What Does CLA Do? Examining the Science Behind this Popular Supplement

October 2, 2025 •

4 min read

In animals like mice, Conjugated Linoleic Acid (CLA) supplementation has been shown to reduce total body fat by up to 60% [3, 13]. Many individuals ask: what does CLA do for human health, and can it replicate these dramatic results? The answer lies in understanding its complex effects on metabolism and body composition [12].

Quick Summary

Conjugated Linoleic Acid (CLA) influences fat metabolism and body composition, potentially aiding in modest fat loss and muscle mass preservation. Its effects, however, are isomer-specific and less pronounced in humans than in animal models. Some concerns regarding metabolic health exist with synthetic forms.

Key Points

CLA is an isomer of linoleic acid found in meat and dairy [2]. Most supplements contain a different isomer mix derived from plant oils [7].
CLA's anti-fat effects are linked to the t10,c12 isomer [3]. This isomer can reduce fat storage by modulating enzymes and promoting fat cell death [14].
Human studies show only modest fat loss [12]. The dramatic effects seen in animal research are not typically replicated in humans [3, 4].
Supplemental CLA may have negative side effects [8]. These include stomach upset and, in some cases, concerns over insulin resistance and liver fat accumulation [8, 9].
Natural food sources are generally safe and provide beneficial isomers [8, 7]. Higher intake from food is associated with better health markers [7].
CLA is not a regulated medication [9]. The quality and isomer ratios in supplements can vary, so caution is advised [9].

Understanding Conjugated Linoleic Acid

Conjugated Linoleic Acid, or CLA, is a group of isomers of linoleic acid, a polyunsaturated omega-6 fatty acid [7]. Unlike harmful industrial trans fats, CLA occurs naturally in meat and dairy products from ruminant animals like cows and sheep [2, 9]. The most common natural form is the cis-9, trans-11 (c9,t11) isomer, which is created during the digestive process in these animals [9].

However, the CLA found in dietary supplements is typically produced by chemically altering linoleic acid from plant-based sources, such as safflower oil [7]. This process creates a different ratio of isomers, most notably containing a higher proportion of the trans-10, cis-12 (t10,c12) isomer, which is linked to some of CLA's more significant—and sometimes adverse—effects [4].

How Does CLA Influence the Body?

CLA's purported effects on body composition and health are not tied to a single mechanism but involve a combination of biological actions. The specific effects often depend on which isomer is dominant, with the t10,c12 isomer playing a key role in fat metabolism changes observed in both animal and some human studies [3, 14].

Modulates Fat Metabolism: The t10,c12 isomer can inhibit lipoprotein lipase, an enzyme crucial for breaking down fat in the blood so it can be absorbed by fat cells. By reducing this activity, less fat is stored in adipocytes [14].
Increases Fat Burning: Some research suggests CLA increases energy expenditure and boosts the body's fat-burning capabilities, even during sleep. This process, known as thermogenesis, helps use fat stores more efficiently for energy [4].
Inhibits Adipogenesis: CLA has been shown to prevent pre-adipocytes (immature fat cells) from differentiating into mature, fat-storing adipocytes. This means the body creates fewer new fat cells [14].
Promotes Apoptosis: In some studies, particularly in animal models, CLA has been linked to increased apoptosis, or programmed cell death, in existing fat cells, leading to a reduction in overall fat mass [14].
Enhances Fatty Acid Oxidation: CLA may increase the activity of CPT-1, an enzyme that transports fatty acids into mitochondria, where they are oxidized for energy [4].

Potential Benefits of CLA

Body Composition

The most studied effect of CLA is its impact on body composition, though results vary significantly between animal and human trials. While animal studies show dramatic reductions in body fat, human studies are less conclusive, often showing only a modest, if any, effect [3, 4]. A meta-analysis published in 2007 found that CLA supplementation produced a modest but significant reduction in body fat, particularly over the first six months [12].

Preserving Muscle Mass

Several studies have shown that CLA supplementation can help preserve or increase lean muscle mass, especially when combined with exercise [5, 6]. This is a potential benefit for those aiming for weight loss, as it helps ensure that weight lost is fat rather than muscle. This effect may be more pronounced in men and those with higher physical activity levels [5].

Immune Function

CLA has demonstrated immunomodulatory and anti-inflammatory properties in various models. Some research indicates that the c9,t11 isomer has anti-inflammatory effects by altering cytokine levels, though these results are often not consistently reproduced in human clinical trials [11].

