Conjugated Linoleic Acid (CLA) is a free fatty acid that is naturally occurring in most dairy products as well as a wide variety of meats. In fact, close to 60% of naturally occurring CLA is found in dairy products, about 37% is found in meats and the rest is found in processed foods (66). CLA was first discovered by accident in 1978 by Michael Pariza at the University of Wisconsin. Although it took several years for scientists to begin looking at the free fatty acid, the first true studies on CLA were performed in the mid 1980's specifically by Ha et al (67). Most of the research and scientific studies on the effects of CLA were done on animals for nearly ten years. As a result, CLA proved to have many benefits on the tested animals such as: improved body composition, enhanced immune function, reduction of atherosclerosis, decrease in cancerous cells, improved food and energy intake, anti-catabolic and anabolic effects as well as many other positive attributes (68).
In two similar studies performed by Blankson et al (69) and Thome et al (70), there is evidence that the use of CLA had a significant reduction in body fat in human subjects although there was no significant decrease in bodyweight. Although this stagnation of bodyweight was not addressed to a thorough degree, the only reasonable explanation for the reduction of body fat accompanied with no decrease in bodyweight would be due to a gain of muscle mass. One of the only disputes that other researchers have with both of these studies is the fact that in both experiments, the use of CLA was combined with a light to strenuous training regime which may have accounted for some of the improved body composition. In other words, both of these studies would suggest that the use of CLA causes a decrease in body fat in obese humans and in healthy exercising individuals currently partaking in an exercise regime.
Another large area of study on CLA in humans deals with the obese and overweight population. This may be due to the belief that CLA can cause a considerable decrease in fat mass and in bodyweight. In two very similar studies, the effects of CLA were observed in the overweight population for 1 year (71) and for 2 years (72). Both studies have almost identical findings. The main difference in the two experiments is that the 2 year study not only examined physical benefits achieved through the use of CLA but also examined the safety, tolerance level, and long term effects of the individuals who used the product. It appears as though CLA had a very significant impact on subjects from both studies.
Subjects who participated in the 12 month study experienced a 6.9% - 8.7% reduction in body fat mass while participants of the 24 month study experienced an average loss of >4.5% in body fat mass. There were no negative side effects or adverse effects experienced in either study which is of significant importance due to the length of the study and the long term use of the supplement. Participants of the 24 month study also had a decrease of plasma total cholesterol and LDL cholesterol although, there was no change in HDL. These findings help to support the claim that CLA can have a positive effect on individuals with atherosclerosis and other coronary issues. Further study is needed to determine its effect on blood triglycerides in humans. It should be noted that both of these studies only involved overweight and obese individuals. So although it is quite clear from the literature that CLA reduces body fat mass in this population, these two studies do not satisfy claims made in regard to a decrease in body fat mass in normal weight individuals and athletes.
Knowing that CLA can have a positive effect on health and body composition in the overweight population is of great importance, especially due to the rapid increase of obesity in society. However, there has been far less research done on the normal-weight and athletic population. Two studies of great relevance to this specific issue were recently performed by Pinkoski et al (73) and Kreider et al (74) and deal with the effects of CLA during resistance training.
In the study performed by Pinkoski et al (73), participants engaged in a consistent three days per week intensive weightlifting routine. Although there were numerous measurements taken of various markers in this study, most of which had no significant change, the most striking results can be seen by looking at the body composition and anabolic/anti-catabolic effects associated with the experiment. At the end of the study, there was a reported lean tissue mass gain of 1.4 kg for the CLA group compared to the placebo group which gained only .2 kg. Also, the CLA group had an average loss of -.8 kg of fat mass compared to the placebo group of -.4 kg. Although these findings seem quite small and almost insignificant, it is of great importance to note that this study only lasted for seven weeks. Many individuals who participate in vigorous lifting workouts often see little to no results after only seven weeks. Had the study gone on much longer, the results may have been far greater. In other words, the CLA group gained close to one half a pound of lean body mass per week while the placebo group gained almost none.
There is valid reasoning that had this study gone on for a longer duration, the results would have been even better, mainly due to other markers of anti-catabolic effects found in this study. The most noteworthy finding here showed that the urinary marker of myofibrillar degradation which directly results in a decrease in the catabolic effect of training on muscle protein and protein synthesis was significantly improved in the CLA group. In simple terms, this would create a more anabolic environment for the body to build muscle while at the same time losing body fat. This could be of significant importance to athletes or bodybuilders looking to shed body fat while maintaining or even gaining muscle mass. The last measurement in this study that had any change in comparison with the placebo group and CLA group was the strength measurements. Almost all strength measurements stayed the same, except for a significant increase in bench press strength only seen in the male subjects of the CLA group. Although there may be several explanations for this occurrence, the details involved with this increased neuromuscular change goes beyond the scope of this paper.
The study performed by Kreider et al (74) is one of the other few CLA experiments performed on experienced exercisers and fit individuals while undergoing an exercise regime. There are some very intriguing results obtained from this study. At first glance, this study would appear to negate any benefits formerly thought to be associated with the supplementation of CLA, especially in the fit or athletic population. In fact, the actual results from this study clearly indicate that there was no significant change in body composition, strength gains, and muscle mass, or bone density when comparing the CLA and placebo groups. However, if one examines the study more closely, there are two very interesting factors to consider in regards to CLA and this particular experiment.
First of all, when the hematological analysis was conducted, it showed that although there were certain significant markers that did not change, there were others that did. The most notable was lessening in the ratio of blood urea nitrogen to creatinine. The experimenters themselves noted that the analysis revealed moderate to large effects for changes in the ratio of blood urea nitrogen to creatinine. This finding may have significance since it is an important marker and indicator for catabolism. In other words, according to this study, this hematological marker may imply that CLA causes a lessening of a catabolic response in the body. It may also promote an anabolic state, thus, resulting in a gain of lean body mass over time. However, the key here may be time itself.
