Over the last several years, there has been a dramatic reduction in the consumption of trans fatty acids (TFAs) as the food industry has increasingly phased out the use of partially hydrogenated vegetable oils. This phase-out followed a requirement to put the TFA content on labels, plus compelling scientific data demonstrating these TFAs raise LDL cholesterol (LDL-C) and lower HDL cholesterol (HDL-C) levels and promote coronary artery disease (CAD). TFAs are also associated with insulin resistance and the development of type 2 diabetes mellitus (DM). There is now a scientific consensus that dietary TFA intake should be minimized as they almost certainly promote atherosclerosis and increased the risk of CAD and likely promote diabetes and perhaps other serious ills. Starting in 2006, the US Food and Drug Administration started to require food labels to list trans fat content and a growing number of consumers started avoiding foods with trans fats on the label. However, the FDA allowed one type of TFA-rich product to avoid being labeled as trans fat content. This product is promoted as a food supplement, and the special treatment offered to conjugated linoleic acid (CLA) supplements allows them to be promoted for weight loss and yet escape the growing stigma associated with TFAs. These industrially-produced CLA supplements are made by the partial hydrogenation of safflower or sunflower oil and typically contain several grams of trans fat, about half of which is a CLA isomer (trans10, cis12-CLA) that is almost absent from the naturally-occurring CLA in ruminant animals.
Today these synthetic CLA supplements are promoted primarily as a weight loss aid, although there are also claims that CLAs reduce CAD, cancer, diabetes, and other ills. Certainly animal studies have shown that CLA supplementation can have an impact on the body composition (it decreas-
ed body fat & increased lean body mass) of various rodents. And some studies suggest that they favorably impact immunity, lipid profile, insulin sensitivity, and cancer development in the short-term (1). However, most of the research suggesting benefits of dietary CLA is preliminary at best. Human trials of CLA supplements to date have shown inconsistent results on body weight and composition (2). Other reviews also show no consistent evidence that CLA supplements are safe and effective weight loss aids in humans (3,4). Still other studies exploring CLA supplement use in humans and body fat loss revealed detrimental effects on insulin sensitivity, blood lipid profile, and liver hypertrophy (5,6).
Indeed, there is growing concern that the mechanism by which CLA supplements may reduce body fat stores is via damage to adipocytes (lipodystrophy), which makes these fat storage cells less capable of holding onto fat. Unfortunately, we know when people develop lipodystrophy (an autoimmune disease that results in the loss of adipocytes), they lose their ability to store fat and soon develop severe insulin resistance and type 2 DM. The results of a recent review suggests that the adverse effects of increasing TFA intake on heart health seen in observational studies may be even greater than predicted, based on their adverse impact on blood lipid levels alone. In this review, CLA supplements produced industrially from linoleic acid-rich oils were also examined. CLA -- either naturally occurring in ruminant animal fat or produced commercially for CLA supplements -- raised LDL-C and total cholesterol to HDL-C ratio. It now appears all TFAs (including CLA) adversely impact blood lipids and likely promote CAD. While additional research is still needed to better understand the effects of naturally-occurring ruminant TFA and particularly CLA on risk factors for CAD, the current data suggests that whatever favorable short term effects CLA has on some CAD risk factors is likely outweighed by its longer term adverse metabolic effects (7).
Most human studies have used large-dose CLA supplements or fortified butter with 5-10 times the amount of CLA naturally found in meat and butterfat. This large amount of CLA would be impossible for people to consume from unfortified dairy products and meats and so may produce metabolic challenges for the human body, which it is ill-prepared to handle biochemically. Most researchers now believe CLA supplements should not be advocated and certainly not prescribed by health professionals until large, randomized, controlled trials are conducted to establish the efficacy of CLA and especially also the safety of taking synthetic CLA supplements in high doses for many years (8).
The same research team that made the headlines with the L-carnitine/red meat causing atherosclerosis showed that the mechanism was via the production of trimethylamine-N-oxide (TMAO). And they published an earlier study showing that choline/betaine/lecithin also are converted to TMAO by bacteria in our intestines and so they all likely promote atherosclerosis too. Indeed, egg yolks have little L-carnitine but are loaded with lecithin, which may explain in part why some studies link egg intake more closely with atherosclerotic disease than one might predict based on their cholesterol and saturated fat content alone. Of course, this data just provides more evidence to question the wisdom of eating eggs and red meats -- not that we really needed more info on their negative health effects.
By James J. Kenney, PhD, FACN
1) 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.
2) Rainer L, Heiss CJ. Conjugated linoleic acid: health implications and effects on body composition. J Am Diet Assoc 2004;104:963-8.
3) Mensink RP. Metabolic and health effects of isomeric fatty acids. Curr Opin Lipidol 2005;16:27-30
4)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.
5) Riserus U, Smedman A, Basu S, et al. Metabolic effects of conjugated linoleic acid in humans: the Swedish experience. Am J Clin Nutr 2004;79 (Suppl):S1146-S8
6) Larsen TM, Toubro S, Astrup A. Efficacy and safety of dietary supplements containing CLA for the treatment of obesity: evidence from animal and human studies. J Lipid Res 2003;44:2234-41.
7) European Journal of Clinical Nutrition advance online publication, 27 March 2013; doi:10.1038/ejcn.2013.43.
8) 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.
Stephanie Ronco has been editing for Food and Health Communications since 2011. She graduated from Colorado College magna cum laude with distinction in Comparative Literature. She was elected a member of Phi Beta Kappa in 2008.