Measurement of the rate of assimilation of oligo- and polysaccharides by 13CO2 breath tests and isotope ratio mass spectrometry.

13CO2 breath tests were developed to study the digestion and absorption of (13C)lactose, (13C)sucrose, (13C)starch and (13C)glucose using naturally enriched substrates. Measurements of 13CO2 breath excretion were done in steady state conditions in order to study the assimilation rate of these carbohydrates as reflected by their oxidation rate. The reproducibility of the tests in the same individual was excellent. The excretion rate of 13CO2 after administration of (13C)glucose and (13C)sucrose was identical, while the excretion rate of 13CO2 after lactose was somewhat slower, although not significantly. In contrast, the excretion of 13CO2 after 50 g (13C)starch was significantly slower than the 13CO2 excretion after 50 g (13C)glucose or 50 g (13C)disaccharides. As the monosaccharide constituents used in this study have comparable absorption rates, the delayed excretion of 13CO2 after a (13C)starch load must be due to a slower digestion. It is concluded that (i) digestion and not monosaccharide transport is the rate-limiting step in the assimilation of polysaccharides and (ii) that the present breath tests are suitable to study the digestibility of naturally labelled carbohydrates.