Comparison of lactulose and inulin as a reference standard for the study of resistant starch fermentation using hydrogen breath test

Disaccharide lactulose is commonly used as a standard to quantitate the colonic fermentation of undigested sugars by means of H2 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 H2 breath studies of starch digestibility. To test this hypothesis, we measured H2 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 H2 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 H2 breath excretion per gram of resistant starch was significantly lower than that for both lactulose and inulin (p<0.02), being 4.7 ppm x h x g-1. In vitro fermentation for 8 hrs with fecal homogenate showed similar mean hydrogen production rates for inulin and lactulose (30.5 vs 27.7 mL/mg fermented carbohydrate), and a significantly lower rate for starch (9.1 mL/mg) (n = 7; ANOVA p = 0.0007). After lactulose, the first increase of hydrogen at least 10 ppm over baseline occurred significantly (p<0.05) earlier in a dose-dependent manner, whereas after inulin it remained unaffected. Our findings suggest that inulin does not present advantages over lactulose in the quantitative estimation of colonic fermentation of resistant starch, although it seems to affect orocaecal transit time to a lesser extent.