What is CLA?
Conjugated linoleic acid, or CLA, is a collective term for a group of isomers of the omega-6 fatty acid, linoleic acid. Isomers are compounds with the same chemical formula but different structural arrangements, and these differences can alter their biological effects. The two most widely studied CLA isomers are cis-9, trans-11 (c9,t11), also known as rumenic acid, and trans-10, cis-12 (t10,c12).
Naturally, CLA is produced by microbes in the digestive systems of ruminant animals like cows, goats, and sheep. It is therefore naturally present in their meat and dairy products, particularly from grass-fed animals, which have higher concentrations. The c9,t11 isomer is the predominant form found in these natural sources.
In contrast, the CLA used in most dietary supplements is manufactured through a chemical process that alters linoleic acid, typically sourced from safflower or sunflower oil. This process yields a roughly 50:50 mixture of the c9,t11 and t10,c12 isomers, a balance that differs significantly from the CLA found in natural foods.
How CLA works to influence body composition
Research into how CLA works has revealed several proposed mechanisms, though findings often differ between animal studies and human trials. Much of the anti-obesity effect is attributed to the t10,c12 isomer, while the c9,t11 isomer has shown stronger anti-cancer and anti-inflammatory properties in some models. The complex interactions of CLA within the body are believed to affect fat and energy metabolism in multiple ways.
Affecting fat cell development and breakdown
One of the most discussed mechanisms involves CLA's effects on adipocytes, or fat cells. The t10,c12 isomer is thought to inhibit the maturation of pre-adipocytes into mature adipocytes, effectively reducing the body's capacity to store new fat. It can also increase the breakdown of stored fat (lipolysis) and potentially trigger apoptosis, or programmed cell death, in existing fat cells.
Influencing energy metabolism
CLA is also hypothesized to boost overall energy expenditure and fat burning through a process called thermogenesis. This is mediated by uncoupling proteins (UCPs) found in mitochondria, which regulate heat production. By increasing UCP activity, CLA may help the body burn more energy, and specifically fat, for heat rather than storing it. Some human studies have shown a modest increase in resting metabolic rate in individuals taking CLA, but the effect on overall body fat has been inconsistent.
Modulating enzymes for lipid metabolism
Further mechanisms involve the regulation of key enzymes responsible for fat synthesis and breakdown. For example, the t10,c12 isomer may decrease the activity of lipoprotein lipase (LPL), an enzyme that helps fat cells absorb fatty acids from the bloodstream. Simultaneously, it may enhance the activity of carnitine palmitoyltransferase-1 (CPT-1), an enzyme that aids in the transport of fatty acids into the mitochondria for oxidation (burning).
Sources of CLA: Food vs. Supplements
| Feature | Natural Food Sources | CLA Supplements |
|---|---|---|
| Primary Sources | Meat and dairy from ruminants (beef, lamb, milk, cheese, yogurt). | Safflower and sunflower oil. |
| Isomer Content | Predominantly cis-9, trans-11 (c9,t11) isomer. | Often a 50:50 mixture of c9,t11 and trans-10, cis-12 (t10,c12). |
| Concentration | Low levels, typically 2-5 mg/g of fat. | High doses, typically 3-6 g per day. |
| Associated Health Effects | Associated with improved metabolic health and lower disease risk in observational studies. | Inconsistent results on weight loss, mixed effects on health markers. |
| Safety Profile | Considered safe as part of a balanced diet. | Can cause digestive issues and potential metabolic side effects in high doses. |
Potential benefits of CLA
While animal studies have shown dramatic anti-obesity and anti-carcinogenic effects, the results in humans are more modest and less consistent.
- Modest Fat Loss: Meta-analyses of human trials suggest that CLA can cause a modest reduction in body fat over the first several months, but the effect often plateaus over time.
- Increased Lean Body Mass: Some studies have noted a modest increase in lean body mass in subjects taking CLA, which can contribute to a healthier body composition.
- Immune System Support: Early research, primarily in animal models, indicated that CLA might have immune-modulating effects. However, more human research is needed to confirm these benefits.
Potential side effects and safety concerns
It is crucial to understand that the high doses of CLA in supplements differ significantly from the small amounts found naturally in food. This difference contributes to potential side effects not seen with dietary intake. Doses of 3-6 grams per day are typically considered safe for short-term use, but long-term effects are not well-established.
- Gastrointestinal Issues: Mild to moderate side effects like nausea, diarrhea, and stomach upset are commonly reported with CLA supplements.
- Insulin Resistance: Some human studies, particularly those using specific isomers like t10,c12, have shown an increase in insulin resistance, especially in individuals who are already overweight or have metabolic syndrome.
- Fatty Liver: Animal studies and some human research have raised concerns that high doses of CLA can lead to the accumulation of fat in the liver.
- Inflammatory Markers: Some research indicates that CLA supplementation may increase certain inflammatory markers in the blood, such as C-reactive protein (CRP).
Conclusion
What is CLA and how does it work? CLA is a complex fatty acid with multiple isomers, found naturally in modest amounts in ruminant products and in higher, different concentrations in supplements. Its mechanism of action involves influencing fat storage, breakdown, and overall energy metabolism, but the effects are highly isomer-dependent and often more dramatic in animal models than in humans. While some human studies show a modest reduction in body fat, particularly in the initial months, results are inconsistent and the long-term health implications, especially at high supplemental doses, remain a concern. Given the potential side effects like digestive issues, insulin resistance, and adverse lipid profile changes, consuming CLA from its natural food sources is often considered a safer and more nutritionally sound approach. As always, consult a healthcare provider before starting any new supplement regimen, particularly if you have underlying health conditions.
Lists of natural sources of CLA
Dairy Products
- Cheese (e.g., Mozzarella)
- Yogurt (Plain)
- Milk
- Butter
Meat Products
- Beef (especially grass-fed)
- Mutton
- Veal
For more information
For a deeper dive into the metabolic effects of CLA, refer to the National Institutes of Health.
