CLA, or conjugated linoleic acid, is a group of fatty acids naturally found in the fat of ruminant animals like cows, goats, and sheep. It is also produced synthetically for dietary supplements, often sourced from vegetable oils like safflower oil. While natural food sources contain primarily the cis-9, trans-11 (c9, t11) isomer, most commercial supplements are an equal 50:50 mixture of c9, t11 and the trans-10, cis-12 (t10, c12) isomers, or contain more of the t10, c12 form. This isomeric difference is critical, as research suggests they have distinct, and sometimes opposing, biological effects in the body.
The Promising, But Conflicting, Animal Research
In animal models, particularly mice, CLA has shown dramatic effects on body composition and metabolic health, suggesting its potential as a powerful supplement for humans. Studies have indicated that CLA supplementation can: reduce body fat by up to 60-70%, inhibit fat cell growth, increase fat burning, and stimulate fat breakdown. These significant findings in animals fueled initial excitement and widespread interest in CLA as a potent weight loss agent. Beyond weight management, preclinical research has also explored CLA’s potential anti-inflammatory and anti-cancer properties, with some studies suggesting beneficial effects on immune function and tumor inhibition in rodents.
The Modest Effects and Significant Concerns in Humans
Despite the promising animal data, human studies present a much less clear and, at times, concerning picture. The observed effects are modest and often inconsistent, and potential health risks associated with supplemental forms are a major consideration.
Modest Fat Loss and Body Composition Changes
Numerous human trials have investigated CLA’s effects on body fat reduction, yet the results are far less impressive than in animals. A 2007 meta-analysis found that CLA produced a very modest fat loss of about 0.09 kg per week compared to placebo, with the effect plateauing after six months. Other reviews have also concluded that while some fat mass reduction may occur, the overall clinical significance is minimal, with little to no effect on total body weight. The modest fat loss, if any, is often not worth the potential side effects.
Metabolic and Inflammatory Risks
A significant concern with CLA supplementation is its potential to negatively impact metabolic health and increase inflammation, largely driven by the t10, c12 isomer found in most supplements.
- Increased Insulin Resistance: Multiple studies have shown that supplemental CLA can worsen insulin sensitivity in individuals with metabolic syndrome or type 2 diabetes, and even in healthy individuals with the t10, c12 isomer. This reduces the body’s ability to absorb sugar effectively and could potentially increase the risk of developing diabetes.
- Fatty Liver: High doses of supplemental CLA have been shown to increase the accumulation of fat in the liver in both animal and human studies, a key step toward metabolic syndrome and other health issues.
- Altered Cholesterol and Inflammation: Some research indicates that supplemental CLA can lower levels of “good” HDL cholesterol and increase inflammatory markers in the blood, such as C-reactive protein (CRP). While some inflammatory response is natural, chronic inflammation is linked to numerous diseases, including obesity, cancer, and heart disease.
Other Reported Side Effects
Beyond metabolic issues, individuals taking CLA supplements have reported various mild-to-moderate side effects, including nausea, diarrhea, upset stomach, and fatigue. For individuals with bleeding disorders, CLA supplements may also slow blood clotting.
CLA from Food vs. Supplements: A Critical Comparison
Understanding the source of CLA—whether from whole foods or supplements—is crucial, as their effects differ significantly due to variations in isomeric composition and concentration.
| Feature | CLA from Food (Ruminant-Derived) | CLA from Supplements (Synthetic) |
|---|---|---|
| Primary Isomer | Mostly cis-9, trans-11 (c9, t11) | Primarily an equal 50:50 mix of c9, t11 and trans-10, cis-12 (t10, c12) |
| Source | Ruminant meat (beef, lamb) and dairy (milk, cheese, butter), especially from grass-fed animals | Safflower or sunflower oil chemically altered to produce CLA isomers |
| Dosage | Low, in natural amounts as part of a balanced diet (e.g., 150–212 mg/day in Americans) | High, often 3.2 to 6.4 grams per day to achieve a dose linked to potential (but modest) effects |
| Health Implications | Not associated with negative health effects like inflammation or insulin resistance. May be part of a healthy diet. | Associated with potential metabolic risks, increased inflammation, and other side effects due to high dose and specific isomer profile. |
Who Should Exercise Caution?
While CLA in food is generally considered safe, the potential risks of high-dose supplementation mean certain individuals should be particularly cautious or avoid CLA supplements entirely:
- Individuals with diabetes or metabolic syndrome: CLA has been shown to worsen insulin resistance and blood sugar control.
- Those with bleeding disorders or on blood-thinning medication: CLA may slow blood clotting.
- People with liver conditions: High doses may increase fat accumulation in the liver.
- Pregnant or breastfeeding women: There is insufficient evidence to confirm safety for this demographic.
- Athletes or individuals seeking performance enhancement: Evidence for significant improvements in body composition or performance is mixed and unreliable in humans, with better alternatives available. For a detailed review, see Examine.com's summary on CLA.
Conclusion: Navigating the Complex Evidence
Ultimately, whether CLA is good or bad depends heavily on its source and context. Getting conjugated linoleic acid naturally from food sources like grass-fed beef and dairy products as part of a balanced diet appears to be safe and may offer minor health benefits. The risks associated with high-dose, isolated CLA supplements, however, are a major concern. The significant difference between promising animal results and modest, often contradictory, human outcomes suggests that supplemental CLA may not be the effective and safe health booster it is marketed to be. For most people, focusing on a nutrient-rich diet with natural CLA sources, rather than relying on high-dose supplements, is the safer and more evidence-backed approach to health and body composition goals.
