CLA Supplements Remain Unproven

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In 2013, the FDA made a preliminary determination that partially hydrogenated vegetable oils (PHVO) containing trans fatty acids (TFA) will no longer going to be classified as “generally recognized as safe” (GRAS). It now appears likely that the FDA will ban the use of these TFA-containing oils, perhaps by 2016, largely because they adversely impact blood lipids in ways linked to promoting more coronary artery disease (CAD) (1). However, even if this ban goes into effect, it will not completely eliminate all TFA from the American diet. Why not? Simply because modest amounts of TFA occur naturally in meat and milk from ruminant animals. Of course, the fats in these animals are much higher in saturated fat and cholesterol that have long been shown to elevate the levels of atherogenic apoB-containing lipoproteins in the blood and CAD. TFAs are also present at very low levels in other edible oils, where they are produced during the high temperature manufacturing process. In addition, food supplement companies will likely be able to petition FDA for specific uses of certain PHVOs. What specific health benefits have TFA-containing fats been promoted to produce?

It turns out that a specific type of TFA-containing fat has long been promoted as a weight loss aid based largely on animal studies that have shown some presumably favorable effects of conjugated linoleic acid (CLA). Conjugated means that its two unsaturated double bonds are separated by a single saturated bond. CLA that occurs naturally in ruminant animal fat is largely the cis-9 and trans 11 isomer of linoleic acid. CLA is also produced by partial hydrogenation of safflower or sunflower oils, in which case it is a 50-50 mix of cis-9 and trans-11 linoleic acid and trans-10 and cis-12 linoleic acid. Food supplements used in studies in rodents and a few short-term clinical trials in humans have mostly used this artificially-produced CLA. The major TFA in PHVOs is elaidic acid although such foods contain many types other types TFAs including small amounts including some CLAs.

Both naturally-occurring and synthetic CLA differ from elaidic acid in that both CLAs contain both a trans and cis bond, whereas elaidic acid has only one trans bond. In addition to decreasing body fat and increasing lean body mass (LBM), some studies did show that CLA may enhance immune function, reduce insulin resistance, promote healthier blood lipid profiles, and even reduce the risk of cancer (2). The majority of the favorable research on the supposed benefits of CLA supplementation is preliminary and focused largely on rodent experiments. Human trials with CLA supplements to date have shown varying results (3).

The few human studies showing that CLA has presumably beneficial effects on body composition have serious limitations. For example, a 12 week supplementation trial in China with CLA did show reduced fat mass in overweight subjects. However, despite modest fat loss, this study reported a modest (but not quite significant) increases in serum triglycerides (+17%) and cholesterol (+3.7%) and a small reduction in HDL-C (-1.4%) (4). These are the same types of adverse changes seen with elaidic acid rich PHVOs. For the most part, review articles have shown no consistent data on the health risk and benefits of CLA supplements as weight loss aids in human subjects. Nor is there any good evidence that CLA supplements have any consistent beneficial effect on plasma lipids and in fact the blood lipid changes seen in response to CLA supplements appear to be detrimental and may very well promote CAD (5). Other studies exploring CLA supplement use in humans and body fat loss actually revealed detrimental effects on insulin sensitivity, blood lipid profile, and liver hypertophy (6). Other studies have shown no significant effect of CLA on immune function and/or biomarkers of inflammation in human subjects (7).

Can CLA Protect Against Breast Cancer?

Some human trials looking at breast cancer risk and tumor size have suggested a potential benefit with CLA supplementation (8). However, the suggested anti-carcinogenic effects seen in rodent studies and studies of cultured human breast cancer cells was not apparent in an epidemiological study done in the Netherlands (9).Clearly more research is needed before any conclusions can be drawn concerning the possible effects of CLA supplements on the risk of developing or treating breast cancer or any other type of cancer in people.
Inconsistent results of CLA supplement trials in human subjects may be due to the failure to control for confounding variables such as length of study, physical activity level, and the amount and type of CLA isomers in the supplements. For the most part, human studies have used supplements with high doses of CLA and/or foods supplemented with amounts of CLA far in excess of what occurs naturally -- even in grass-fed meat and dairy foods. Despite the claims of promoters of high-dose CLA supplementation, it is still far too early to conclude that such supplements can aid weight loss, increase lean body mass, reduce inflammation or insulin resistance or protect people from developing breast or other cancers.

Bottom Line:

The FDA appears poised to reverse its serious error of classifying TFA as GRAS, which should lead to the elimination of PHVOs in foods. Only if large placebo-controlled clinical trials can demonstrate the safety and efficacy of CLA supplements should the FDA consider exempting them from its looming ban of PHVOs. In the meantime, responsible health professionals should not be advocating CLA supplements. Nor should anyone suggest the modest CLA content of fatty dairy products or meat from ruminant animals is heart-healthy because their CLA content magically eliminates the well-established nonHDL-cholesterol raising effects ruminant animal fat, which is largely due to their relatively high saturated fat and cholesterol content.

By James J. Kenney, PhD, FACN


  2. Tricon S, Burdge GC, Kew S, et al. Opposing effects of cis-9, trans-11 and trans-10, cis-12 conjugated linoleic acid on blood lipids in healthy humans. Am J Clin Nutr 2004; 80:614-20.
  3. Rainer L, Heiss CJ. Conjugated linoleic acid: health implications and effects on body composition. J Am Diet Assoc 2004;104:963-8.
  4. Chen SC, Lin LH, Huang HP, et. al. Effect of conjugated linoleic acid supplementation on body fat composition in a Chinese population. Nutr. 2012;28:559-65.
  5. Mensink RP. Metabolic and health effects of isomeric fatty acids. Curr Opin Lipidol 2005;16:27-30.
  6. Terpstra AHM. Effect of conjugated linoleic acid on body composition and plasma lipids in humans: an overview of the literature. Am J Clin Nutr 2004;79:352-61.
  7. Tricon S, Burdge GC, Williams CM, et al. The effects of conjugated linoleic acid on human health-related outcomes. Proc Nutr Soc 2005;64:171-82.
  8. Diary Farmers of Canada Promotional Information. Proceedings of the Fifth Annual Health and Nutrition Symposium - Examining the Latest Scientific Evidence on Trans Fats; 2005 Nov 9; Toronto. Toronto: The Dairy Farmers of Canada, 2005.
  9. Voorips LE, Brants HAM, Kandinaal AFM, et al. Intake of conjugated linoleic acid, fat, and other fatty acids in relation to postmenopausal breast cancer: the Netherlands Cohort Study on Diet and Cancer. Am J Clin Nutr. 2002;76:873-882.
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