Conjugated linoleic acids (CLAs), a group of fatty acids that have been reported in animal studies to increase bone density, promote fat and weight loss and serve as antiobesity, antiatherogenic, and antidiabetic agent. Therefore, CLA has gained reputation among bodybuilders as a weight loss and general health supplement. Supplement companies also heavily market CLA as supplement that reduces catabolism, burns fat, increases strength and muscle mass. The effectiveness of most supplements on body composition is small with CLA being one of them as when looking at available literature its effects are highly unreliable.
Effects of Conjugated Linoleic Acid on Lean Mass
Cornish and coworkers  hypothesized that the combination of conjugated linoleic acid, creatine, and protein would be superior to creatine and protein combined or protein by itself. Researchers tested this hypothesis in novice lifters who have been performing resistance training for at least 12 months. Subjects were randomly assigned to three groups: CCP (6 g/day CLA, 9 g/day creatine, 36 g/day whey protein), CP (9 g/day creatine, 36 g/day whey protein, 6 g/day placebo), or P (45 g/day whey protein, 6 g/day placebo) during 5 weeks of strength training.
Analyses showed bench-press and leg-press strength increased more in the CCP group than in the CP + P groups combined (p < .05). Study also reported that group ingesting conjugated linoleic acid, creatine and protein increased lean tissue mass more than the other two groups combined. Creatine and protein group increased lean body mass significantly (p < 0.05) more than protein only group. Study therefore concluded that CLA supplementation during strength training might have more of an effect than creatine and protein on measures of strength . The study was partially sponsored by a company that produces CLA.
A study by Mark Tarnopolsky and colleagues  also noted beneficial effects of conjugated linoleic acid and creatine combination on resistance-training. However, there was no individual supplement groups for comparison and we all know that creatine alone causes significant gains compared to placebo. A well-designed study  in 23 experienced bodybuilders (in average 5 years experience) reported that 6 g/day of CLA for 28 days provides no ergogenic value to experienced lifters. There were no changes in total body mass, fat-free mass, fat mass, percent body fat, bone mass, strength, serum substrates, or general markers of catabolism and immunity during training.
Based on these three studies in athletic population it is obvious that CLA is highly unreliable supplement and no firm conclusions can be drawn.
CLA tested in vitro (Leydig cells) increased the synthesis of testosterone via unknown mechanism . However, when same researchers tested CLA in human subjects (6g of CLA daily for 3 weeks) testosterone remained unchanged compared to placebo. Furthermore, CLA also didn’t affect estradiol, cortisol, and SHBG blood levels either before or after a resistance exercise bout. CLA provided no anabolic relevance.
A popular belief among bodybuilders is also that CLA keeps cortisol at bay, however there is no scientific study to back this up.
Effects of CLA on Body Fat Mass
Studies conducted on various animal reported reduced fat mass from 27 to 60% [5,6]. As usual results obtained from animal research don’t always translate well to human subjects. There are many studies that evaluate CLA as a fat loss supplement, however majority of studies show no statistically significant effects of CLA on fat loss. Even in studies where its effects are statistically significant CLA exhibits poor reliability and potency .
Conjugated Linoleic Acid Side Effects
There were no indications of altered oxidative stress or altered kidney function with CLA and/or creatine supplementation . Grethe Berven and associates  investigate the safety of conjugated linoleic acid (CLA) in healthy volunteers. Subjects were receiving 3.4 g CLA for 12 weeks and results showed that CLA is a safe substance in healthy populations. One year of supplementation even with a high dose (7.5g CLA) was not associated with any clinically relevant toxicology signs. However, reduction in HDL cholesterol was noted and increase in triglycerides were seen as statistically significant and an increase in white blood cell .
There is also a concern that conjugated linoleic acid ingestion by extremely overweight people might cause aggravate insulin resistance, which may increase their risk of developing diabetes . In contrast to in vitro studies which observed anti-oxidative effects of CLA , studies in humans noted that especially trans-10 cis-12 (t10c12) CLA, is pro-oxidative .
(Other common names: ALC, Cis-9,trans-11 Conjugated Linoleic Acid, Cis-Linoleic Acid, CLA, CLA-Free Fatty Acid, CLA-Triacylglycerol, LA, Linoleic Acid, Trans-10,cis-12 Conjugated Linoleic Acid)
- Cornish, Stephen M., et al. “Conjugated linoleic acid combined with creatine monohydrate and whey protein supplementation during strength training.” International journal of sport nutrition 19.1 (2009): 79.
- Tarnopolsky, Mark, et al. “Creatine monohydrate and conjugated linoleic acid improve strength and body composition following resistance exercise in older adults.” PLoS One 2.10 (2007): e991.
- Kreider, Richard B., et al. “Effects of conjugated linoleic acid supplementation during resistance training on body composition, bone density, strength, and selected hematological markers.” The Journal of Strength & Conditioning Research 16.3 (2002): 325-334.
- Macaluso, Filippo, et al. “Effect of conjugated linoleic acid on testosterone levels in vitro and in vivo after an acute bout of resistance exercise.” The Journal of Strength & Conditioning Research 26.6 (2012): 1667-1674.
- DeLany, James P., et al. “Conjugated linoleic acid rapidly reduces body fat content in mice without affecting energy intake.” American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 276.4 (1999): R1172-R1179.
- Park, Yeonhwa, et al. “Changes in body composition in mice during feeding and withdrawal of conjugated linoleic acid.” Lipids 34.3 (1999): 243-248.
- Conjugated Linoleic Acid – Examine.com
- Berven, Grethe, et al. “Safety of conjugated linoleic acid (CLA) in overweight or obese human volunteers.” European Journal of Lipid Science and Technology 102.7 (2000): 455-462.
- Whigham LD, et al. Safety profile of conjugated linoleic acid in a 12-month trial in obese humans. Food Chem Toxicol. (2004)
- Risérus, Ulf, et al. “Supplementation With Conjugated Linoleic Acid Causes Isomer-Dependent Oxidative Stress and Elevated C-Reactive Protein A Potential Link to Fatty Acid-Induced Insulin Resistance.” Circulation 106.15 (2002): 1925-1929.
- Leung, Yan Ho, and Rui Hai Liu. “Trans-10, cis-12-conjugated linoleic acid isomer exhibits stronger oxyradical scavenging capacity than cis-9, trans-11-conjugated linoleic acid isomer.” Journal of agricultural and food chemistry 48.11 (2000): 5469-5475.
- Basu, S., et al. “Conjugated linoleic acid induces lipid peroxidation in men with abdominal obesity.” Clinical Science 99.6 (2000): 511-516.