Enzymatic Digestion of Starches That Happens in the Small Intestines

Potatoes are high in starch.

Potatoes are high in starch.

Starch is not just for making your shirts crisp; it is also a type of carbohydrate commonly found in foods such as potatoes, rice and grains. Potatoes and some other root vegetables are the richest sources of starch, whereas baked goods and pasta contain much less. Starch digestion begins in your mouth, although the majority of enzymatic action takes place in the small intestine.

Initial Digestion of Starch

All carbohydrate digestion starts in the mouth with the secretion of the enzyme alpha-amylase into saliva. Alpha-amylase, sometimes also called salivary amylase, starts to reduce starch from a complex sugar into less complex sugars such as maltose. By the time starch enters the stomach, alpha-amylase is deactivated by the acidity of the gastric juice and no longer works. In the stomach, small amounts of gastric amylase continue breaking down the starch, although the majority of the digestion takes place in the small intestine.

Digestion in the Duodenum

The duodenum is the name for the first foot or so of the small intestine. As the partially digested starch enters the duodenum, a hormone called secretin is immediately secreted from the intestinal walls to trigger the release of alkaline bicarbonate solution from the pancreas. Bicarbonate neutralizes the stomach acid that’s mixed with the starch. Once neutralized, another hormone called cholecystokinin is secreted from the intestinal wall, which further triggers the pancreas to release digestive enzymes. Intestinal amylase and pancreatic amylase fully break down the starch into maltose, which is then reduced by the enzyme maltase into a simple sugar called glucose. One molecule of maltose is cleaved to become two molecules of glucose.

Absorption in the Jejunum and Ileum

The jejunum is the name for the second part of the small intestine, and it has a lining that is specialized for absorbing sugars from carbohydrates and amino acids from protein. The ileum is the final segment, and it also absorbs nutrients. Both segments have many small involutions or finger-like projections called villi, which are connected to small blood vessels. Villi increase the surface area of the small intestine. Consequently, glucose and other nutrients are readily absorbed through the villi and are quickly deposited into the bloodstream. The end product of starch metabolism is glucose, which is the sugar of the blood that every cell needs for fuel to do their jobs.

Digestion in the Large Intestine

Sometimes starchy carbohydrates, such as potatoes and beans, are not well digested in the small intestine. Reasons for this include not enough acidity in the stomach and/or reduced enzyme secretions in the duodenum. Once in the large intestine, friendly bacteria further break-down the partially digested starch via fermentation. Bacterial fermentation produces gas, which leads to bloating and flatulence. Thus, gas production is usually a sign that your digestive process is not as good as it could be. Consult your doctor about causes of poor digestion.

 

References

  • Encyclopedia of Human Nutrition; Benjamin Caballero et al.
  • Human Metabolism: Functional Diversity and Integration; J. Ramsey Bronk
  • Textbook of Functional Medicine; David S. Jones

About the Author

Sirah Dubois is currently a PhD student in food science after having completed her master's degree in nutrition at the University of Alberta. She has worked in private practice as a dietitian in Edmonton, Canada and her nutrition-related articles have appeared in The Edmonton Journal newspaper.

Photo Credits

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