The History of CLA: How Conjugated Linoleic Acid Became a Popular Supplement
Conjugated linoleic acid, better known as CLA, was one of the defining weight-loss supplements of the late 1990s and 2000s. Its rise began not in a gym but in a cancer research lab studying hamburgers. Our editors follow CLA from that unexpected starting point through the mouse studies that excited marketers to the human trials that cooled expectations.
An accidental discovery in grilled beef
In the late 1970s, food scientist Michael Pariza at the University of Wisconsin was investigating whether cooking meat created mutagens, substances that can damage DNA. While analyzing extracts from grilled ground beef, his team found something unexpected: a fraction that seemed to inhibit mutagenesis rather than promote it.
It took several years to identify the compound. In 1987, Pariza and colleagues published their findings showing that the protective factor consisted of conjugated dienoic derivatives of linoleic acid, a family they named conjugated linoleic acid.
Chemically, CLA is a group of isomers of linoleic acid, an omega-6 fatty acid. The word "conjugated" refers to the arrangement of two double bonds separated by a single bond. The two most studied isomers are cis-9, trans-11 and trans-10, cis-12.
CLA occurs naturally in meat and dairy products from ruminant animals such as cows, sheep and goats, where bacteria in the rumen produce it. Grass-fed animals tend to have higher levels than grain-fed ones.
From cancer research to body composition
Through the late 1980s and early 1990s, CLA was studied mainly for potential anticancer effects in animal models. Results in rodents were intriguing, and the compound attracted attention as a possible protective nutrient in the diet.
A turning point came in 1997, when Park, Pariza and colleagues reported that mice fed CLA had markedly less body fat and relatively more lean mass than control animals. The fat reduction in mice was dramatic, and the media and supplement industry took notice.
Mechanistic studies suggested CLA might reduce fat storage by affecting enzymes involved in lipid uptake into fat cells, increasing fat oxidation and promoting the death of fat cells in rodents. Researchers later found that the trans-10, cis-12 isomer was mostly responsible for these effects.
The leap from mice to humans was the critical assumption. Mice have a very different metabolism and much higher relative doses were used, but marketing rarely mentioned these caveats.
The supplement boom
By the late 1990s, companies were producing CLA from safflower and sunflower oil through chemical processing that yields a roughly equal mix of the two main isomers. Branded ingredients became widely available, and CLA softgels appeared in fitness magazines, diet programs and pharmacies.
The marketing story was compelling: a natural fatty acid from food that helped burn fat and preserve muscle without stimulants. That fit perfectly with growing consumer interest in "natural" weight management.
In the United States, certain CLA ingredients later received GRAS (generally recognized as safe) status for use in specific foods and beverages, which enabled CLA-fortified milk, juices and snack products. Being safe for use in food is not the same as being proven effective for weight loss, though advertising often blurred that line.
CLA was frequently combined with other ingredients in fat-burner formulas, which made it harder for consumers to know what produced any effect they noticed.
What human trials revealed
Dozens of human trials followed. A 2007 meta-analysis by Whigham and colleagues concluded that CLA at around 3.2 g per day produced a modest fat loss, averaging well under a kilogram over several weeks compared with placebo.
A 2012 meta-analysis of longer trials by Onakpoya and colleagues reached a similar conclusion: a small, statistically significant reduction in fat mass whose clinical relevance was questionable.
Safety signals also emerged. A study by Risérus and colleagues found that the trans-10, cis-12 isomer worsened insulin sensitivity in obese men with metabolic syndrome. Other studies reported changes in blood lipids and markers of inflammation, as well as fatty liver in rodents.
| Period | Main claim | What evidence showed |
|---|---|---|
| 1980s | Anticancer factor in beef | Promising animal data, no human proof |
| Late 1990s | Dramatic fat loss | Large effects in mice |
| 2000s | Fat loss in people | Small average effects in meta-analyses |
| 2010s | Safety questions | Isomer-specific metabolic concerns |
In Europe, the EFSA evaluated several health claims for CLA and did not find sufficient evidence to approve them, which further dampened its reputation.
CLA's place today
CLA has not disappeared, but it no longer dominates the fat-loss market. It now tends to appear as a supporting ingredient rather than a headline product.
- Its effect on fat mass in humans is small and inconsistent.
- Most benefits seen in animals have not replicated in people.
- The synthetic isomer mix differs from natural CLA in food.
- Some individuals may experience digestive upset or unfavorable metabolic changes.
People with diabetes, insulin resistance or fatty liver disease should be especially cautious and speak with a doctor before taking CLA. It is not prohibited in sport, though contamination of multi-ingredient fat burners remains a risk.
The story of CLA is a classic example of how early laboratory findings can be amplified into consumer expectations long before human research catches up.
Editorial Team Conclusions
CLA began as a curiosity in grilled beef, studied for possible protective effects against cancer.
Striking results in mice transformed it into a best-selling fat-loss supplement in the late 1990s, but human trials showed only small average effects.
Isomer-specific safety concerns, particularly regarding insulin sensitivity, mean CLA deserves a cautious, evidence-based look rather than enthusiasm.
To continue, consider our articles on whether CLA makes sense during a bulking phase, how to evaluate fat-burner claims, and the role of omega-3 fats in body composition.
References
- Ha YL, Grimm NK, Pariza MW. Anticarcinogens from fried ground beef: heat-altered derivatives of linoleic acid. Carcinogenesis. 1987;8(12):1881–1887.
- Park Y, Albright KJ, Liu W, Storkson JM, Cook ME, Pariza MW. Effect of conjugated linoleic acid on body composition in mice. Lipids. 1997;32(8):853–858.
- Whigham LD, Watras AC, Schoeller DA. Efficacy of conjugated linoleic acid for reducing fat mass: a meta-analysis in humans. Am J Clin Nutr. 2007;85(5):1203–1211.
- Onakpoya IJ, Posadzki PP, Watson LK, Davies LA, Ernst E. The efficacy of long-term conjugated linoleic acid (CLA) supplementation on body composition in overweight and obese individuals: a systematic review and meta-analysis of randomized clinical trials. Eur J Nutr. 2012;51(2):127–134.
- Risérus U, Arner P, Brismar K, Vessby B. Treatment with dietary trans10cis12 conjugated linoleic acid causes isomer-specific insulin resistance in obese men with the metabolic syndrome. Diabetes Care. 2002;25(9):1516–1521.
- Kennedy A, Martinez K, Schmidt S, et al. Antiobesity mechanisms of action of conjugated linoleic acid. J Nutr Biochem. 2010;21(3):171–179.
