Adverse Effects of Conjugated Linoleic Acid

Conjugated linoleic acid (CLA) is a non-essential fatty acid primarily found in the meat and dairy products of ruminants. It's believed to have antioxidant and anti-cancer properties, and is also believed to reduce body fat and increase lean muscle mass. Kangaroo meat is considered to have the highest concentration of CLA. In the U.S., beef is the biggest source of CLA, and grass-fed beef has 300 percent to 500 percent more CLA than grain-fed. Eggs are also a good source of CLA. CLA is considered to have many health benefits, and the only consistently reported side effect is mild gastrointestinal discomfort upon taking it. However, there have been studies that suggest CLA may also have serious adverse effects, especially in people with diabetes and metabolic syndrome.

Insulin Resistance and Low HDL in Human Studies

• In a 2002 study of obese men with signs of metabolic syndrome, CLA supplementation showed a reduction in body fat and weight but also showed an increase in insulin resistance and blood sugar, and a drop in HDL (good cholesterol) levels. Although this study illustrates serious adverse effects of CLA supplementation, it illustrates those effects within one special population. Whether or not CLA would have the same effect on obese males without diabetes and/or signs of metabolic syndrome remains to be seen. However, a 2003 study in mice revealed similar results.

Insulin Resistance In Mouse Studies

• Mice were divided into two different groups. One group was fed a CLA-rich diet for 4 weeks, then fed a CLA-free diet for another 4 weeks. The other group had a CLA-free diet for 2 weeks, then a diet with CLA for another 2 weeks. During that period, some of the mice were also given injections of an anti-diabetes drug to boost insulin sensitivity.
  The first group of mice lost significant amounts of body fat but gained excessive amounts of fat in the liver and experienced insulin resistance. When CLA was removed, the animals gained weight but the levels of fat in the liver dropped.
  The mice that were given the anti-diabetes drug did not lose weight but also did not become insulin resistant.

Body Fat Levels in Rat Studies

• The rat study used rats specially bred to gain weight. The rats were fed a high-fat diet for 4 weeks. Then, for the next 4 weeks, half the rats were given a low-fat diet with CLA supplementation. The remaining rats had a low-fat diet without CLA. The supplement did not cause fat loss, but excess fat didn't accumulate in the liver, either.

Contradictory Studies

• Although all of the above studies would indicate that loss of body fat with CLA leads to an increase in hepatic fat, none of the studies included exercise as a component and many of the studies focused on special populations.
  A 2007 study on healthy obese individuals showed a decrease in body fat, an increase in lean mass and no decrease in HDL levels. A 2008 study in healthy men showed no adverse effect on HDL levels, insulin sensitivity or blood sugar levels.

Conclusion

• There are studies showing decreased HDL and insulin resistance as potential adverse effects of CLA. However, because these studies were performed on special populations, it's not clear if these same effects are present in obese but otherwise healthy populations. The adverse effects of CLA may be related to CLA in general or to the physical condition of the person taking CLA, and the amount taken. For these reasons, people considering CLA supplementation, especially those with diabetes and/or metabolic syndrome, should consult their doctors and monitor blood levels.