Risks and Considerations with Supplementation

Side Effects

While CLA is generally well-tolerated in amounts found in food, high supplemental doses can lead to side effects. Common issues include stomach upset, diarrhea, nausea, and fatigue [8]. More serious side effects have been reported, particularly linked to the synthetic t10,c12 isomer found in supplements [4].

Insulin Resistance: High doses of the t10,c12 isomer have been shown to potentially worsen insulin sensitivity and increase fasting blood glucose levels in abdominally obese men and individuals with metabolic syndrome [4, 8].
Fatty Liver: Animal studies have linked high doses of supplemental CLA to increased fat accumulation in the liver, a risk factor for metabolic syndrome [9].
Inflammation and Oxidative Stress: Concerns exist that CLA supplementation can increase markers of inflammation and oxidative stress, contradicting some of its purported benefits [4, 8].

FDA Regulation and Safety

Since 2008, the FDA has given mixed CLA a "Generally Recognized as Safe" (GRAS) status for use in certain foods, but this does not regulate the supplements widely sold [9]. This means that quality and isomer ratios can vary between products. Doses up to 6 grams per day for up to a year have been studied and generally considered safe, but the risk of side effects increases with higher doses [7]. Long-term safety, especially with high doses and synthetic isomers, is not well-established [9].

Natural vs. Supplemental CLA


Feature	Natural Food Sources (e.g., grass-fed beef, dairy)	Supplemental Sources (e.g., safflower oil-derived)
Isomer Composition	Primarily the c9,t11 isomer [9]	A roughly 50:50 mix of c9,t11 and t10,c12 isomers [9]
Intake Level	Relatively low (approx. 150-200 mg/day for Americans) [7]	High (typically 3-6 grams/day) [10]
Associated Health Effects	Linked to improved metabolic health and lower disease risk in observational studies [7]	Mixed results; associated with modest fat loss but also potential negative effects on glucose and lipid metabolism [4, 8]
Potential Risks	Generally considered safe at food levels [8]	Side effects more common due to higher doses and isomer balance [8]

Conclusion

While animal studies paint a compelling picture of CLA's potential to dramatically reduce body fat and improve body composition, human research has been far less consistent. For many, the weight loss effects are modest and may not be worth the potential side effects associated with high supplemental doses of synthetic isomers. The different isomers of CLA found in food versus supplements appear to have distinct biological activities, with the t10,c12 isomer linked to both anti-fat and potentially adverse metabolic effects. For those considering a CLA supplement, it is crucial to manage expectations, be aware of the possible side effects, and understand that it is not a magic solution for weight loss. The most impactful way to manage body composition remains a balanced diet and consistent exercise routine. For more information on dietary supplements, the NIH Office of Dietary Supplements provides a comprehensive overview: Dietary supplements for weight loss.

Frequently Asked Questions

CLA is a type of omega-6 polyunsaturated fatty acid found naturally in the meat and dairy products of ruminant animals. It is also sold as a dietary supplement, though the isomer composition often differs significantly from its natural form [2, 7].

Scientific evidence suggests that CLA may produce a modest reduction in body fat mass in humans, particularly over the first six months of supplementation [12]. The effects are generally small and not as dramatic as those observed in many animal studies [3, 4].

Yes, CLA is a mixture of different isomers. The most common natural isomer is c9,t11, while supplements often contain a roughly 50:50 mix of the c9,t11 and t10,c12 isomers [9]. The t10,c12 isomer is most associated with fat loss effects [3].

Common side effects associated with CLA supplements include gastrointestinal issues like stomach upset, diarrhea, nausea, and fatigue. Some studies, particularly with the t10,c12 isomer, have raised concerns about potential insulin resistance and liver fat [8].

In research studies, doses of CLA supplements have typically ranged from 3 to 6 grams per day [10]. It is recommended to take CLA with meals for better absorption [8]. It is best to consult a healthcare provider for personalized dosage recommendations.

Yes, CLA is naturally present in foods like grass-fed beef and dairy products, including milk, cheese, and yogurt [2, 7]. However, the amount from diet is typically much lower than the dosages used in supplement studies [7].

The long-term safety of CLA supplementation, especially with the higher doses and different isomer mixes, is not well-established [9]. Some studies have raised concerns about adverse metabolic effects over time, such as insulin resistance and altered lipid profiles [4].