Conjugated Linoleic Acid (CLA) Research: The Past, the Present, and the Future

Updated on July 10, 2018
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S E Hurst graduated from the University of Tennessee with a PhD in Comparative and Experimental Medicine in 2012.

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How CLA acts on the human body is still under extensive investigation, but several studies have been done using animal models. The two major isomers proven to have biological effects are the cis-9 and trans-11, and the trans-10, cis-12 CLA isomers. Researchers are taking this information and using it to find possible treatments and/or methods of prevention for some medical issues.

Differences in linoleic acid and conjugated linoleic acid (CLA) isomers.
Differences in linoleic acid and conjugated linoleic acid (CLA) isomers. | Source
CLA supplementation in rabbits showed a reduction in plague formation.
CLA supplementation in rabbits showed a reduction in plague formation. | Source

CLA and arteriosclerosis

A study involving rabbits was conducted to test the effect of CLA on arteriosclerosis, the buildup of fats, cholesterol and other substances in and on artery walls, which can restrict blood flow1. Dr. David Krithcevsky and his team fed rabbits a high cholesterol, plaque-forming diet for three months. After the three months, one-third of the rabbits were killed to measure the buildup of plaque in the arteries. For the next few months, the remaining rabbits were fed a normal diet supplemented with a 50:50 mixture of the two biologically active CLA isomers. The dosage for one group was 0.1% CLA and the other group was given 1.0% CLA as measured by the weight of food administered. At the end of the study, the rabbits fed 0.1% CLA had a slight reduction of plaque, whereas the group fed 1.0% CLA experienced a 30% reduction of plaque. Since a 50:50 mixture was used, the active isomer for arteriosclerosis treatment is still unknown.

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CLA supplementation was shown to effect the immune response in multiple species including chickens.
CLA supplementation was shown to effect the immune response in multiple species including chickens. | Source

CLA and the immune system

At the University of Wisconsin-Madison, Dr. Mark E. Cook and his associates tested CLA’s immune system enhancing abilities on chickens. Poultry producers vaccinate chickens every year to protect against certain epidemics. Every time the vaccine is given, some growth is halted, costing producers about one billion dollars in extra handling costs1. This halted growth comes from the inflammatory response triggered by the immune system. The inflammatory action produces proteins called cytokines, which induce some muscle wasting. To fight against this response, scientists fed animals a 50:50 mixture of the cis-9, trans-11 and trans-10, cis-12 CLA isomers, and then injected the animals with an immune stimulant. For rats, mice, chicks, and pigs, the wasting was blocked temporarily without reducing the ability of the immune system to fight infection.

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CLA supplementation improved immune system function in guinea pigs and healthy men.
CLA supplementation improved immune system function in guinea pigs and healthy men. | Source

CLA and allergies

In a series of clinical trials, CLA increased the body’s production of infection-fighting white blood cells and depressed the immune system stimulation when not needed by the body. These findings sparked interest that CLA might control allergies. To test their hypothesis, researchers sensitized guinea pigs to specific allergens associated with asthma1. When the airways of the guinea pigs were exposed to the allergens after taking a CLA supplement, less airway constriction was observed. From this result, scientists believed CLA lessened the activity of cyclooxygenase-2 (COX-2), which causes the allergic reaction and inflammation. These same isomers were used to test the effects on immune function in healthy men2. Conducting the study over a twelve-week period with men ages 31 to 69 showed that the 50:50 combination of cis-9, trans-11 and trans-10, cis-12 CLA isomers improved the ability of the immune system to fight new infections as demonstrated following a Hepatitis B vaccination.

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CLA and cancers

Researchers have also been looking into the effects of CLA on various cancer models. Scientists at Cornell University enriched butter with the cis-9, trans-11 CLA isomer and fed it to rats. The rats were then injected with a known carcinogen for breast cancer. For the CLA-enriched butter group, there was a reduction in the number of terminal end buds by 50% (where mammary tumors form) and an increase in programmed cell death3. The same conclusion was reached by Dr. Pariza and his research with rat models1. Researchers do state, however, that tracking cancer is somewhat impractical because some malignancies can take years to form.

