The findings, published in the Journal of Nutrition, come from the extension of a 12-month study on 157 adults. It found that a daily intake of 3.4g of Tonalin brand CLA, made by Cognis, produced an average 9 per cent reduction in body fat mass.
While the second year of study was not carried out in double-blind conditions, and therefore cannot provide such strong evidence of efficacy, it does reflect high compliance levels for the product and its long-term safety.
"The high compliance and low drop-out rates indicate that long-term CLA supplementation was well-tolerated by subjects," write the authors.
After the initial 12 month trial, published last spring in the American Journal of Clinical Nutrition, 134 of the participants volunteered to continue for another 12 months. Ethical reasons stopped the researchers from continuing the double-blind procedure.
"All patients knew they were taking CLA which minimises our findings somewhat," said study co-ordinator Dr Jean-Michel Gaullier from Scandinavian Clinical Research in Kjeller, Norway.
"However the placebo group from the first year of study, which started the CLA treatment during the second year, showed the same decrease in body fat mass as those taking CLA in the first year. This confirmed our results from the initial trial," he told NutraIngredients.com.
He added: "Those who continued to take the CLA maintained the lower amount of body fat mass, without decreasing it further, but without regaining the fat."
As a general rule, a population with this kind of age profile should gain around 1kg each year, explained Dr Gaullier.
The researcher says that a 9 per cent body fat decrease represents about 2 kg in weight for a person weighing around 80kg to begin with, or the kind of weight loss that can be seen with a long-term exercise programme.
People on certain diets can lose up to 10 kg in the best case scenario but the benefit of CLA is that it does not affect the basal metabolic rate, according to Dr Gaullier. Other weight loss products reduce metabolic rates and this leads to weight regain.
CLA supplementation also appeared to lower levels of leptin, a hormone associated with increased weight. Over the course of 24 months, leptin levels dropped 20-35 per cent as study subjects lost body fat.
The researchers say that risk of side effects from CLA is no greater than in those taking the placebo. (They can only be properly measured in the first year, during placebo control). Most of those reported were unrelated to the use of CLA, with the remaining 6 per cent described as 'mild' reactions, most frequently gastrointestinal in nature, and seen equally across both CLA and placebo groups.
"The side effects were mainly due to capsules or the amount of oil ingested everyday that can affect some people," said Dr Gaullier.
The new results suggest that people with the highest body mass index (BMI) and body fat mass, especially women with a 25 to 30 BMI, lose the most body fat on CLA supplementation.
Further research is needed to find out if obese people benefit even more.
"The best market for CLA would be those who are overweight but not yet obese. There is no treatment for these people so CLA could be interesting for them," Dr Gaullier noted.
CLA is thought to work by lowering the amount of fat normally stored after meals, helping the body to break down and efficiently use fat.
Changes during the past 30 years in how cattle are raised (moving to grain feed over grass), coupled with the trend toward low-fat dairy and reduced beef consumption, have drastically reduced the amount of CLA humans acquire through diet.