What is CLA, and which foods contain it?

CLA, or conjugated linoleic acid, is not one fatty acid but a group of forms (isomers) of linoleic acid, the 18-carbon omega-6 fatty acid found in many plant oils. In CLA, the two double bonds sit next to each other, separated by a single bond, which is what "conjugated" means. CLA is made mainly in the rumen, the first stomach of cattle, sheep and goats, so it is found in their milk, cheese, butter and meat. The main form, cis-9, trans-11 CLA (rumenic acid), makes up as much as 80% to 90% of the CLA in these foods. US Department of Agriculture data list 0.267 g of CLA per 100 g of salted butter, 0.166 g in cheddar cheese and 0.013 g in whole milk. There is no Thai Reference Daily Intake (Thai RDI) for CLA.

In short:

• What it is: a group of isomers of linoleic acid with conjugated double bonds, all C18H32O2
• Main forms: cis-9, trans-11 (rumenic acid) and trans-10, cis-12
• Where it is made: in the rumen of cattle, sheep and goats, and in their tissues from trans-vaccenic acid
• Foods: butter, cream, cheese, milk, lamb and beef; very little in plant oils, pork, chicken and fish
• What changes the amount: the animal's feed (grass, oilseeds), the fat content of the food, and cheese ripening
• Thai RDI: none; counted within total fat (65 g a day)

This article explains what CLA is, where it comes from and how much foods contain. It does not discuss what it does in the body. For linoleic acid and the omega-6 family, see What is the difference between omega-3 and omega-6?

What is CLA?

Linoleic acid and conjugated linoleic acid

Linoleic acid is a fatty acid with 18 carbon atoms and two double bonds. In ordinary linoleic acid, the double bonds are at positions 9 and 12, both in the cis shape, and a CH2 group sits between them. A 2024 review in the journal Heliyon describes CLA as a general term for all the geometric and positional isomers of linoleic acid, and a 2023 review in Food Research International describes it as a group of isomers of linoleic acid with a conjugated double bond. "Positional" means the double bonds have moved (for example to positions 9 and 11, or 10 and 12), and "geometric" means each can be cis or trans.

In chemical names, Z means cis and E means trans. All CLA isomers have the same formula as linoleic acid; only the position and shape of the double bonds differ. Trans-vaccenic acid is included because it is the starting material from which animal tissues make rumenic acid.

The two main isomers

The Heliyon review describes cis-9, trans-11 CLA and trans-10, cis-12 CLA as the most abundant isomers. In foods from ruminants, one of them dominates: a 2010 review in Nutrition Journal reports that cis-9, trans-11 CLA, also called rumenic acid, accounts for up to 80-90% of the total CLA in ruminant products.

How does CLA fit the definition of trans fat?

CLA isomers contain a trans double bond, but a 2013 review from the University of Alberta explains that CLA is currently excluded from the Codex Alimentarius definition of trans-fatty acids, the international definition used for food labelling rules. In FoodData Central, for example, cheddar cheese has separate rows for CLA and for total trans fatty acids.

Where does CLA come from?

Made in the rumen

Cattle, sheep and goats are ruminants: their first stomach, the rumen, holds bacteria that break down grass and feed. These bacteria also change the unsaturated fatty acids in the feed. The Nutrition Journal review explains that rumen bacteria, notably Butyrivibrio fibrisolvens, convert linoleic acid and alpha-linolenic acid step by step towards stearic acid, a saturated fatty acid, in a process called biohydrogenation, and that this process depends heavily on rumen pH. CLA and trans-vaccenic acid are formed as intermediate steps.

Made in the animal's tissues

Not all CLA comes straight from the rumen. The Nutrition Journal review notes that synthesis of CLA from trans-vaccenic acid has been documented in rodents and dairy cows, among others. A 2016 review from Chiang Mai University and Kasetsart University, which focuses on Thailand and Asia, likewise describes vaccenic acid as the precursor of rumenic acid.

