They are added to extra healthy juices, bread and desserts: nutritional fibres, called fructo-oligosaccharides, or FOS, on the label. A few years ago Wageningen researchers showed that this kind of fibre is not so healthy at all. PhD researcher Wendy Rodenburg of the Department of Human Nutrition has found out why.
It’s an attractive theory, but Wageningen researchers have been questioning whether it holds water for a number of years. In 2005, a predecessor of Rodenburg demonstrated that, in rats on a diet containing synthetic fibres, salmonella actually penetrates the gut more easily. ‘An explanation offered was that the sugar chains all ferment in the same location in the large intestine. Because bacteria convert the chains into fatty acids, the intestine becomes acid there. A slightly acid environment is not unhealthy, but too much fatty acid weakens the gut wall, making it easier for salmonella to break through it.
Rodenburg therefore investigated which genes in the gut cells of laboratory rats become active if the animals eat FOS. ‘Genes involved in the conversion of nutrients into energy become more active. The acidification that takes place in the gut probably means that the gut cells need more energy to maintain themselves, and that makes them more vulnerable.’
When they are exposed to salmonella, but not to sugar chains, the resistance genes, such as the gene for interleukin 1-beta, become more active. This would indicate that the rat’s body has detected the pathogen and is trying to fight the infection. If the rats were given food containing FOS as well, the resistance genes became even more active. ‘The sugar chains make the infection worse, it appears,’ says Rodenburg.
Resistance genes have been examined in various studies on the effects of indigestible sugar chains. When researchers observed them becoming more active, this was taken to be evidence of the health-promoting effect of sugar chains. ‘But this appears to be a mistaken interpretation,’ says Rodenburg. ‘I hope that my PhD research will contribute to the development of better biomarkers, thereby improving ways of measuring the influence of food products on gut health.’
Rodenburg conducted her research at RIKILT Institute of Food Safety and Nizo-Food Research, and it was financed by the Top Institute Food and Nutrition (TIFN). / Willem Koert
Wendy Rodenburg receives her PhD on 5 February. She was supervised by Professor Martijn Katan, chair of Nutrition Studies at the Vrije Universiteit Amsterdam, former chair of Human Nutrition at Wageningen University.