Methods of the nutritional quality evaluation of dietary fibre are specified in the submitted paper. Soluble dietary fibre fermentation has a preventive effect, it is however also influenced by insoluble fibre. The presented method of methane and hydrogen estimation is suitable for the correct interpretation of the effect of fibre on the digestive system. Methane and hydrogen are final products of the anaerobic fermentation process of fibre in the colon. The estimation of methane concentration in expired air is described. Gas chromatography is used. The variation coefficient of a estimation was 2,05 %. The expired air was collected in a special receptacle, which was filled with a saturated solution of sodium chloride and acidified with hydrochloric acid. The use of a mouth piece for gas sampling was preferred to a mask in this study for the breath test (it was tested by comparing 10 healthy volunteers).Methanogenesis the last stage of anaerobic fermentation is described. Methaneestimation in expired air informs on the fermentation of undigested food residua in the colon and reflects physiological and pathological conditions of the digestion system. The transit time was determinated using this method after administration of 20 g lactulose and there was an increased methane concentration in expired air after 100–200 minutes in all cases.Thearea under the concentrationpeak reflects dietary fibre fermentation in vivo. Introduction of breath test is useful as it is a non-invasive method. Breath tests were included into quality evaluation of nutritional supplements.

        Key words: breath tests, methane, dietary fibre, anaerobic fermentation, gas chromatography.