Disaccharide lactulose is commonly used as a standard to quantitate the colonic fermentation of undigested sugars by means of H-2 breath measurements. However, its high hydrogen production rate during fermentation may make it inappropriate for mimicking the fermentation of more complex carbohydrates, such as starch, Undigestible carbohydrates with a higher molecular weight might be more suitable than lactulose as a standard in H-2 breath studies of starch digestibility, To test this hypothesis, we measured H-2 breath in 8 healthy volunteers after a standard meal supplemented with 5g or 10g of lactulose or inulin, an undigestible oligosaccharide with an average degree of polymerization 4.5 times higher than that of lactulose, The results were then compared with those obtained after a standard meal containing a known amount (6.1 g) of resistant starch from high-amylose corn starch, Median H-2 breath excretion per gram of reference carbohydrate was lower after the 5g dose of inulin than after the 5g dose of lactulose (19.1 vs 26.6 ppm x h x g(-1); Wilcoxon's rank test p = 0.021) but similar after the two 10 g doses (inulin 22.4; lactulose 23.6; p = 0.234), Median Hz breath excretion per gram of resistant starch was significantly lower than that for both lactulose and inulin (p