Estimates of starch digestion in the rat small intestine differ from those obtained using in vitro time-sensitive starch fractionation assays

The objectives of this study were as follows: 1) to determine the rate and extent of starch disappearance from the small intestine of the rat fed selected starch sources, 2) to determine the ratios of the major starch fractions [rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS)] in those starch sources using two in vitro methods and 3) to compare the two data sets to determine the accuracy of the in vitro methods. Diets were prepared using cornstarch, potato starch, amylomaize, maltodextrin, modified maltodextrin or pullulan. Starch sources and diets were analyzed for starch fractions by two in vitro methods. Diets were fed to rats, intestinal contents were collected and the ethanol-induced precipitate from the contents was analyzed to obtain a digestion curve that was mathematically modeled for comparison to results obtained using the two in vitro methods. Only the cornstarch diet had a defined amount of RDS, SDS and RS. The RDS concentration obtained from the intestinal contents of the rats fed the cornstarch diet differed (P < 0.05) from that determined by one in vitro method but was consistent with the value obtained using the other in vitro method. All other digestible starch values obtained differed (P < 0.05) among methods except for that of amylomaize. Starch fractions in starch sources obtained using in vitro procedures differed (P < 0.05) from values obtained for diets. The rate of disappearance differed (P < 0.05) between in vivo and in vitro procedures. There was minimal agreement between in vitro methods tested, and there was also minimal agreement between in vitro and in vivo results. Classification of starch into RDS and SDS components cannot be accomplished for a variety of starch sources, with cornstarch being the major exception.