Prior to the start of a 7-days supplementation period (one of the three probiotic supplements or placebo), blood samples will be obtained and small intestinal permeability will be assessed by measuring the urinary extraction ratios of ingested water-soluble, non-degradable test probes, with and without indomethacin challenge (to compromise the gut). After each supplementation period the gut will be stressed again by the indomethacin protocol, followed by measurement of small intestinal permeability, whereupon blood samples will be taken and duodenal tissue samples will be obtained by standard gastroduodenoscopy. Each supplementation period will start 1 month after finishing the pervious test episode, in randomized sequence.