Conjugated linoleic acid, or CLA, has been offered in supplement form for several years. It’s a combination of different isomers that are made from the essential fatty acid linoleic acid. Microbes living in the guts of ruminant animals, such as cows, convert linoleic acid into different isoforms of CLA through a process known as biohydrogenation. In cows linoleic acid, a polyunsaturated fatty acid, is converted into CLA, which is a trans fatty acid. While most artificially produced trans fatty acids have potent negative effects on health and are linked to cardiovascular disease and cancer, CLA is just the opposite in that it offers protection against those same diseases.
The type of CLA found in supplements is prepared by processing linoleic acid under alkaline conditions. That yields a mixture of two primary and several secondary CLA isomers. The two main isomers are referred to as 9,11, or rumenic acid, and 10,12, and both are considered biologically active forms of CLA. Rumenic acid is the dominant CLA isomer found in natural foods that contain CLA, such as milk and meat.
If you don’t take CLA supplements but eat meat and drink milk, your average CLA intake will be 152 to 212 milligrams daily. Precisely how much you’ll get from food depends on such factors as the animal’s breed, nutritional status and age. For example, cows that are kept in feedlots and given a primary grain diet produce meat and milk that is nearly devoid of CLA. Milk from cows that are allowed to graze in a pasture and eat mainly grass has about five times more of it. That could mean a big difference in how milk affects health.
The difference between milk from grass-fed and feedlot cows was illustrated in a newly published study involving 4,000 people living in Costa Rica, a country where milk and meat are produced from grass-fed cows.1 Scientists found that people with the highest blood content of CLA had a 49 percent lower risk of heart attack than those who had the lowest CLA content. The findings held true even when other cardiovascular-risk factors, such as high blood pressure and smoking, were filtered into the analysis. The researchers said that the CLA content of milk from grass-fed cows was more than enough to counter the possible negative cardiovascular effects of the saturated fat in whole milk.
CLA may protect against heart attacks through a variety of mechanisms. It boosts antioxidant activity in the body’s natural antioxidant enzymes, such as catalase and glutathione peroxidase. Animal studies show that CLA also offers protection against the other major killer, cancer, not only through antioxidant activity but also by boosting the activity of a tumor-suppressing gene. In addition, rumenic acid helps fight tissue inflammation, which is important because cancer is an inflammatory-based disease. When CLA was first isolated in 1987, its initial beneficial health property was that it provided potent cancer protection for animals.
The primary controversy concerning CLA has been over its role in body composition changes. Some studies suggest that it may encourage bodyfat loss and have anticatabolic effects in muscle. That’s the basis for its use as a bodybuilding supplement, and it’s included in many sports supplement products. The controversy concerns conflicting findings about the effects of CLA in animals and humans.
Many animal studies confirm CLA’s role in fat loss, but studies with humans have been paradoxical, with some showing definite fat-loss effects and others showing little or no effects. One reason is that the different isomers of CLA have opposing effects on bodyfat loss. The 10,12 isomer is thought to be responsible for fat loss and lean-mass gain, but it also causes insulin resistance and negative changes in blood fats, particularly in middle-aged men who have a lot of fat around their waists. By contrast, rumenic acid may help increase bodyfat, but it also reduces insulin resistance.
As to why some human studies link CLA supplements to fat loss and others don’t, you need to look at which isomer was used in the study. For fat loss, 10,12 is clearly superior to 9,11. Another factor that can affect study results is the dose of CLA and how long it was used by the subjects. Results can also be significantly affected by the sex, age, weight and even the metabolic status of subjects. Studies of animals that showed successful bodyfat loss used doses that averaged 20 times higher than those used in human studies.
How can CLA favorably affect bodyfat loss? Some rodent studies have revealed CLA’s role depressing appetite, but no study thus far has shown that effect in humans. In rodents CLA changes the ratio of various peptides that control appetite in the hypothalamus section of the brain; however, not all studies that found a fat-loss effect in animals also showed a drop in appetite, indicating that other mechanisms are involved in CLA-related fat loss. In animal studies the 10,12 isomer of CLA inhibited an enzyme involved in fat synthesis. CLA also helps activate PPAR receptors that affect bodyfat synthesis and cancer by controlling the activity of certain genes in response to fatty acid intake.
CLA may fuel energy expenditure. In animals it increases the activity of thermogenic proteins that convert calories into heat, which raises the metabolic rate. It also increases the activity of the enzyme that helps carnitine transport fat into the mitochondria of cells, where it is oxidized. Once again, human data in this area is contradictory, with some studies showing an increase in metabolism from CLA and others showing little or no effect. One human study that lasted 13 weeks found that while CLA boosted metabolism, the subjects still didn’t lose any bodyfat.
Other studies show that CLA supplementation increases lean body mass, which can include bone and muscle. That would mean a higher resting metabolic rate. In one study human sedentary subjects took five grams a day of a CLA supplement for seven weeks, while others took a placebo. Both groups trained with weights. Those in the CLA group showed a significant gain in lean mass compared to those in the placebo group, along with greater bodyfat loss. The men in the CLA group reported greater strength gains than those in the placebo group. The CLA group also had lesser amounts of a muscle-protein-breakdown product, pointing to an anticatabolic effect of CLA. In a later study experienced resistance-trained athletes took six grams a day of CLA for a month while continuing to train as usual. A few of them took a placebo. Researchers found no differences in performance, fat loss or markers of muscle breakdown, although some in the CLA group did report minor beneficial changes.
CLA shares a property with testosterone in that it prevents the maturation of fat cells. The 10,12 version of CLA can even reverse the process by increasing inflammatory effects in fat cells—more or less the way it causes insulin resistance. The isomer also blunts the activity of adiponectin, a beneficial fat-cell protein that, among other things, lowers insulin resistance and spurs fat burning. The isomer inhibits the activity of GLUT-4, the primary protein transporter of glucose into muscle. The insulin resistance that results from 10,12 CLA leads to an increase of free fatty acids in the blood. If those fatty acids are burned through exercise, all is well. If they aren’t, they can act as substrates and cause elevated blood glucose and excess fat in the blood, which is related to cardiovascular disease. The 10,12 form can also bring on intense oxidation events in the body and may interfere with the function of the omega-3 fat docosahexaenoic acid, a.k.a. DHA, in some tissues; it lowers the heart content of DHA 25 percent in mice.
Most the of the problems caused by 10,12 are balanced by the presence of the 9,11 isomer. That likely explains why commercial CLA supplements blend both isomers. The idea is to offer the significant health benefits attributed to CLA minus the potential problems associated with using either isomer alone. What remains to be determined now is the precise beneficial dose of CLA. In the meantime, the usual suggested daily dose for health purposes averages three grams. Alternatively, you can eat kangaroo meat, which contains the highest natural concentration of CLA in any food. That should keep you jumping.
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1 Smit, L.A., et al. (2010). Conjugated linoleic acid in adipose tissue and risk of myocardial infarction. Am J Clin Nutr. In press.
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