The physiological effects of dietary fibres in humans are due to their physico-chemical properties. However, it is difficult to predict these effects simply by measuring certain characteristics in vitro. Studies in human subjects are still required to assess the effectiveness of new substrates. The aim of the present study in healthy human subjects was to evaluate the effects of two novel fibres, potato (PF) and maize (MF), on fasting and postprandial blood concentrations of carbohydrate and lipid metabolites as well as on stool ouput and transit time. The chemical composition, water-binding capacity (WBC) and fermentative properties of the fibres were also characterized in order to determine their possible involvement in digestive and metabolic effects. Stools, as well as breath and blood samples, were collected after consumption for 1 month of either a basal diet (control) or a basal diet supplemented with fibre (lSg/d). MF resisted fermentation better than PF and had lower digestibility. However, both fibres increased faecal output of dry matter, neutral sugars and water. There was an inverse relationship between stool weight and orofaecal transit time, although only MF significantly reduced transit time. Orocaecal transit was lengthened by PF, probably because of its high WBC. PF ingestion also decreased postprandial plasma levels of total and esterified cholesterol but had no effect on fasting concentrations. In contrast, MF lowered fasting cholesterolaemia and increased free:esterified cholesterol. These particular physiological and fermentative properties suggest that PF and MF would be suitable ingredients in a healthy diet. Dietary fibre: Stool output: Transit time: Plasma Upids IThe mechanisms by which a particular dietary fibre acts on human physiology are mainly related to its physico-chemical properties. For instance, solubility seems to be particularly involved in the metabolic effects of fibre. Soluble fibres can apparently reduce serum cholesterol concentration, whereas insoluble ones are usually ineffective (Glore et al. 1994). Other properties, such as viscosity and the ability to sequester bile salts, can also influence glucose and cholesterol metabolism (Wolever, 1995). The effects of dietary fibre on digestive functions also depend on several physico-chemical properties of substrates. It is now recognized that fibre can affect stool weight and transit time through two main mechanisms. First, it may act by increasing lumen volume. This mechanism depends on the amount of indigestible residue in the colon, the water-binding capacity (WBC) of the residue, the stimulation of microbial growth, and the production of gas (Cummings, 1986). The resulting increase in stool volume may cause colonic propulsion through excitation of enteric mechanoreceptors, thus decreasing transit time. Second, fibre may act directly on colonic motility either by mechanical stimulation of submucosal neural receptors (Cherbut & Ruckebusch, 1985) or by an effect of fermentation end-products, e.g. short-chain fatty acids (S...