Many of the apparent health benefits of conjugated linoleic acids (CLA) have only been demonstrated in animal studies, said Dr. Michael de Vrese, of Germany's Federal Research Centre for Nutrition and Food, at last week's Drinktec expo in Munich.
"In human trials, results were not so clearly favourable," said De Vrese in a talk on the potential for functional dairy drinks development. "In humans there will have to be a number of big studies," he added.
Talking specifically about the relationship between CLA and weight loss, de Vrese said that in food he would have "some reservations over using CLA in health claims for weight loss.
"Only specific CLA types have proved effective in some trials, while some have not proved effective in any trials."
De Vrese's comments add a new twist to the debate over what foods fortified with CLA can do for consumers' health.
There are 28 different types of CLA, though the type most commonly found in meat and dairy products is generally referred to as 'cis-9, trans-11 CLA'.
Some studies on animals have shown CLA to be effecting in fighting and preventing certain cancers, while trials on both humans and animals have also shown CLA can help to reduce body fat in the long term.
Yet, despite CLA's natural roots, it has so far mainly been used by the supplement industry, with its potential application in functional foods still only in the developing stages.
De Vrese's cautious approach to the subject echoes a report by the French food standards agency, AFSSA, earlier this year, which said: "Based on current knowledge, AFSSA considers that the addition of certain mixtures of CLA in foods is not justified, whether in form of supplements or food ingredients".
One leading CLA producer, Loders Croklaan Lipid Nutrition, said it had entered discussions with AFSSA over the issue and that the French agency had not used the most recent data.
Manufacturers and ingredients firms are keen to bring CLA into the fortified food arena as this segment offers significantly larger potential than the supplements sector.
Yet progress has been limited so far, with no CLA-fortified foods available in France, despite the country being Europe's biggest market for CLA supplements. More than 20 firms offer the fatty acid in capsule form.
Spain has so far led the way in Europe through CLA-enriched dairy foods and cookies.
Another problem is that CLA has been stuck in a regulatory grey area in Europe. It is considered a 'new' ingredient by food makers yet has been available in foods before the European Union's 1997 novel foods regulation, suggesting that it should not require safety approval.