Understanding Conjugated Linoleic Acid (CLA)
Conjugated Linoleic Acid, or CLA, refers to a group of isomers of linoleic acid, which is a polyunsaturated omega-6 fatty acid. It is created naturally in the digestive systems of ruminant animals, like cows, sheep, and goats, by bacteria that break down the fats in the animals' diet. Once synthesized, the CLA is then stored in the meat and milk of these animals. The most studied and biologically active isomer in humans is the cis-9, trans-11 form, also known as rumenic acid.
The amount of CLA in these animal products is not fixed; it is heavily influenced by the animal's diet. Grass-fed animals, which consume a diet rich in fresh pasture, produce meat and dairy with significantly higher levels of CLA compared to their grain-fed counterparts.
The Role of Ruminant Diet in CLA Production
The diet of a ruminant animal directly impacts its CLA content. A grass-fed diet, high in linoleic and alpha-linolenic acids, leads to increased CLA production during ruminal biohydrogenation. When animals are switched to a grain-heavy diet, their CLA levels decrease. This makes the highest sources of CLA inherently linked to the animal's natural, grass-based feeding habits.
Why Lamb is a Top Source of CLA
Among all ruminant meats, lamb is consistently cited as having one of the highest concentrations of naturally occurring CLA. The concentration of CLA in lamb fat can be particularly potent, and it remains stable even after cooking. Fattier cuts of lamb will naturally contain more CLA, as the fatty acid is stored within the fat cells. However, it's important to note that the CLA content in all ruminant meats is highly dependent on the animal's diet, and grass-fed options will always contain more.
Dairy Products as Potent CLA Sources
Dairy products are also a significant source of CLA in the human diet. As with meat, the CLA content in dairy is affected by the cow's diet and varies seasonally. Milk and butter from grass-fed cows, for instance, contain substantially more CLA than those from grain-fed cows. Certain cheeses also provide a rich source. Here's a look at some of the highest-CLA dairy products:
- Butter: Grass-fed butter, especially from spring and summer milk, can be one of the densest sources of CLA, potentially containing more than 10 mg of CLA per gram of fat.
- Cheese: Fatty, ripened cheeses such as Swiss, Blue, and Brie can be rich sources, with some studies showing CLA content up to 7.96 mg per gram of fat.
- Milk and Yogurt: Full-fat versions of these products from grass-fed animals also contribute a healthy dose of CLA to the diet.
The Importance of Fat and Seasonality
Because CLA is a fatty acid, its concentration is highest in the fatty parts of meat and dairy products. Leaner cuts of meat will contain significantly less CLA. Furthermore, the seasonal changes in pasture quality affect CLA levels, with higher concentrations often observed in products from animals grazing on fresh, nutrient-dense grass during warmer months.
Other Sources of CLA
While ruminant meat and dairy are the most concentrated natural sources of CLA, other foods contain smaller amounts. These include:
- Poultry, such as turkey and chicken
- Pork
- Some vegetable oils, like safflower and sunflower oil
- Certain mushrooms and seeds, though in much smaller quantities.
Comparison of Top Natural CLA Sources
To give a clear perspective, here is a comparison of common CLA sources based on typical reported values (expressed as mg per gram of fat):
| Food Source | Typical CLA Content (mg/g fat) | Factors Influencing CLA Level | Notes |
|---|---|---|---|
| Lamb (Grass-fed) | 4–19+ | Primarily diet (grass vs. grain) | Considered the highest natural source |
| Butter (Grass-fed) | 6–12+ | Seasonality, diet | Higher in spring/summer butter |
| Cheese (Ripened) | 3.5–7.9 | Milk source (sheep, cow), ripening time | Highest in sheep milk cheese |
| Beef (Grass-fed) | 4–10+ | Diet, cut of meat | Significantly higher than grain-fed beef |
| Milk (Grass-fed) | 5+ | Diet, seasonality | Can be enriched by feeding practices |
| Beef (Grain-fed) | < 4 | Diet | Low levels |
| Pork / Chicken | < 1 | Diet | Trace amounts |
Supplementation vs. Food Sources
It's important to distinguish between naturally occurring CLA from food and the CLA found in dietary supplements. Supplements are typically manufactured by chemically altering linoleic acid from vegetable oils, such as safflower oil. The isomer balance in supplements differs significantly from the natural balance in food, and high doses from supplements have been associated with potential side effects. In contrast, the CLA from natural food sources is generally considered beneficial and part of a healthy diet.
Conclusion
While various foods contain conjugated linoleic acid, the most concentrated and potent natural sources are found in the meat and dairy products of ruminant animals. What is the highest source of CLA? Lamb, especially from grass-fed animals, and high-fat dairy products like grass-fed butter and certain aged cheeses consistently demonstrate the highest concentrations. For consumers seeking to increase their natural CLA intake, prioritizing these grass-fed animal products over grain-fed alternatives is the most effective dietary strategy.
For more detailed information on CLA biosynthesis and the factors affecting its levels in dairy products, Penn State Extension provides an excellent overview: Conjugated Linoleic Acid (CLA) in Animal Production and Human Health.
