The study investigated the effects of various dietary fibres and fibre analogues on glucose tolerance in diabetic volunteers. Volunteers underwent 50-g glucose tolerance tests (GTT) with and without the addition of guar, pectin, gum tragacanth, methylcellulose, wheat bran, or cholestyramine, each equivalent to 12 g of fibre. The results showed that all substances significantly reduced blood glucose and serum insulin concentrations during the GTT. Guar was the most effective in flattening the glucose response, and this effect was not abolished by hydrolysed non-viscous guar. The reduction in the mean peak rise in blood glucose concentration correlated positively with the viscosity of the substance (r = 0.926; P < 0.01), and also with the delay in mouth-to-caecum transit time (r = 0.86; P < 0.02). Viscous dietary fibres are therefore likely to be the most therapeutic in modifying postprandial hyperglycaemia.The study investigated the effects of various dietary fibres and fibre analogues on glucose tolerance in diabetic volunteers. Volunteers underwent 50-g glucose tolerance tests (GTT) with and without the addition of guar, pectin, gum tragacanth, methylcellulose, wheat bran, or cholestyramine, each equivalent to 12 g of fibre. The results showed that all substances significantly reduced blood glucose and serum insulin concentrations during the GTT. Guar was the most effective in flattening the glucose response, and this effect was not abolished by hydrolysed non-viscous guar. The reduction in the mean peak rise in blood glucose concentration correlated positively with the viscosity of the substance (r = 0.926; P < 0.01), and also with the delay in mouth-to-caecum transit time (r = 0.86; P < 0.02). Viscous dietary fibres are therefore likely to be the most therapeutic in modifying postprandial hyperglycaemia.