Very little in plants

The Food Research International review says the main natural sources of CLA are dairy products, beef and lamb, and that only trace amounts occur naturally in plant lipids. This is why CLA follows the fat of ruminant foods rather than plant oils, even though plant oils are the richest sources of ordinary linoleic acid.

Which foods contain CLA, and how much?

CLA per 100 g of food

The figures below come from the US Department of Agriculture's FoodData Central database (SR Legacy, and Foundation Foods for whole and nonfat milk), which lists CLA as "18:2 CLAs". The fat column shows how much of each food is fat.

Foods from ruminants (lamb, butter, cream, cheese, beef, whole milk) are at the top. Chicken, eggs, salmon and pork have much less, even when they contain a similar amount of fat.

The fat matters

Because CLA is part of the fat, the leaner the cut or the lower the fat in the dairy food, the less CLA it has. In the table, raw T-bone steak with its fat has 0.079 g per 100 g, while the lean part alone has 0.020 g. Whole milk has 0.013 g per 100 g, while nonfat milk has none listed. Butter and cream, which are mostly milk fat, have more CLA per 100 g than milk.

What changes the amount of CLA in milk and meat?

The animal's feed

The Nutrition Journal review reports that grass-fed ruminants have been shown to produce 2 to 3 times more CLA than ruminants fed in confinement on high-grain diets. The review compares CLA per gram of fat; it also notes that grass-fed beef is lower in fat overall, so higher-fat portions of grain-fed beef can contain a similar amount of CLA. The Heliyon review lists feeds rich in polyunsaturated fat, such as oilseeds, vegetable oils, fish oil and microalgae, and grass-based feeding as ways to increase CLA in milk and dairy products.

A Thai study of beef cattle

A study from Khon Kaen University with partners in the Netherlands and Belgium fed 12 Thai Native and 12 Holstein bulls a ration with either whole cottonseed or sunflower seed for 90 days. In both the fat under the skin and the fat within the muscle, the cottonseed ration gave a lower proportion of cis-9, trans-11 CLA and a higher proportion of saturated fatty acids than the sunflower seed ration. The intramuscular fat of the Thai Native bulls had more polyunsaturated and less saturated fat than that of the Holstein bulls, whatever the ration. The study covered 24 animals on two rations, so it shows the direction of the effect rather than the CLA content of Thai beef in general.

Cheese-making and storage

A 2025 review from Aristotle University of Thessaloniki in Greece says the CLA in cheese depends on the CLA in the milk, because the fat carrying the CLA moves from the milk into the cheese. Certain lactic acid bacteria can add CLA during ripening by converting linoleic acid: in the studies reviewed, starter bacteria raised CLA by 1.19, 1.6 and 6.6 times compared with control cheeses in ovine, miniature and Cheddar cheeses. On the other hand, lipid oxidation, from fluorescent light or exposure to air, can lower CLA during storage.

Is there a recommended daily amount of CLA?

No. CLA is not a nutrient with a Thai Reference Daily Intake (Thai RDI), so there is no recommended daily amount for it in Thailand. The figures above show how much CLA foods contain, not how much anyone needs. CLA is counted within dietary fat: the Thai RDI for total fat from all foods is 65 g a day, and for saturated fat 20 g. Butter, cream and cheese, among the foods with the most CLA per 100 g, are also high in fat. For the other Thai reference amounts, see What are the Thai daily reference amounts for each nutrient?, and for the types of fat in cooking oils and coconut milk, see How do cooking oils differ, and what fats do they contain?

Why the figures are only a guide

FoodData Central describes foods produced in the United States and New Zealand, and the studies show that feed, breed, the cut and the fat content all change CLA. Thai milk, cheese and beef may contain different amounts.

Who should talk to a doctor or pharmacist?

People with a milk allergy

Many of the foods richest in CLA are dairy foods. The US FDA lists milk among the nine major food allergens. People with a milk allergy should talk to a doctor about dairy foods.

Pregnancy and regular medication

If you are pregnant or breastfeeding, or take medication regularly, talk to a doctor or pharmacist before taking dietary supplements.

Dietary supplements are not intended to prevent, treat or cure any disease.