Conjugated Linoleic Acid and Body Fat Reduction
By Dónal P. O'Mathúna, PhD. Dr. O'Mathuna is Senior Lecturer in Ethics, Decision-Making & Evidence, School of Nursing, Dublin City University, Ireland; he reports no financial relationships relevant to this field of study.
Much has been written about obesity and the range of health problems associated with it. Along with body weight itself, the proportion of fat in people's bodies is important. Methods of reducing body fat, and thereby increasing fat-free mass (FFM), are generating much interest. Physical exercise is the standard, and can use fat stored in the body and increase muscle mass and FFM, but other considerations include the use of specific supplements. Conjugated linoleic acid (CLA) has received much professional and popular interest as a dietary approach to reducing the accumulation of body fat and improving the body's overall composition.1
Interest in CLA developed after it was noted that animals fed high-fat diets had very different body compositions depending on whether the diet was supplemented with CLA. Mice fed diets containing about 1% CLA gained much less body fat, although their body weight was about the same as that of mice not given CLA.2 However, the results varied depending on the species of animal, with results in humans being much more modest.3 Nonetheless, great interest has developed in the potential role of CLA in reducing the accumulation of body fat.
Pharmacology
CLA is not one compound, but a group of closely related omega-6 fatty acids. Linoleic acid is an omega-6 fatty acid found in some vegetable and fish oils. It contains two double bonds that are 'unconjugated,' meaning they are separated by two or more single bonds. Conjugated double bonds are separated by only one single bond. Linoleic acid is converted into conjugated linoleic acid by bacteria in cows' guts as they ruminate on vegetable oils.2 Humans primarily obtain CLA from dairy products and beef fat, although CLA can also be manufactured via a simple chemical reaction within the body. The amount of CLA in cow's milk varies enormously depending on the cow's diet (grazing or feed) and the time of the year (highest when cows graze on rapidly growing grass).2
When linoleic acid is converted into CLA, the two double bonds can end up in a number of different locations along the molecule. Each of these compounds, called isomers of CLA, has its own biological activity. Dietary CLA is thus a complex mixture of isomers, though two forms tend to predominate in dairy foods and supplements.4 One called t10,c12-CLA is primarily responsible for the effect on body fat composition, whereas c9,t11-CLA has different biological effects.1 Foods and supplements vary in their ratios of various isomers, which complicates attempts to study the effects of CLA on humans.
Mechanism of Action
CLA appears to have little or no effect on fat already accumulated in the body.1 The t10,c12-CLA isomer inhibits an enzyme involved in moving lipids into fat cells. This isomer appears to have a number of other effects on lipid cells as well, both encouraging their destruction and slowing their maturation, though much remains to be understood about these effects. Since physical activity can also reduce body fat and increase FFM, CLA supplementation has been recommended as an adjunct to exercise.4
Clinical Studies
Use of CLA has been promoted primarily on the basis of animal studies showing that CLA supplementation led to reduced weight gain. However, other animal studies have shown no effects from CLA supplements.5 Where effects did occur, they were dependent on the relative amounts of isomers present in the CLA mixture. In contrast, relatively few trial of CLA have been carried out on humans.
In one trial, 60 overweight or obese men were randomly divided into five groups to receive either placebo or CLA (1.7, 3.4, 5.1, or 6.8 g/day).6 After 12 weeks, fat mass was significantly reduced in the 3.4 and 6.8 g CLA/day, but not in the other groups. No significant differences were found for body mass, body mass index (BMI), or serum lipids.
Another study involved 20 healthy adults of normal weight who exercised in the gym for 90 minutes, three times a week.7 They were randomly assigned to take CLA capsules (0.6 g tid) for 12 weeks. Body fat, but not body mass, was significantly reduced in the CLA group. Another study randomly assigned 17 healthy women of normal weight to receive either CLA or placebo.8 They lived in a controlled metabolic unit for 94 days. No significant differences were found between the groups for changes in body weight, fat mass, FFM, or percentage body fat. Similar non-significant results were found for CLA supplementation in experienced resistance-training men (mean age 23 years).9
Three randomized controlled trials of CLA were conducted by a Swedish research team.10 In the first, 53 healthy adults were randomly assigned to take CLA (4.2 g/day) or the same amount of olive oil for 12 weeks. The proportion of body fat in the CLA group decreased by 3.8 percent, significantly more than in the control group (P = 0.05). No significant differences were found for body weight, BMI, or serum lipids. In the second study, 25 obese men were randomly assigned to the same dose of CLA or placebo for 4 weeks. Sagittal abdominal diameter (SAD), a measure of visceral fat, was reduced with CLA compared to placebo (P = 0.04), with no differences in body mass or BMI. In the third study, 60 obese men were randomized to receive either CLA mixture (3.2 g/day), purified t10,c12-CLA, or placebo for 12 weeks. No significant differences in body composition or mass were found, but both forms of CLA significantly reduced HDL-cholesterol levels (P < 0.05) and the t10,c12-CLA also increased insulin resistance (P < 0.01) and glycemia (P < 0.001).
A larger and longer study randomly assigned 180 overweight adults to receive either CLA as fatty acids (3.6 g CLA/day), CLA as triglycerides (3.4 g CLA/day), or placebo for 1 year.11 Those receiving either form of CLA had significantly reduced body fat mass compared to placebo (P = 0.002). Body mass and BMI were significantly reduced in the triglyceride CLA group, but not the fatty acid CLA group. The fatty acid form led to significantly elevated LDL levels while the triglyceride CLA had significantly reduced HDL levels.
