CLA and the Liver and Kidneys: Is It Safe?
Conjugated linoleic acid, or CLA, has been sold for decades as a "natural fat burner." Its effect on body fat in humans turned out to be small, but another question has received less attention: what does long-term CLA use do to the liver, the kidneys, and metabolic health? Our editorial team reviewed animal data, human trials, and the regulatory picture.
What CLA is and why isomers matter
CLA is not a single molecule but a group of isomers of linoleic acid in which the double bonds are "conjugated," meaning they are separated by only one single bond. Small amounts occur naturally in beef, lamb, and dairy fat, where the dominant isomer is cis-9, trans-11, also called rumenic acid.
Commercial supplements are made by chemically processing safflower or sunflower oil. This produces a roughly 50:50 mixture of two isomers: cis-9, trans-11 and trans-10, cis-12. The second isomer is barely present in food but is the one responsible for most of the fat-reducing effects seen in animals.
Unfortunately, the same trans-10, cis-12 isomer also appears to drive most of the adverse metabolic effects. This is the central tension of CLA: the component that "works" in animal models is also the one that raises safety questions, so the effects and the risks are hard to separate.
In the United States, certain CLA preparations received "Generally Recognized as Safe" (GRAS) status for use in specific foods, with intake limits. That status reflects the doses studied, not unlimited consumption, and it does not cover every supplement on the market.
The liver: alarming animal data, calmer human data
The strongest warning signals come from mice. In several experiments, diets containing trans-10, cis-12 CLA caused dramatic loss of body fat but also severe fatty liver, enlarged liver size, and high insulin levels. Researchers have interpreted this as fat being redistributed from adipose tissue into the liver.
Mice, however, are unusually sensitive to CLA, and the doses used were high relative to body weight. In hamsters and rats the effects were weaker or inconsistent. Translating these findings directly to humans is not justified, but they explain why scientists continue to monitor liver outcomes in clinical trials.
Human studies lasting up to two years, such as the trial by Gaullier and colleagues, generally did not show clinically significant changes in liver enzymes in healthy overweight adults. Some trials did note small changes in markers of inflammation or liver function, but not liver damage in the usual clinical sense.
There are scattered case reports in the medical literature describing liver injury in people taking CLA-containing products. Such reports cannot prove causation, especially when multi-ingredient "fat burners" are involved, but they reinforce the principle that people with existing liver disease should be cautious.
Insulin resistance and lipids
A notable human finding concerns insulin sensitivity. In a study by Risérus and colleagues, obese men with metabolic syndrome who received purified trans-10, cis-12 CLA developed worsened insulin resistance and increased markers of oxidative stress. The mixed-isomer product had milder effects.
Insulin resistance matters for the liver because it is a major driver of non-alcoholic fatty liver disease. Even if CLA does not directly injure liver cells in humans, worsening glucose metabolism in susceptible people could theoretically push them toward fatty liver over time.
Effects on blood lipids are inconsistent. Some trials reported a slight drop in HDL cholesterol or a rise in lipoprotein(a), while others found no meaningful change. For someone with a family history of cardiovascular disease, periodic lipid testing during prolonged CLA use is a prudent step.
The kidneys: what is known
Compared with the liver, kidney safety has received much less study. Most human trials measured creatinine as part of routine safety labs and did not report clinically relevant changes in people with normal kidney function.
No consistent mechanism has been identified by which CLA would directly damage the kidneys. Indirect effects through insulin resistance, blood pressure, or inflammation are theoretically possible but have not been demonstrated in human outcome data.
People with chronic kidney disease were generally excluded from CLA trials. That means there is simply no evidence about safety in this population, which is not the same as evidence of safety. Anyone with impaired kidney function should consult a nephrologist before taking it.
| Organ / marker | Animal findings | Human findings |
|---|---|---|
| Liver fat | Marked increase in mice | No clear clinical increase in trials |
| Liver enzymes | Elevated in some models | Mostly stable; rare case reports |
| Insulin sensitivity | Reduced in mice | Worsened in some obese men |
| Kidney function | Limited data | No consistent signal in healthy adults |
Who should avoid CLA and how to use it sensibly
Given the modest fat-loss benefit, roughly a fraction of a kilogram over months in meta-analyses, the risk-to-benefit calculation matters. People in certain groups have little to gain and potentially more to lose.
- People with diagnosed fatty liver disease, hepatitis, or elevated liver enzymes.
- People with prediabetes, type 2 diabetes, or metabolic syndrome.
- People with chronic kidney disease.
- Pregnant or breastfeeding women, due to lack of data.
- Anyone taking blood thinners, as CLA may affect clotting in theory.
Healthy adults who decide to try CLA should stay within the 3–6 g per day range used in most human trials, choose third-party tested products, and avoid stacking it with stimulant-based fat burners. Liver enzymes, glucose, and lipids can be checked at baseline and after a few months.
It is also worth remembering that diet and training produce far larger changes in body composition than CLA ever has. Supplements belong at the margins of a program, not at its center.
Conclusions of the editorial team
CLA is not acutely toxic to the liver or kidneys in healthy humans at studied doses, but animal data and insulin-resistance findings justify caution.
The trans-10, cis-12 isomer is the main source of both the effect and the concern, which makes it difficult to obtain one without the other.
The modest fat-loss benefit makes CLA a poor choice for people with existing metabolic or liver problems.
Related reading we suggest: our guide to liver enzyme tests, an overview of evidence-based fat-loss supplements, and an article on omega-3 versus omega-6 fats in athletic diets.
References
- 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.
- 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.
- Gaullier JM, Halse J, Høye K, et al. Supplementation with conjugated linoleic acid for 24 months is well tolerated by and reduces body fat mass in healthy, overweight humans. J Nutr. 2005;135(4):778–784.
- Onakpoya IJ, Posadzki PP, Watson LK, et al. 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.
- Clément L, Poirier H, Niot I, et al. Dietary trans-10,cis-12 conjugated linoleic acid induces hyperinsulinemia and fatty liver in the mouse. J Lipid Res. 2002;43(9):1400–1409.
- LiverTox: Clinical and Research Information on Drug-Induced Liver Injury. Conjugated linoleic acid. Bethesda (MD): National Institute of Diabetes and Digestive and Kidney Diseases.
