Conjugated linoleic acid (CLA) is a fatty acid found in beef, veal, and dairy products that some studies have shown to guard against cancer, increase lean muscle mass, and aid in fat loss. As a synthetic nutritional supplement it is generally safe, but there are some possible side effects. Some of the less serious side effects are heartburn, nausea, fatigue, and possible allergic reactions. More serious conjugated linoleic acid side effects are increased insulin resistance, increased blood glucose levels, and lowered HDL, which is good cholesterol. Conjugated linoleic acid also appears to raise c-reactive protein, which is a sign of increased inflammation in the body.
There was once an abundance of conjugated linoleic acid in the meat of ruminants like sheep and cattle. Since today’s livestock are fed a grain-based diet instead of grazing on grass, CLA levels in beef, veal, and dairy products are much lower than they were prior to the twentieth century. Since the modern diet is deficient in conjugated linoleic acid, many bodybuilders and dieters have turned to CLA supplements.
A 2002 study of obese men with metabolic syndrome showed that CLA supplementation caused increased insulin resistance, making conjugated linoleic acid side effects possibly dangerous for the obese or people with diabetes or with a pre-diabetes condition. The same study showed that certain types of CLA increased c-reactive protein levels. This marker indicates an increased risk of diabetes, high blood pressure and heart disease, making CLA possibly unsafe for those at increased risk of heart disease.
In some studies in hamsters and rats, certain types of CLA lowered cholesterol, while other types had no effect. Among three chicken studies, two showed chickens fed CLA had increased cholesterol levels, while one study showed lower levels. A separate, human study showed CLA lowered HDL cholesterol levels.
Conjugated linoleic acid side effects have not been studied on pregnant and breastfeeding women. It is best for those women to avoid CLA supplements until more research is performed. Many of conjugated linoleic acid side effects reported in studies were in special populations, like obese men and diabetics. Healthy adults may not respond the same way to CLA and could possibly reap the benefits without the side effects. While the results of all these contradictory studies may be confusing, it may be best to pass on CLA supplements until more is known for certain about benefits and conjugated linoleic acid side effects.