Studies done at the Wageningen Center for Food Sciences in the Netherlands have shown that FOS weakens the normally impermeable intestinal barrier allowing the salmonella infection to reach areas outside of the intestine.
The unexpected findings, first suggested in 2003, are among the first to raise questions about the benefits of prebiotics, shown to stimulate the growth of probiotic bacteria and therefore thought to enhance the resistance to disease.
Writing in this month's issue of the Journal of Nutrition (vol 135, issue 4, pp837-842), Sandra Ten Bruggencate and colleagues say that a 60g/kg dose of FOS for two weeks increased intestinal permeability and mucin excretion in rats, and also increased the presence of toxic compounds in faecal water.
After infection with the salmonella bacteria, mucin excretion and intestinal permeability in the FOS groups increased even further in contrast to the control group.
"We think that fermentation of the prebiotic increases cytotoxic components that could damage the intestinal barrier," Ten Bruggencate told NutraIngredients.com. "But we are not sure what the components are."
"It could also be that when the prebiotic enters the instestine, more microorganisms use it as food, producing byproducts that could be toxic."
Mucin excretion also suggests that the intestine is producing extra 'slime' to protect it against the cytotoxic components.
In addition, the researchers report that the FOS increased translocation of salmonella to extraintestinal sites.
Salmonella food poisoning is an unpleasant illness and, although most people make a full recovery, it can be serious for vulnerable groups such as the elderly, babies or people in poor health.
The number of cases in Europe is going down due to industry control programmes but England and Wales still had a reported 9757 cases in 2003.
However it is unclear yet whether the effects of prebiotics seen in rats also mean an increased risk of infection for humans. Results of a new trial on 34 men, not yet published but due to be presented at digestive disease week in the US in May, suggest a much more moderate effect on the human gut than in rats.
Furthermore, regulations on research procedure will not allow the Dutch team to infect volunteers with salmonella so they could not investigate the potential for increased risk of the food poisoning.
"We only saw increased mucin excretion in the humans but no signs of the other adverse effects that we had seen in rats - increased intestinal permeability and toxic compounds in the faecal water," said Ten Bruggencate.
She added that the human subjects had been given 20g of FOS per day, as much as any human would be able to consume.
The results are unclear for the food industry, which is seeing good growth from these ingredients. Currently worth €87 million in the European marketplace, they are set to reach €179.7 million by 2010, according to Frost & Sullivan.
"You need some real evidence in order to avoid these products," Ten Bruggencate said.
She noted that there are very few studies investigating the potential side effects of prebiotic ingredients.
"I could only find one in the literature, from a Finnish group looking at cancer risk."