Conjugated linoleic acid (CLA) is a unique and somewhat controversial nutrient. Linoleic acid is an essential polyunsaturated omega-6 fatty acid. It necessary for the proper functioning of the human body but cannot be produced internally. It must be obtained from the diet.
The word conjugated refers to the unique structure of this particular form of linoleic acid. A conjugated fatty acid has at least one pair of double bonds separated by a single bond. While this fact is true of all CLA, it must be noted that the nutrient comes in at least 28 different forms (isomers) because the double bonds can be found in different locations on the fatty acid chain.
The molecules around a double bond can be arranged in one of two ways:
- As cis bonds, which have a bent configuration.
- As trans bonds, which have a straighter configuration.
All CLA isomers have both a cis bond and a trans bond. Because in this case the trans bond is part of a larger conjugated system, CLA is not listed on nutrition facts labels as a trans-fatty acid.
The two most significant isomers of CLA are called “c9, t11 CLA” (also called rumenic acid) and “t10, c12 CLA,” the numbers referring to the positions of the double bonds in the chain. The latter is rarely found in large quantities in nature.
Sources of CLA
CLA occurs naturally, but it can be produced artificially, as well.
Microorganisms in the rumens of ruminants produce CLA naturally. The CLA then becomes part of the animal’s fat and can be consumed by humans in the form of meat, although full-fat dairy products are particularly potent sources of CLA. The c9, t11 isomer is the predominant form in these foods, accounting for about 80% to 90% of the CLA in dairy products. The milk of grassfed ruminants contain 500% more CLA than that of grain-fed animals, while a grassfed hamburger contains 250% more CLA than is found in a feedlot-produced hamburger of equal leanness. However, CLA is typically removed altogether during processing in low-fat dairy products. CLA levels also vary with species, sheep milk containing the most CLA, followed by cow milk, and then goat milk.
Wild game often contains high levels of CLA, too. Meat from deer, elk, and antelope (all ruminants) is comparable to grassfed beef in CLA amounts.
CLA can be found in some vegetable oils and the meat of non-ruminant animals, such as swine, chicken, and fish. However, it is present in almost negligible quantities. Turkey is the exception, being surprisingly high in CLA.
The CLA supplements popular with dieters and bodybuilders often contain the artificial form of the nutrient, created by chemically altering the linoleic acid found in sunflower and safflower oils. Usually, both c9, t11 and t10, c12 are present in equal amounts.
The Good and Bad of CLA
Now for the controversial part—the many health claims surrounding CLA.
Advocates of CLA consumption and supplementation make many claims for their favorite nutrient. Depending on the information source, we may read that CLA:
- Enhances immune function.
- Reduces food allergies.
- Inhibits some types cancer, particularly breast cancer.
- Strengthens bones.
- Fights asthma.
- Reverses arteriosclerosis.
- Increases muscle mass.
- Assists in weight loss.
- Reduces total and LDL cholesterol.
- Improves insulin response.
Warnings about the risks of CLA are equally diverse and powerful:
- Sparks inflammation.
- Leads to heart disease.
- Creates gastrointestinal discomfort.
- Lowers HDL cholesterol.
- Causes or worsens diabetes and metabolic syndrome.
Perusal of the many scientific studies available on CLA suggests that part of the difficulty may lie in the source of the nutrient.
One study compared the effects of the c9, t11 isomer with those of the t10, c12 isomer in mice. While, t10, c12 proved to be more effective at reducing body fat than its natural counterpart, it actually increased the fat buildup in the liver, a condition linked with metabolic syndrome in humans. Also, t10, c12 (unlike c9, t11) dramatically reduced the levels of omega-3 fatty acids in the heart. Omega-3 fatty acids are linked with heart health.
A study on healthy humans failed to reveal any connection between either type of CLA and body composition or insulin resistance, but it did demonstrate that t10, c12 increases while c9, t11 decreases the ratios of LDL to HDL and of total to HDL cholesterol in the blood. Another study conducted on obese men did suggest a connection between t10, c12 and insulin resistance.
On the other hand, scientists have so far failed to confirm any link between c9, t11 and heart problems, provided that the CLA is consumed in normal quantities:
Some studies suggest that at lower doses, rTFA [trans-fatty acids, such as CLA, produced by ruminants] do not affect lipids and lipoproteins, but at higher doses, which are not attainable by diet, rTFA may have similar effects as iTFA [trans-fatty acids produced through industrial processes such as partial hydrogenation of vegetable oils].
Gebauer, Sarah K., et. al. Effects of Ruminant trans Fatty Acids on Cardiovascular Disease and Cancer: A Comprehensive Review of Epidemiological, Clinical, and Mechanistic Studies. Adv Nutr vol. 2: 332-354, 2011.
The same scientific review, while noting a lack of clinical studies in humans, also confirmed the breast-cancer-fighting benefits of c9, t11 in animals:
Animal studies generally show a benefit of c9,t11-CLA in mammary cancer and suggest that the anticarcinogenic effects of c9,t11-CLA may vary across species, with rats being more responsive than mice.
At present, many of the other claims made for naturally occurring c9, t11 CLA remain to be conclusively evaluated.
Conjugated Linoleic and Linolenic Acid Production by Bacteria: Development of Functional Foods
More about CLA and its sources.
Conjugated linoleic acid content of milk from cows fed different diets
Source of information on effect of grass-based diet on CLA in milk. Abstract only.
A Literature Review of the Value-Added Nutrients found in Grass-fed Beef Products
A look at the benefits of grassfed beef, including enhanced CLA levels. Includes diagrams of CLA structure.
Fatty acid analysis of wild ruminant tissues: Evolutionary implications for reducing diet-related chronic disease
Includes facts about CLA levels found in various parts of elk, antelope, and mule deer.
Fatty acid composition of liver, adipose tissue, spleen, and heart of mice fed diets containing t10, c12-, and c9, t11-conjugated linoleic acid
Study comparing the effect of the two isomers on mice as mentioned above.
Opposing effects of cis-9,trans-11 and trans-10,cis-12 conjugated linoleic acid on blood lipids in healthy humans
Illustrates the difference in the two isomers.
Supplementation With Conjugated Linoleic Acid Causes Isomer-Dependent Oxidative Stress and Elevated C-Reactive Protein: A Potential Link to Fatty Acid–Induced Insulin Resistance
This study suggests that t10, c12 may produce insulin resistance in humans that are already obese.
Effects of Ruminant trans Fatty Acids on Cardiovascular Disease and Cancer: A Comprehensive Review of Epidemiological, Clinical, and Mechanistic Studies
The review quoted above that suggests that c9, t11 is safe and possibly beneficial when eaten in normal quantities.