Conjugated Linoleic Acid (CLA): Efficacy and Safety
Direct Answer
CLA supplementation lacks convincing evidence for clinical efficacy in humans and should not be recommended for weight loss, body composition changes, or metabolic disease treatment based on current evidence.
Evidence Summary
Body Composition and Weight Loss
The most robust human data demonstrates CLA is ineffective for preventing weight regain or reducing body fat:
A 1-year randomized controlled trial (n=101) showed no benefit: CLA supplementation (3.4 g/day) for one year failed to prevent weight or fat mass regain compared to placebo in obese individuals who had previously lost >8% body weight. The CLA group regained 4.0 kg body weight and 2.1 kg fat mass versus 4.0 kg and 2.7 kg respectively in placebo group (not statistically significant) 1.
Animal studies do not translate to humans: While rodent studies show dramatic reductions in fat mass and increases in muscle mass with large CLA doses, 13 randomized placebo-controlled clinical trials in humans do not support these findings 2.
Human evidence remains ambiguous: Current evidence is equivocal regarding whether CLA supplementation has any significant effect on body composition in humans 3.
Metabolic and Lipid Effects
Potential metabolic harm outweighs uncertain benefits:
The 10-trans,12-cis CLA isomer may induce insulin resistance and hyperlipidemia in both animal and human studies 2.
While some animal models show favorable lipid changes, beneficial effects have not been established in humans 3.
Some human subjects experienced declines in HDL cholesterol and increases in lipoprotein(a), which are adverse changes 3.
Safety Concerns
Long-term safety data reveals concerning signals:
Increased leukocyte counts: The 1-year trial demonstrated a significant increase in white blood cell count with CLA supplementation, though no difference in reported adverse events or insulin resistance indexes was observed 1.
Effects on lactation and pregnancy: CLA supplementation in lactating women reduces milk fat content. Cordial blood CLA concentrations correlate negatively with both gestational length and birth weight 2.
One positive safety study exists: A 12-month trial in obese humans (6 g/day Clarinol) showed no adverse effects on liver function, glucose, insulin, lipids, or blood counts, with fewer adverse events than placebo 4. However, this conflicts with other safety signals.
Theoretical Mechanisms Without Clinical Validation
Animal and in vitro studies suggest CLA may affect:
- Eicosanoid and cytokine metabolism 5
- Gene transcription related to inflammation and carcinogenesis 5
- Cell-mediated immune response 5
However, these mechanisms have not translated to clinically meaningful benefits in humans 5, 3.
Clinical Recommendation
Do not recommend CLA supplementation for:
- Weight loss or body composition improvement 1
- Metabolic disease management 2
- Lipid profile optimization 3
- Any therapeutic indication in pregnant or lactating women 2
The evidence basis is weak at best for human applications, with concerning safety signals that outweigh any theoretical or animal-model benefits 2, 1.