ten times that amount of fat during a normal meal.
Potential problems, if any, could exist from altering fatty acid content of the phospholipids in cell membranes. Research suggests that diet can influence membrane fluidity by altering the structure of the phospholipids. Furthermore, membrane fluidity can influence the activity of membrane-bound enzymes. Whether CLA affects the activity of membrane enzymes is only theoretical, but deserves attention.