Conjugated linoleic acid (CLA) is an isomer of linoleic acid. Linoleic acid is an omega-6 fatty acid, one of two fatty acids considered essential since they cannot be made in the body and must be consumed in the diet.

The chemical formula of CLA is the same as linoleic acid; however, its chemical structure is different. CLA is commonly found in two shapes: trans-10, cis-12-CLA (10-CLA) and cis-9, trans-11-CLA (9-CLA). CLA is found in milk, butter, and meat fats. Roughly 75%-90% of the CLA found in milk is of the 9-CLA variety.

CLA is believed to improve body composition by lowering fat mass and increasing fat-free mass. There are a variety of proposed mechanisms, which include decreased enzyme activity leading to a decreased uptake of triglycerides by fat cells. CLA is also believed to influence transcription factors that regulate production of new fat cells. Lastly, CLA is thought to increase β-oxidation, the metabolic pathway used to burn fat.

CLA is a popular supplement among bodybuilders, fitness competitors, and other athletes interested is reducing body fat and increasing lean.

A significant amount of research has been done on CLA, producing a wide variety of results. Early studies in animals were positive: CLA use led to losses in body fat and increases in lean body mass, especially in mice. However, studies in humans were not nearly as conclusive or effective. Research suggests that 10-CLA is most effective in producing changes in body fat; however, when supplemented alone, it produces insulin resistance. This negative affect is counteracted when 10-CLA is supplemented with 9-CLA. A dosage of 3.2 g/day of 50:50 (10-CLA:9-CLA) is suggested, and higher doses are not recommended. Higher intakes of CLA increase liver and spleen size and can result in insulin resistance. In addition, some studies have found CLA to increase makers of inflammation, such as the C-reactive protein.