My mother always used to tell me to chew my food. Did she know that salivary alpha-amylase was hydrolyzing carbohydrates in my mouth? She probably had a good idea. Still, I swallow too quickly. So little gets digested and the food ends up in my stomach.
Alpha-amylase can't work in the stomach because the pH is too acidic (1p69). So digestion of carbohydrates finally continues when it enters the duodenum (1p69). Here stomach acid is neutralized (1p45) and alpha-amylase enters again secreted from the pancreas (1p69). Around half of carbohydrates are digested in the duodenum (1p48).
Although salivary amylase doesn't do much in our mouth, the reason it's there may give us clues in its role in evolution of human digestion. In fact, for breaking up starches quickly to obtain glucose, alpha-amylase is used for digestion by almost the entire animal kingdom, including amoebas and sponges (2). Biologists are now studying the how amylase genes are copied to find out more (3).
1. Gropper SS, Smith JL, Groff JL. Advanced Nutrition and Human Metabolism. Belmont, CA: Thomson Wadsworth, 2009.
2. Da Lage JL, Danchin EG, Casane D. Where do animal alpha-amylases come from? An interkingdom trip. FEBS Lett 2007;581:3927-35.
3. Myers PZ. Amylase and human evolution. Pharyngula. Available at: http://scienceblogs.com/pharyngula/2008/12/amylase_and_human_evolution.php.