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CLA and body fat composition

CLA has also been shown in animal models to inhibit skin, mammary, and colon tumor growth as well as increasing apoptosis (programmed cell death) in mammary, liver, and adipose tissue4. The reduction of adipose tissue prompted researchers to search for the effects of CLA on body fat composition. For 28-32 days, mice were fed a diet containing 1.0% CLA4. At the end of the experiment, total fat was reduced by over 50% and there was an additional reduction of leptin (a hormone that regulates food intake). The effects of this study proved to be gender specific. As expected, male mice had a higher metabolic rate and increased muscle mass. For both groups, the fat loss was sustained even after removal of CLA from the diet.

Anticipating the same results in humans, 20 volunteers ages 18 to 30 were recruited from a fitness center. Volunteers were given CLA supplements and ordered not to change dietary or lifestyle habits. Without these changes, each participant taking CLA lost an average of six pounds of body fat more than the placebo group. They also regained less fat and retained more muscle mass after stopping treatment. Dr. Delbert Dorchshceld of the University of British Columbia believes that “as [CLA] reduces body fat, it increases lean tissue. The action indirectly boosts a human’s metabolism as lean tissue burns more calories than fatty tissue” 5. Another trial involving 80 clinically obese men and women showed parallel results6. Three grams of over-the-counter CLA were given three times of day to half the group (equal number of men and women) and the other group was given three grams of safflower oil capsules three times a day. Both groups received instruction on caloric restriction and exercise. After six months of treatment, all had lost weight at an average of five pounds each. An increase in muscle mass was also reported for one-third of the CLA group.

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CLA and type 2 diabetes (T2D)

Dr. Pariza and his colleagues believe that CLA must force adipose cells to remain small by inhibiting the enzymes that fill them6. These changes in body composition help to mediate improvements in health. Such improvements include increased insulin sensitivity, lowered blood pressure, and improved vasodilatation function, all of which are closely associated with diabetes and heart disease7. Although no human trials have been performed to test CLA effects in heart disease, several researchers have used humans to test CLA’s effects on type 2 diabetes (T2D). Dr. Martha Belury and her team gave capsules of three grams of CLA or three grams of safflower oil capsules to nine people diagnosed with T2D. The duration of the trial was eight weeks1. For the group receiving CLA, glucose concentration was moderately improved and only this group experienced a lowering of the leptin concentration. It is believed that an elevated level of leptin is associated with obesity. Belury suggests CLA may act via a mechanism to alter body-fat accumulation. If CLA could be proven to effectively treat diabetes, then it could replace many antidiabetic drugs, which can have toxic side effects.

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For these benefits to reach the masses, the products consumed must contain recommended levels of CLA. However, the processing of these ruminant products may greatly alter the content of CLA as well as alter the structures of the CLA isomers themselves8. Since the effects of this processing are also major concerns for consumers and professionals9, the effects of pasteurization on CLA should be explored.

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References

1. Raloff, J. (2001). The good trans-fat; Will one family of animal fats become a medicine? Science News.

2. Albers, R., van der Wielen, R.P.J., Brink, E.J., Hendriks, H.F.T., Dorovska-Taran, V.N., & Mohede, I.C.M. (2003). Effects of cis-9, trans-11, and trans-10, cis-12 conjugated linoleic acid isomers on immune function in healthy men. European Journal of Clinical Nutrition, 57, 595-603.

3. Bauman, D.E., Barband, D.M., Dwyer, D.A., & Grinan, J.M. (2000). Technical note: Production of butter with enhanced CLA for use in biomedical studies with annual models. Journal of Dairy Science, 83, 2422-2425.

4. Belury, M. A. (2002). Dietary conjugated linoleic acid in health: Physiological effects and mechanisms of action. Annual Review of Nutrition, 22, 505-531.

5. Journal of international medical research features study on body fat impact of conjugated linoleic acid (CLA): Fifth study in eighteen months confirms body fat activity of CLA in humans. (2002).

6. Raloff, J. (1999). Better butter? This one may fight cancer. Science News.

7. Stewart, K.J. (2002). Exercise training and the cardiovascular consequences of type 2 diabetes and hypertension. Journal of the American Medical Association, 288, 1622-1631.

8. Campbell, W., Drake, M.A., & Larick, D.K. (2003). The impact of fortification with conjugated linoleic acid (CLA) on the quality of fluid milk. Journal of Dairy Science, 86, 43-51.

9. Creamer, L. K, Pearce, L.E., Hill, J. P. & Boland, M.J. (2002). Milk and dairy in the 21st century. Journal of Agricultural and Food Chemistry, 50, 7187-7193.

© 2018 Sarah Hurst

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