Conjugated linoleic acid (CLA) refers to a group of fatty acids formed from the polyunsaturated fat linoleic acid and naturally found in meat and dairy. Supplements, made from safflower or sunflower oil, typically contain a mix of CLA forms in different proportions from that in animal foods. CLA is promoted mostly for weight loss, with lab studies showing it reduces body fat in animals. But human trials have yielded mixed results—and any weight loss seen has been modest at best. In a 2012 study from China in the journal Nutrition, for example, overweight and obese people who took CLA for 12 weeks had only slight reductions in body fat and weight. A 2010 report from Health Canada said there is insufficient evidence that CLA helps with weight loss.
Observational studies have linked high intakes of CLA from high-fat dairy foods (like cheese) to a reduced risk of some cancers—but it's not known if the supplements would have this potential effect. Also, there are concerns about possible adverse effects from the supplements, including increases in inflammation, decreased HDL ("good") cholesterol, and reduced insulin sensitivity. CLA has been shown to cause liver damage in animals. It's not clear which formulas (there are many) may be riskier.
Our take: Don't take CLA supplements, especially if you have diabetes or liver disease. The risks outweigh the small potential benefits. CLA naturally found in food is fine—and may even be good for you. Full-fat dairy foods and meat from grass-fed animals are the richest food sources.