June 1, 2000
Conjugated Linoleic Acid -
By: Andrea Platzman, R.D.
Most fats take a lot of flack for the risks they can pose to health, especially when consumed in the large quantities typical of the American diet. Certain fatty acids, however, may have quite a different effect. Conjugated linoleic acid (CLA) appears to be one of these.
Linoleic acid is an 18-carbon polyunsaturated omega-6 fatty acid with double bonds at the cis-9 and cis-12 positions (-C=C-C-C=C-). The double bonds in conjugated linoleic acid, however, are separated by only one single bond (-C=C-C=C-). There are eight potential isomers, but the cis-9, trans-11 version is the one found most frequently in foods. Linoleic acid is found primarily in vegetable oils, while CLA is found primarily in dairy products and ruminant tissue.
In milkfat, CLA concentrations hover around 3 to 5 mg per gram, according to Larry Satter, director of the U.S. Dairy Forage Research Center in Madison, WI. Interestingly, in an experiment with a range of feeds, cows eating nothing but grass produced five times as much CLA in their milk as those deriving calories from the usual high-protein diet.
CLA and cancer
In animal studies, CLA has been shown to inhibit the proliferation of human malignant melanoma and colorectal, breast and lung cancer cells. Most of this research has been done with a mixture of CLA isomers. A recent rat study, however, used high-CLA butterfat with predominantly cis-9, trans-11 isomers to see if its biological activities would be similar to a mixture of isomers. Undertaken by Clement Ip et al and published in the December 1999 Journal of Nutrition, this study looked at a number of rat mammary-gland responses, including density of mammary carcinogenesis target cells, which are known as terminal end bud, or TEB, cells. While butterfat CLA and the isomer mixture appeared to have largely similar effects, rats consuming the CLA-enriched butterfat appeared to accumulate more CLA in the mammary gland and other tissues. The researchers hypothesize that vaccenic acid in butterfat may serve as a precursor for CLA synthesis.
Additionally, this fatty-acid version was shown to reduce the proliferation of TEB cells by 30%. "This research demonstrates for the first time that natural CLA in foods is biologically active, and that we can use a designer-foods concept to enhance the natural level of anticarcinogens in foods," says Dale Bauman, Ph.D., Liberty Hyde Bailey Professor of animal science at Cornell University, Ithaca, NY.
CLA has been found to lower cholesterol while also reducing the size of lesions found in arteries. It works in two ways to protect the heart and arteries: by lowering the amount of fats in the bloodstream, and by preventing fats from sticking to artery walls.
Additionally, since this fatty acid tends to be incorporated more abundantly into the cellular and mitochondrial membranes of the heart muscle, and the heart relies more on fatty acids than on glucose for energy, a greater abundance of CLA in this area may improve the efficiency of fat transport and fat metabolism in the cardiac mitochondria. The result is likely a strengthened heart.
Less fat, more lean mass
CLA supplementation appears to help improve the ratio of lean body mass to fat, decreasing fat, especially on the abdomen, as well as enhancing muscle growth. Michael Pariza, Ph.D., department chair, food microbiology and toxicology, University of Wisconsin-Madison, noted during a seminar entitled "Positive Health Benefits of Conjugated Linoleic Acid, a Functional Food Component of Meat and Dairy Programs," given during the 1999 Institute of Food Technologists annual meeting in Chicago last July, that the CLA isomer that appears to affect body fat is trans-10, cis-12.
Mice fed CLA-supplemented diets have exhibited 60% lower body fat and approximately 14% increased lean body mass. Although Pariza's recent human trial using CLA from safflower oil showed no overall effect on body weight or body composition, when weight was regained, the placebo group's gain consisted of 74% fat and 26% lean body mass, while the CLA group's gain consisted of 55% fat and 45% lean body mass.
Thus, CLA is probably not a weight-loss agent, but rather a weight-gain deterrent. One explanation for this effect may be CLA's enhancement of insulin sensitivity, thereby allowing fatty acids and glucose to pass more easily through muscle-cell membranes and away from fat tissue, resulting in an improved muscle-to-fat ratio.
"Since there are a number of different chemical forms of CLA that occur naturally in foods, taking a supplement may not have the same effects as eating foods that naturally contain CLA," states Mark Kantor, Ph.D., associate professor and extension specialist, University of Maryland, College Park. "If one extrapolates the information from the animal data, a person weighing 150 lbs. would need to consume about 3 grams per day of CLA to receive any potential benefits."
Although much promising research has been conducted on CLA, most of the studies thus far have involved animals. The next step will be further and more-detailed human clinical studies that explore additional CLA activities, such as an enhanced immune-system response. Meanwhile, the current research introduces a range of possibilities for food and beverage manufacturers in the ever-expanding functional-foods arena.
Andrea D. Platzman, a registered dietitian, writes regularly for nutrition publications. She earned a master's degree in nutrition from New York University, and has a culinary and business background.
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