As previously mentioned, there were two factors in this study that may have profound significance as to the implications of CLA when used with fit or athletic population. The second one deals with the length of the study. This study was only conducted for 4 weeks. This brief time period is simply not long enough to determine the effectiveness of CLA supplementation, especially in the fit and athletic population. To say that there was no change in body mass, strength, and muscle gains during the study is somewhat deceiving. In fact, there are very few aids or exercise programs themselves that produce changes and measurable benefits after only 4 weeks. Had the study been prolonged for at least another month or two, the findings may have indicated a direct change in body composition and strength gains. In fact, the decrease of blood urea nitrogen to creatinine as mentioned earlier may produce measurable gains of lean body mass and strength if this process were to take place for a longer period of time. In other words, for physiological and biochemical shifts to have a significant effect on the body, it may take much longer to occur than just 4 weeks.
As with much research done on supplements and ergogenic aids, a great deal of it is performed on animals. This is done to test out a new product where negative side effects and possible harm to humans is unknown as well as for issues of practicality. Simply put, it is much easier to use fifty rats than it is to recruit fifty human participants. In regards to CLA, there have now been hundreds of studies conducted on animals such as rodents and cows. Unlike most studies conducted on an individual product or substance, the results for CLA usage on animals are very conclusive and similar throughout. Quite simply, CLA has a positive effect on lean body mass, fat mass, and other markers of health.
Rather than endlessly listing hundreds of similar studies performed on animals fed with CLA, simply describing the results of several prominent and more recent experiments should be sufficient. A study performed by Bhattacharya et al (75) showed that CLA decreased body weight and fat mass in mice while at the same time being fed a high fat diet. Yet, another similar study conducted by several of the same scientists, Bhattacharya et al (76) concluded that mice fed with CLA decreased their fat mass, had an increase in muscle mass, a decrease in serum leptin levels, and an increase in energy expenditure. Quite simply both of these studies show that CLA has a very positive effect on body composition, decreases appetite as seen from serum leptin levels, and increases the metabolism as seen from the increased energy expenditure. Again, these results are in regards only to mice. These studies did not involve human subjects.
Another very profound study performed on mice while being fed CLA was conducted by Banu et al (77). This experiment not only showed a decrease in body weight, but also showed an increase in bone mass and bone density. This is of great significance due to the fact that weight loss in general is often associated with a decrease in bone mass and bone density. Another similar study performed by Viswanadha et al (78), showed that CLA caused mice to decrease their total fat mass while increasing their total weight. Although not all physiological markers were measured, the scientists concluded that this increase in weight was from either an increase in muscle mass or bone mass, or perhaps both.
Summary and Recommendation
Although there is a significant amount of positive research surrounding CLA, there is still great debate on how CLA affects the physiology and biochemistry of the body to induce the above mentioned results. Some researchers believe that it helps keep the adipose tissues small by possibly blocking certain enzymes that are responsible for the enlargement of fat cells. Some feel it is directly involved in fat metabolism by helping the body use its own fat cells as its primary source of fuel. Others believe that CLA's most notable influence is the effect it may have on corticosteroids. It is theorized that CLA may help counteract corticosteroids such as cortisol which is responsible for muscle breakdown and body fat storage. Hopefully, further research will give greater insight not only into the effectiveness of CLA, but also to the specific physiological and biochemical pathways that are responsible for understanding how it works in the human body.
All in all, it appears as though CLA is a beneficial supplement and has the potential to assist many individuals with their health and fitness goals. Although it appears to be beneficial to most populations, the studies support its greatest benefits in the overweight and obese population. This is not to say that CLA will not aid athletes and fit individuals in their goals for lean body mass and strength, they simply will not have as great of changes in their bodies as would a population of lower fitness levels. They will, however, receive many of the same benefits, just to lesser degree. Some of these known benefits that humans can gain from supplementing with CLA include a decrease in fat mass, increase in muscle mass, increase in strength, possible increase in muscular and cardiovascular endurance, increased bone mass and bone density, increased immune system function, decrease in overall body weight, increased metabolism, and increase in energy expenditure and metabolism (73).
Other possible benefits that have been witnessed in animals, but have yet to be seen with human studies include a decrease in total cholesterol as well as a decrease in LDL and an increase in HDL, improved levels of blood glucose levels, and a decrease in blood triglyceride levels . If these benefits were to eventually be reported in human studies, then this would prove that CLA not only helps with physical appearance and fitness levels but it may also play a role in decreasing obesity, cancer, heart disease, and diabetes (73). Time will tell whether or not the same benefits produced in animal studies will have as significant of an impact on humans. If it does, then CLA may be a huge key to the future for health and fitness supplementation.
There appear to be no toxicity issues or related concerns when CLA is taken by healthy populations. However, as reported by consumerlab.com, there is some information suggesting that CLA may worsen blood sugar control in diabetics and obese individuals without diabetes. For those who are willing to give CLA a try, it is important to utilize CLA correctly as well as consume the best brand names in order to receive the full benefits of the supplement. A safe and effective dosage would range anywhere between 1500-3000mg per day depending on overall body size (71, 72, 73, 74). Dosages should be evenly spread out three times a day and should be consumed with meals.
Some of the better and more reliable CLA brand name products as reported by consumerlab.com include: AST Sports Science CLA, Metagenics Ultra CLA, Natrol Tonalin CLA, Natural Max Ultra Supertone CLA, Nature's Sunshine CLA, Nutrilite CLA, Ultimate Nutrition Patented CLA, and Vitamin World CLA.
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