Who Should Not Take CLA: Conjugated Linoleic Acid Contraindications
Conjugated linoleic acid, better known as CLA, has been sold as a "natural" fat-loss aid for more than two decades. It sounds harmless, as it is found in beef and dairy, but supplemental CLA is a different product with its own risks. Our editorial team reviewed which people should avoid CLA and why.
Food CLA versus supplement CLA
CLA is a group of isomers of linoleic acid, an omega-6 fatty acid, in which the double bonds are arranged in a "conjugated" pattern. In ruminant animals, bacteria in the stomach produce CLA, so it naturally appears in beef, lamb, milk and cheese. The dominant isomer in food is cis-9, trans-11, sometimes called rumenic acid.
Commercial supplements are made by chemically processing safflower or sunflower oil. They usually contain roughly equal amounts of two isomers: cis-9, trans-11 and trans-10, cis-12. This matters because the trans-10, cis-12 isomer is the one most associated with both fat-loss effects in animals and metabolic side effects in humans.
Typical food intake provides well under a gram of CLA per day, while supplements used in trials provide around 3–6 g per day. So taking CLA capsules is not equivalent to eating more dairy; it delivers a much higher dose of an isomer mix that is uncommon in the diet.
In the US, CLA products are sold as dietary supplements, and certain CLA preparations have also received "generally recognized as safe" (GRAS) status for use in some foods following notification to the FDA. GRAS status refers to defined uses and amounts, not to unlimited self-dosing.
How much fat does CLA actually burn?
CLA became popular after striking results in mice, which lost substantial fat and sometimes developed fatty liver and insulin resistance at the same time. Human results have been much more modest.
A meta-analysis by Whigham, Watras and Schoeller (2007) estimated an average fat loss of about 0.09 kg per week with CLA at roughly 3.2 g per day. A later meta-analysis by Onakpoya and colleagues (2012) found a small reduction in fat mass compared with placebo but described the clinical relevance as questionable.
These effects are small compared with the variation produced by changes in diet, physical activity and sleep. Even in a 24-month trial by Gaullier and colleagues (2005), which reported reduced body fat and good tolerability, the absolute changes were modest.
The modest benefit is an important context for the risks below. When a supplement offers little, even a moderate risk can tip the balance against using it for certain people.
Insulin resistance, diabetes and fatty liver
The main reason for caution is the effect of the trans-10, cis-12 isomer on glucose metabolism. A study by Risérus and colleagues (2002) in obese men with metabolic syndrome found that this purified isomer increased insulin resistance, raised blood glucose and increased markers of oxidative stress and inflammation.
Mixed-isomer supplements have produced less consistent effects, and many studies in healthy people found no clinically significant change. However, given the findings in metabolically vulnerable groups, people with type 2 diabetes, prediabetes or metabolic syndrome are usually advised to avoid CLA supplements.
Animal studies repeatedly show that the trans-10, cis-12 isomer can promote fat accumulation in the liver. Human data are limited, but people with non-alcoholic fatty liver disease, now often called metabolic dysfunction-associated steatotic liver disease, have little to gain and a plausible risk to lose.
| Group | Reason for concern | Advice |
|---|---|---|
| Type 2 diabetes, prediabetes | Possible worsening of insulin sensitivity | Avoid |
| Metabolic syndrome | Insulin resistance and inflammation in trials | Avoid |
| Fatty liver disease | Liver fat increase in animal studies | Avoid |
| Pregnancy and breastfeeding | Reduced milk fat; no safety data in pregnancy | Avoid |
| Anticoagulant or antiplatelet therapy | Possible additive effect on bleeding | Ask doctor |
| Heart disease, dyslipidemia | Mixed effects on HDL and inflammatory markers | Ask doctor |
Pregnancy, breastfeeding and bleeding risk
There are no adequate safety studies of CLA supplements in pregnancy, and the developing fetus depends on a carefully balanced supply of fatty acids. Avoidance is the reasonable default.
During breastfeeding, the concern is more concrete. A study by Masters and colleagues (2002) found that maternal supplementation with CLA reduced milk fat content. Because milk fat is a major energy source for infants, nursing mothers should not use CLA supplements.
CLA may have mild antiplatelet effects, based on laboratory studies. The clinical importance is uncertain, but people taking warfarin, direct oral anticoagulants, clopidogrel or high-dose aspirin, and those with bleeding disorders, should consult a doctor before using it. Stopping CLA before surgery is also sensible.
Research on blood lipids has produced mixed results. Tricon and colleagues (2004) found that the two main isomers had opposing effects on the ratio of LDL to HDL cholesterol, with the trans-10, cis-12 form being less favorable. People with existing cardiovascular disease should therefore discuss CLA with their physician.
Side effects and how to use CLA more safely
The most common side effects in trials are gastrointestinal: stomach upset, nausea, loose stools and bloating. Taking capsules with meals usually helps. Some people also report fatigue.
For healthy adults without the conditions listed above who still want to try CLA, consider these points:
- Studies typically used about 3–6 g per day of mixed isomers; higher amounts add risk without proven extra benefit.
- Combine it with a structured diet and training program, since CLA alone has little impact.
- Consider checking fasting glucose and lipids before and after a few months of use.
- Stop if you notice digestive problems or unexpected changes in lab results.
- Choose products tested by an independent laboratory for purity and oxidation.
Products labeled as "Tonalin CLA" or similar are mixed-isomer preparations. Very few consumer products specify the ratio of isomers, which makes comparisons between brands difficult.
Conclusions of the editorial team
CLA supplements deliver a small fat-loss effect at best. Their trans-10, cis-12 isomer can worsen insulin sensitivity in metabolically vulnerable people.
People with diabetes, prediabetes, metabolic syndrome or fatty liver, as well as pregnant and breastfeeding women, should avoid CLA. Those on blood thinners or with heart disease should consult a doctor first.
For related reading, see our articles on green tea extract and liver safety, on evidence-based fat-loss strategies, and on omega-3 versus omega-6 balance.
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.
- 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.
- 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.
- Masters N, McGuire MA, Beerman KA, Dasgupta N, McGuire MK. Maternal supplementation with CLA decreases milk fat in humans. Lipids. 2002;37(2):133–138.
- 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(3):614–620.
