The rat study brought about more favorable results as the animals didn't lose weight overall, but did show a decreased accumulation of fat in their livers. In the mice study, however, the fatty acid was fed to the animals, who lost weight but then accumulated fat in their livers.
These results draw attention to potential side effects of a supplement ingredient that has been heavily backed by scientific studies and marketed to help reduce body fat as well as reduce the risk of diabetes. Yet the clout of the study involving mice - the one with more discouraging results - is questionable given their behavior is said to be less close to humans then that of rats.
The mouse study appears in a recent issue of the Journal of Lipid Research, while the rat study will appear in an upcoming issue of the Journal of Nutritional Biochemistry.
"Many people take CLA as a supplement in hopes of trimming body fat, and it seems to work," said Martha Belury, the lead author of both studies and an associate professor of human nutrition at Ohio State University. "But we're not sure what else it does to the body."
"It seems that these mice and rats represent a continuum of possible side effects induced by CLA," said Belury. "The question is, are humans more like mice or rats? We're probably somewhere in between."
Formed by bacteria in ruminant animals that take fatty acids from plants, CLA is found predominantly in dairy products such as milk, cheese and meat.
In the first study, the mice that were fed a CLA-supplemented diet lost weight quickly, but also accumulated excessive amounts of fat in their livers. Excessive fat accumulation in the liver is linked to insulin resistance and is a characteristic of Type 2 diabetes.
In the second study, CLA didn't help rats lose weight they had gained prior to taking the supplement. However, it decreased the amount of fat accumulated in the rats' livers due to this weight gain. As a result, the rats were less resistant to insulin.
According to Belury, up to 75 percent of people with obesity and diabetes develop an illness called non-alcoholic fatty liver disease in which fat accumulates in the liver and can ultimately make a person insulin resistant.
CLA may or may not have a similar effect on humans, according to Belury, and it will take time to determine how the human body responds to it. Belury and researchers from Ohio State's medical center are currently conducting a clinical trial of the effects of CLA on women with diabetes.
However, a human six-month randomised, double-blind, placebo-controlled study, published last year in the International Journal of Obesity (doi: 10.1038/sj.ijo.0803437), reported that CLA safely reduced the body fat mass of 40 overweight and obese subjects by one kilogram (2.2 lbs) and their body weight by 0.6 kg (1.3 lbs). In comparison, those in the placebo group gained 0.7 kg (1.5 lbs) of body fat mass and 1.1 kg (2.4 lbs) of body weight.
Moreover, no impact of CLA was found on the resting metabolic rate, insulin resistance, or blood lipid levels. Also, there was no difference in the number of adverse events when compared to placebo.