The group that had already been taking CLA lost no more weight or body fat, but they maintained the body fat losses seen in the previous year. People who began taking the supplement during year two of the study lost an average of 3.5 pounds and also saw reductions in overall body fat.
The safety picture for those who took the supplement for two years was somewhat reassuring. Total cholesterol and LDL cholesterol were reduced slightly, while HDL cholesterol, triglycerides, and fasting blood glucose levels remained unchanged.
But there were consistent increases in lipoprotein levels associated with CLA use. Lipoprotein levels are believed to be independent predictors of heart disease risk.
CLA use was also associated with increases in white blood cells and blood platelet counts, which suggested an inflammatory response to use of the supplement. Inflammatory responses like these are believed to lead to blood vessel damage seen in atherosclerosis and cardiovascular disease.
Researcher Jean-Michel Gaullier and colleagues concluded that the role of CLA in cardiovascular risk is still "equivocal."
"Further studies are needed to determine if there is an effect of CLA on cardiovascular risk and inflammation in humans," they wrote.
In a review of the CLA research, published last year, California nutritionists Lisa Rainer, MS, RD, and Cynthia J. Heiss, PhD, concluded that while the animal studies on CLA are promising, the research in humans remains inconclusive.
"The existing studies of CLA supplementation in human beings are difficult to interpret because of the different parameters measured and the variances in dosage, duration of administration, and subject characteristics," they write.
In an interview with WebMD, Rainer said she would not recommend CLA supplementation for weight control on the basis of the studies she has seen.
"More trials need to be done in human beings before we really know the long-term benefits and safety," she says.
But Pariza, who began taking CLA when it became available in supplement form almost a decade ago, believes strongly that long-term use by healthy people is not only safe but beneficial.
He says CLA may have merit as a weight loss supplement when combined with another weight-reducing treatment or may singularly promote loss of body fat and maintenance of muscle. He says the supplement also can subsequently reduce the risk of weight regain.
SOURCES: Gaullier, J-M. The Journal of Nutrition, April 1, 2005. Jean-Michel Gaullier, PhD, Scandinavian Clinical Research AS, Kjeller, Norway. Michael W. Pariza, PhD, professor, department of food microbiology and toxicology; and director, Food Research Institute, University of Wisconsin at Madison. Lisa Rainer, MS, RD, registered dietitian, Northridge, Calif.