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 (15 g/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.

The exact mechanisms by which non-starch polysaccharides increase stool output are unknown. In the present study the hypothesis that the site of fermentation and short-chain fatty acid (SCFA) accumulation is related to the action of non-starch polysaccharides (NSP) on stool output was tested. The basal diet (45 g NSP/kg) of forty-three male Wistar rats was supplemented with 50 g/kg of either guar, karaya, tragacanth, gellan, xanthan or ispaghula for 28 d. A further twenty-three rats were maintained on the basal diet for the same time period. Faeces were then collected over 2 d and caecal contents obtained post-mortem. Caecal and faecal wet and dry weights and SCFA were measured. Each supplement had a different effect on the caecal and faecal contents but they appeared to fall into three groups when compared with the basal diet. In group 1, guar gum affected only caecal SCFA. It had no effect on stool output or faecal SCFA. In group 2, karaya increased caecal SCFA and tragacanth, karaya and xanthan increased faecal SCFA and faecal water. In group 3, ispaghula and gellan had no consistent effect on caecal or faecal SCFA concentrations but increased Total faecal SCFA output and increased faecal wet and dry weight. Although the knowledge that SCFA are rapidly absorbed in the large intestine has led us to believe that they play no role in determining faecal output, these results suggest that in some cases where NSP are slowly fermented, and increase faecal SCFA, the role of the SCFA may need to be reassessed.