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updated 7/30/2004 2:19:45 PM ET 2004-07-30T18:19:45

Does something that helps decrease body fat, increase lean muscle, and fight off cancer sound like a dream come true? It may not be. That something may be conjugated linoleic acid, known as CLA. This group of polyunsaturated fatty acids, which are building blocks of fat, are similar in structure to the essential fatty acid linoleic acid. Although animal studies show CLA improves the body’s physical makeup and helps prevent cancer, human studies are few and vary in results.

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Because cud-chewing animals like cows and sheep form CLA naturally, meat and milk products from beef and lamb are the major sources of CLA in our diets. Since CLA is part of the fat in these foods, reduced-fat dairy products have less health-damaging saturated fat, as well as less CLA. While CLA occurs in a variety of slightly different varying chemical forms, only some of them appear to be biologically active. Most animal experiments use a supplement containing a synthetic mixture of CLA variations.

Animal research positively demonstrates CLA’s ability to decrease body fat and increase muscle. This news has enticed both dieters and athletes to try CLA supplements. Athletes, however, seem less likely to benefit from CLA, because the effects seem to be greater in those who are overweight, or possibly still growing.

Inconsistent results
Human studies are evenly split between finding CLA helpful and showing no effect. A new study of 180 overweight adults gave some CLA supplements and others placebos. All of the participants were free to eat and exercise as they chose. After one year, people who had been taking CLA supplements lost an average of two to four pounds of body fat, while those receiving placebos lost nothing. The supplements even appeared to boost lean muscle mass among some of the adults. The exercise level between the two groups failed to explain the small changes in body composition.

CLA may interrupt several different stages of cancer development. Studies show that CLA can affect the metabolism of carcinogens, protect DNA, slow the growth of cancer cells, promote their destruction, and possibly block the spread of established tumors. In humans, some observational evidence links CLA with breast cancer protection. But controlled studies that could justify a stronger conclusion have only been done in animals. These studies have also produced inconsistent results.

More human studies needed
CLA could improve a person’s immunity. But there are two other areas of possible concern. First, although animal studies show that CLA helps insulin work more effectively, other studies suggest insulin resistance and blood sugar control may be worsened. Second, in some animal studies, CLA seems to decrease fat buildup in blood vessels, but other animal and human studies suggest cholesterol levels and other aspects of heart health become worse.

Most researchers say it is too soon to recommend CLA supplements. More human studies must verify the benefits, as well as the risks. The best chemical forms and dose amounts must also be determined. Our normal eating habits provide less than one gram of CLA per day, but studies use doses of three to four grams.

Some people have advocated eating higher-fat dairy products to get more CLA. However, CLA is less than one percent of the fat content of these foods. Saturated fat will add up faster than the CLA in these foods. The best plan may be to wait.

One recent review of CLA and its anti-cancer effects notes that CLA can accumulate in the body. Studies may now find out whether small amounts of CLA from limited servings of reduced-fat dairy products might add up over time to offer enough cancer and health protection.

Nutrition Notes is provided by the American Institute for Cancer Research in Washington, D.C.

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