Sucrose (table sugar) is too large to be absorbed from the small intestine. For this reason, sucrose must be digested to its simpler forms, glucose and fructose, before it can be absorbed into the bloodstream and used as an energy source.3 Sucrase is the only naturally-occurring enzyme in humans that can break sucrose down into its simpler forms.
Starches are even larger compounds than sucrose. However, there are several intestinal enzymes that play a role in the digestion of the starches we eat, including: sucrase-isomaltase, maltase-glucoamylase, and α-amylases
So, if you have CSID and the enzyme sucrase-isomaltase is missing from your small intestine or isn’t working or isn’t working well, you may or may not have GI symptoms after eating foods that contain starches. While there is only one naturally-occurring enzyme (sucrase) in our bodies that digests sucrose, there are multiple enzymes in our bodies that act to digest starches. Because only sucrase can digest sucrose, eating foods that contain sucrose usually causes worse GI symptoms in patients with CSID than eating foods that contain starches because some of those starches may be broken down in part by one of the two enzymes other than sucrase-isomaltase.