BACKGROUND Diets with a low glycemic index (GI) have been shown to improve glucose tolerance in both healthy and diabetic subjects. Two potential mechanisms are discussed in relation to long-term metabolic effects: a decreased demand for insulin in the postprandial phase and formation of short-chain fatty acids from fermentation of indigestible carbohydrates in the colon. OBJECTIVE The objective was to study the effect of the GI and the indigestible carbohydrate--resistant starch (RS) and dietary fiber (DF)--content of cereal-based breakfasts on glucose tolerance at a second meal (lunch) in healthy subjects. METHODS The effects of 7 test breakfasts with known GIs (GI: 52-99) and RS + DF contents (2-36 g) were evaluated. White-wheat bread was used as a reference breakfast (high GI, low RS + DF content). Glucose and insulin responses after the second meal were measured in healthy subjects. In addition, the satiating capacity of 4 of the 7 test breakfasts was estimated before and during the second meal. RESULTS Two of the 4 low-GI breakfasts improved glucose tolerance at the second meal. Only these 2 breakfasts were capable of postponing the in-between-meal fasting state. There was no measurable effect of fermentable carbohydrates on glucose tolerance at the second meal. The highest satiety score was associated with the barley breakfast that had a low GI and a high RS + DF content. CONCLUSIONS Glucose tolerance can improve in a single day. Slow absorption and digestion of starch from the breakfast meal, but not the content of indigestible carbohydrates in the breakfast meal, improved glucose tolerance at the second meal (lunch).