Bottom Line: Conjugated Linoleic Acid (CLA) has shown some cancer fighting properties in animal studies, however studies in humans are lacking. CLA has also been shown to reduce both good and bad levels of cholesterol.
A group of substances called polyunsaturated fatty acids. Conjugated Linoleic Acid (CLA) is made by microbes that live within the gut of certain animals. It is therefore found throughout the body was well as in dairy products and beef. The walls of all cells within the body contain a variety of fatty acids and CLA is readily incorporated into those cell walls upon ingestion. Because CLA is an antioxidant, it may help protect the body from some cancer causing substances. Because CLA is incorporated into the walls of cells, it may modify the signaling between cells, although the affect of this feature in humans in unknown.
Two forms of CLA exist in nature. The form most often found in food, called cis-9, trans-11 is thought to help disrupt the replication of some cancer cells. The other form, which is called trans-10, cis-12, has been shown to reduce HDL ("good") cholesterol levels as well as affect insulin reduction of glucose in the blood.
To Prevent and Treat Cancer Animal studies have shown CLA to reduce cancer proliferation and have general antioxidant activities, however no studies in humans corroborate that evidence.
To Treat High Cholesterol Studies show that while CLA reduces total cholesterol levels, it reduces HDL, or good cholesterol levels as well.
To Promote Healthy Weight CLA appears to improve body fat mass in very obese people, however results are mixed for non-obese people.
Weight Maintenance: A few studies have looked at CLA and its use in reducing weight in obese people. In one study, sixty volunteers with body mass index values (weight divided by height) between 25 and 35 kg/m2 were randomly assigned one of four dosages of CLA or placebo per day for 12 weeks. Volunteers taking the CLA were found to have a higher reduction in body fat mass than those taking placebo. Other measures of weight, such as lean body mass were unaffected.
Another study of fifty-three healthy men and women randomly assigned participants into two groups. One group received CLA for 12 weeks and the other received placebo for 12 weeks. Similar to the study above, subjects in the CLA group observed a significant reduction in proportion of body fat than those taking placebo. Body weight and body mass index were unaffected.
Cholesterol: Sixty obese volunteers with health characteristics that put them in a high risk for heart disease (high blood pressure, insulin resistance, high cholesterol and obesity) participated in a study of CLA and cholesterol. Volunteers were randomly assigned to receive CLA or placebo for 12 weeks. Volunteers who took CLA observed a reduction in high density lipoprotein ("good") cholesterol. Volunteers who took the trans-10, cis-12 form of CLA also observed an unexpected observation of a diminished ability of insulin to reduce glucose levels in their blood.
A group of polyunsaturated fatty acids (PUFA). Conjugated Linoleic Acid (CLA) is commonly found in diary products and beef. This substance has purported benefits for cancer prevention, weight control, and high cholesterol. Animal studies have suggested that CLA may play a role in reducing tumor proliferation in certain cancer cell lines (2)(3)(4). CLA was shown to cause apoptosis in white adipose tissue in mice (5). Human studies of CLA and weight reduction are mixed. Studies of CLA in obese men showed a reduction in body fat mass (6) and body mass index (1). CLA showed little effect on body composition in non-obese women (7) and healthy adults (8). In addition, possible insulin resistance (9), reduction in glycemia and plasma leptin (1) were also observed after supplementation. CLA may affect total cholesterol levels (6)(9)(10). CLA does not significantly reduce proteolysis in muscles (10). Reported adverse events include minor gastrointestinal symptoms (9) and one case of severe fatigue (6).
CLA is a strong antioxidant and is readily incorporated into cell membrane phospholipids. It is thought that replacing other PUFAs with CLA may reduce oxidative stress and modulate intracellular signaling (11). These effects may inhibit carcinogenesis. CLA has been shown to reduce the synthesis of prostaglandins, especially PGE2(12)(13) and decrease stearoyl-CoA desaturase activity (14). It may affect cellular response to tumor necrosis factor-alpha (TNF-alpha) (15). In vitro studies have shown that it inhibits the proliferation of MCF-7 breast cancer cells (3). CLA supplementation increases liver stores of vitamin A in mice. The role of this effect on carcinogenesis remains unclear (16). CLA was also shown to cause apoptosis in white adipose tissue in mice (5). The two major isomers of CLA, trans-10, cis-12 (t10c12) and cis-9, trans-11 (c9t11), have different physiological properties. The cis-9, trans-11 isomer, the principal dietary form, was shown to induce tissue inhibitor of metalloproteinase mRNA in SGC-7901 human gastric carcinoma cells. This may play a role in inhibiting the tumor metastasis cascade (2). The inhibition was thought to come from blocking of the cell cycle with reduced expressions of cyclin A, B1 and D1 and enhanced expressions of CDKI (17). However, the trans-10, cis-12 isomer has been shown to decrease serum HDL cholesterol levels (9), inhibit the activity of stearoyl-CoA desaturase (12), inhibit insulin sensitive in new adipose cells (1) and decrease Insulin-Like Growth Factor-II secretion at both transcriptional and post-transcriptional levels (18).
Absorption Cooking or heating seems to increase rather than decrease CLA content in foods (1)(11). The trans-10, cis-12 isomer is metabolized more rapidly than the cis-9, trans-11 isomer (12). Although CLA may be generated from linoleic acid (LA) by intestinal microorganisms, consumption of excess LA does not appear to increase CLA concentrations in humans (19). Distribution In rats fed with CLA-containing diets, CLA accumulates in the mammary fat pad greater than in the liver or plasma (4).
CLA may increase insulin resistance causing increase in blood glucose level. CLA may affect cholesterol levels (9) as well as decrease creatinine, bilirubin, creatine-phosphokinase and platelets. CLA may increase potassium levels(6).
Blankson H, Stakkestad JA, Fagertun H, Thom E, Wadstein J, Gudmundsen O. Conjugated linoleic acid reduces body fat mass in overweight and obese humans. J Nutr 2000;130:2943-8. Sixty volunteers with body mass index values between 25 and 35 kg/m2 were randomized into five groups to receive either 1.7, 3.4, 5.1 or 6.8 grams of CLA or 9 g placebo per day for 12 weeks. A significantly higher reduction in body fat mass (BFM) was found in the CLA groups when compared with the placebo group, however no additional effects on BFM were observed with doses higher than 3.4 grams per day. No significant differences were observed in lean body mass, blood safety variables or blood lipids. Non clinically significant changes in blood lipids, potassium, creatinine, platelets and bilirubin were observed as well. One severe case of fatigue and some mild-to-moderate gastrointestinal symptoms was reported. as well as some mild-to-moderate gastrointestinal symptoms.
Smedman A,.Vessby B. Conjugated linoleic acid supplementation in humans--metabolic effects. Lipids 2001;36:773-81. Fifty-three healthy men and women participated in a trial of CLA and reduction in body fat. Subjects were randomly assigned to supplementation with 4.2 grams per day of CLA or placebo for 12 weeks. Subjects in the treatment arm showed a significantly decreased proportion of body fat when compared with the control group. No changes were seen on body weight, body mass index, abdominal diameter, serum lipids or glucose metabolism in the treatment group as compared to the placebo group.
