Understanding in vivo digestion of starch in the small intestine.

Starch is the major energy source in food and feed. In humans and animals, starch is digested mostly in the small intestine. The mechanism, however, is still poorly understood, since in vivo digesta are usually isolated from the terminal ileum, colon, and feces, and never from the small intestine. The objective of this study was to understand the mechanism of in vivo starch digestion in the small intestine and compared it with in vitro digestion. In vivo digesta were isolated from the small intestine of pigs fed with diet containing 51% maize starch. The small intestine was divided into four sections, and the digesta were labeled as SI-1, -2, -3, and -4 from the stomach end to the cecum end. The amount of starch in each section was analyzed using the Megazyme total starch assay kit. The morphology of isolated starch granules was studied using scanning electron microscopy. Starch structural analysis was carried out using size exclusion chromatography. The amount of starch in the small intestine decreased as it progressed from SI-1 to SI-4. The starch granules in SI-2 were more severely hydrolyzed than those in SI-1, whereas no starch granules were observed in SI-3. The size distribution of starches in SI-1 and -2 and in vitro digesta showed 2 peaks similar to amylopectin and amylose peaks, whereas that of starch in SI-3 showed several peaks in the smaller molecule/amylose region. The in vitro and in vivo size distributions were qualitatively similar, although there were significant quantitative differences, showing that appropriate in vitro studies can be used to gain realistic information. The insight gained from this study is useful in understanding health issues related to starch metabolism, such as hyperglycemia and diabetes. It also set a platform for studying nutritionally important starches, such as resistant starch and slowly digestible starch.