A randomized, cross-over study involved 17 overweight or obese men.12 They ate controlled diets with either CLA-supplemented butter (2.59 g CLA/day) or low-CLA butter (0.24 g CLA/day). Each diet was maintained for 4 weeks, with an 8-week washout period in between. No differences were found between the two diets for body fat composition or cardiovascular disease risk factors.
Another study examined the combined effect of CLA and creatine supplementation along with resistance training. Thirty-nine community-dwelling, healthy adults (aged 65-85 years) were randomized to receive either creatine (5 g/day) with CLA (6 g/day) or placebo for 6 months.4 No participants had engaged in an exercise program during the previous 2 years, and all participated in supervised resistance training twice a week for the duration of the study. All measures of strength increased for both groups. Isokinetic (resistance at a constant speed) strength increased significantly more in the supplement group (P < 0.05), but isometric (performed at static position) strength gains did not differ between the groups. The supplement group had significantly better increases in FFM and decreases in total fat mass (P < 0.05).
Adverse Effects
The small number of studies addressing this issue have found no adverse effects when animals have been fed up to 5% CLA in the total diet.1 However, some commentators have claimed that adverse effects of CLA on animals have been largely ignored.5 Some animal studies have found that CLA induces insulin resistance, which was also noted in one human study.10 A number of human trials have found adverse trends in serum cholesterol levels. Studies in mice have been reported where fatty liver deposits developed in association with CLA administration, but this may be a unique effect in mice because they metabolize CLA differently.1 Overall, little is known about the potential risks to humans from CLA supplementation. In human trials, the most common adverse effects are gastrointestinal in nature.
Formulation
A range of doses from 2-7 g/day have been used in trials. The optimal dose is not well established, but 3 g daily is commonly recommended.2 Many different formulations are available, with the most common one containing 45% t10,c12-CLA, 45% c9,t11-CLA, and 10% other isomers.
Conclusion
The results of trials of CLA in humans have been somewhat inconsistent, with surprisingly smaller effect sizes on body fat than seen with animals. Like with animals, CLA does not impact total body mass or BMI. However, a modest reduction in body fat has been found in several studies. A recent meta-analysis found 18 randomized, double-blind studies of the impact of CLA on body composition.13 A median dose of 3.2 g CLA/day produced a reduction in body fat of 0.05 kg/week, with the effect leveling off after about 6 months.
Recommendation
The evidence base to support CLA supplementation is relatively small, but growing. CLA does not help with weight loss, but may play a role in reducing body fat. These beneficial effects may be limited to about 6 months. Much further research is needed to determine whether the effects of CLA supplementation are enhanced by exercise. However, studies involving adults of normal weight have found fewer benefits. Whether different effects result from consuming CLA as a dietary supplement compared to a component of dairy and beef products has yet to be examined. The results of some studies raise concerns about people consuming CLA supplements, especially those at risk for diabetes or cardiovascular disease. Further research into CLA use by humans is needed before routine use for weight management can be recommended.
References
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2. Kelly GS. Conjugated linoleic acid: A review. Altern Med Rev 2001;6:367-382.
3. Navarro V, et al. The body fat-lowering effect of conjugated linoleic acid: A comparison between animal and human studies. J Physiol Biochem 2006;62:137-147.
4. Tarnopolsky M, et al. Creatine monohydrate and conjugated linoleic acid improve strength and body composition following resistance exercise in older adults. PLoS ONE. 2007;2:e991.
5. Wang Y, Jones PJ. Dietary conjugated linoleic acid and body composition. Am J Clin Nutr 2004;79: 1153S-1158S.
6. Blankson H, et al. Conjugated linoleic acid reduces body fat mass in overweight and obese humans. J Nutr 2000;130:2943-2948.
7. Thom E, et al. Conjugated linoleic acid reduces body fat in healthy exercising humans. J Int Med Res 2001;29:392-396.
8. Zambell KL, et al. Conjugated linoleic acid supplementation in humans: Effects on body composition and energy expenditure. Lipids 2000;35:777-782.
9. Kreider RB, et al. Effects of conjugated linoleic acid supplementation during resistance training on body composition, bone density, strength, and selected hematological markers. J Strength Cond Res 2002; 16:325-334.
10. Risérus U, et al. Metabolic effects of conjugated linoleic acid in humans: The Swedish experience. Am J Clin Nutr 2004;79:1146S-1148S.
11. Gaullier JM, et al. Conjugated linoleic acid supplementation for 1 y reduces body fat mass in healthy overweight humans. Am J Clin Nutr 2004;79:1118-1125.
12. Desroches S, et al. Lack of effect of dietary conjugated linoleic acids naturally incorporated into butter on the lipid profile and body composition of overweight and obese men. Am J Clin Nutr 2005;82:309-319.
13. Whigham LD, et al. Efficacy of conjugated linoleic acid for reducing fat mass: A meta-analysis in humans. Am J Clin Nutr 2007;85:1203-1211.
O’Mathuna DP. Conjugated Linoleic Acid and Body Fat Reduction. 2008;11:28-31.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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