What is a conjugated linoleic acid?
Conjugated linoleic acid (CLA) is a naturally occurring polyunsaturated omega-6 fatty acid, derived from the more common linoleic acid. The 'conjugated' prefix refers to the arrangement of the molecule's two double bonds, which are separated by a single bond, unlike the two-plus single bonds found in typical polyunsaturated fatty acids. CLA exists in many isomeric forms, meaning the double bonds can be located at different positions along the carbon chain and have different geometric arrangements (cis or trans).
Among the numerous isomers, two have been extensively studied for their biological activity: cis-9, trans-11 CLA and trans-10, cis-12 CLA. While both can be found in ruminant products, their relative abundance differs significantly between natural food sources and commercially produced supplements.
The primary example: cis-9, trans-11 CLA (Rumenic Acid)
The most abundant and well-known example of a conjugated linoleic acid found naturally in food is cis-9, trans-11 CLA, commonly known as rumenic acid. This isomer typically makes up 75% to 90% of the total CLA content in dairy and meat products from ruminant animals.
Rumenic acid is formed in the digestive tract of ruminants (like cattle, goats, and sheep) through a process called biohydrogenation. When these animals consume linoleic acid from their diet, specific rumen bacteria, such as Butyrivibrio fibrisolvens, convert it into CLA. Rumenic acid is then absorbed and incorporated into the animal's tissues and milk fat. Its presence is significantly higher in animals that graze on fresh pasture compared to those on a grain-fed diet.
Sources of naturally occurring CLA
Dietary sources are the primary way humans get rumenic acid. The highest concentrations of this conjugated linoleic acid are found in:
- Dairy products: Milk, butter, cheese, and yogurt from pasture-fed animals are rich sources. For example, sheep's milk can have a higher CLA content than cow's milk.
- Meat from ruminants: Beef and lamb contain significant amounts of CLA, with grass-fed varieties having higher levels than their grain-fed counterparts.
Less significant amounts are found in meat from non-ruminants like poultry and pork. The CLA content in these animal products can vary greatly depending on the animal's diet, with fresh grass and certain plant seeds (like safflower and flaxseed) increasing CLA levels.
Comparing natural and supplemental CLA
While natural sources provide rumenic acid, dietary supplements offer a different composition, which can have varying physiological effects. Supplemental CLA is typically produced by chemically altering linoleic acid from vegetable oils, such as safflower oil, resulting in a different isomeric profile.
Comparison of Natural vs. Supplemental CLA
| Feature | Natural CLA (e.g., from grass-fed beef) | Supplemental CLA (e.g., from safflower oil) |
|---|---|---|
| Primary Isomers | Primarily cis-9, trans-11 (rumenic acid) | Approximately 50% cis-9, trans-11 and 50% trans-10, cis-12 |
| Composition | High proportion of c9, t11-CLA (75-90%) | ~1:1 ratio of the two major isomers |
| Source | Ruminant animal products (dairy, beef, lamb) | Chemically altered vegetable oils |
| Health Effects | Associated with anti-inflammatory and other benefits | Isomer-specific effects; t10,c12 linked to fat reduction but also potential adverse effects |
| Potential Adverse Effects | Lower risk, tied to a balanced diet | Higher doses and different isomer ratios associated with altered glucose metabolism and insulin resistance in some studies |
The diverse biological effects of CLA isomers
The most studied CLA isomers, c9, t11-CLA (rumenic acid) and t10, c12-CLA, have been shown to have distinct effects on human health, contributing to the often-conflicting results seen in CLA research.
- Rumenic Acid (c9, t11-CLA): This isomer is primarily responsible for the anti-inflammatory properties attributed to CLA. Research indicates its potential in managing inflammatory conditions such as inflammatory bowel disease and arthritis, largely by regulating cytokines and signaling pathways like NF-κB. It is also associated with anti-carcinogenic effects observed in some animal studies.
- trans-10, cis-12 CLA (t10, c12-CLA): This isomer is predominantly associated with the anti-obesity effects seen in animal models, particularly the reduction of body fat. Studies suggest it may reduce body fat by increasing lipolysis (the breakdown of fats) and inhibiting lipogenesis (the formation of fats). However, some human and animal studies show that high doses of this isomer may lead to adverse effects, including altered glucose metabolism, insulin resistance, and fatty liver. This highlights the importance of the specific isomer profile when evaluating CLA supplements.
Conclusion
An excellent example of a conjugated linoleic acid is rumenic acid (cis-9, trans-11 CLA), the most prevalent isomer found in nature. This fatty acid is a component of dairy and meat from ruminant animals, especially those fed a pasture-rich diet. The health effects of CLA are complex and isomer-specific, with rumenic acid linked to anti-inflammatory benefits, and the t10, c12-CLA isomer often found in supplements linked to fat reduction but also potential metabolic complications at high doses. The distinction between the isomeric profile of naturally occurring CLA and supplemental CLA is a critical consideration for those interested in its health implications. For further reading on fatty acid metabolism, explore the role of CPT-1 and PPARγ in lipolysis.
Note: While animal studies have shown significant effects, human studies have produced mixed and sometimes conflicting results. Consulting with a healthcare professional before taking CLA supplements is recommended.
