To evaluate the mechanism whereby the addition of fibre to the diet lowers postprandial plasma glucose levels, the effects of ingestion of 10 g guar gum, a non-absorbable storage polysaccharide, on carbohydrate utilization and absorption were examined in healthy subjects. Total glucose utilization independent of gastrointestinal absorption of glucose was determined by the hyperglycaemic clamp technique in which constant hyperglycaemia is maintained with a variable intravenous infusion of glucose. Net splanchnic glucose uptake was also measured. As compare to a control study without guar, ingestion of guar failed to increase total glucose utilization or the uptake of glucose by splanchnic tissues. In contrast, when guar was ingested together with 25 g D-xylose, a non-metabolizable pentose, blood xylose levels were 25-40% lower than when xylose was ingested alone (P less than 0.01). We conclude that the plasma-glucose-lowering effects of high-fibre diets are a consequence of decreased carbohydrate absorption rather than increased total glucose utilization or augmented uptake of glucose by the liver.