Starch is a type of complex carbohydrate, which means that it’s made of long chains of sugars called polysaccharides. Foods high in starch, such as potatoes and some other tubers and root vegetables, are not as easily digested as simpler carbohydrates such as fruit, baked goods and pasta. Starch is only partially broken down in the mouth -- most of the digestive changes that happen to starch occur in your intestines.
Saliva is a specialized fluid secreted from salivary glands near your mouth and tongue. Saliva is made primarily of water, but it also contains some mucus, electrolytes, antibacterial compounds and substances called enzymes. The enzymes found in saliva are needed to begin the digestion of carbohydrates, including starch, and fats. Saliva also functions as a lubricant, which helps chewed-up food slide down your esophagus and into your stomach, as well as a protectant for the delicate membranes that line your mouth and upper gastrointestinal system. A lack of saliva production, known as xerostomia or dry mouth, can significantly reduce digestive efficiency.
The main enzyme in saliva is called salivary amylase, also known as alpha-amylase or ptyalin. Like all enzymes, the purpose of salivary amylase is to break down larger compounds into smaller parts. More specifically, salivary amylase initiates the digestion of starch by breaking chemical bonds and reducing starch to smaller, simpler sugars such as maltose. Salivary amylase cleaves the polysaccharides of starch into the disaccharide maltose, which is made of two molecules of glucose sugar. Salivary amylase is only effective at reducing carbohydrates and doesn’t change fat or protein. Other enzymes are required for fat and protein digestion.
Digestion in the Mouth
The exact percentage of starch digested in the mouth is difficult to estimate because it depends on many factors. For example, a person who chews food thoroughly and secretes lots of salivary amylase into her mouth will break down a higher percentage of starch compared to a person who is quicker to swallow or produces much less saliva. The pH of saliva, which is a measure of its acidity, and the concentration of salivary amylase also vary from person to person.
Final Changes to Starch
Starch continues to be changed in your stomach and small intestine, where it’s metabolized by other forms of amylase, such as gastric amylase and pancreatic amylase. In most people, starch is nearly completely changed into maltose somewhere along the small intestine, at which point the enzyme maltase cleaves into glucose. Glucose is absorbed through the intestines and is deposited directed into your bloodstream, which is why it’s often called blood sugar. In some people with weaker digestion, a significant proportion of undigested starch makes its way to the large intestine, where “friendly” bacteria ferment it and produce carbon dioxide and nitrogen gases. Common symptoms of this process include bloating, abdominal pain, flatulence and diarrhea.
- Textbook of Functional Medicine; David S. Jones
- Human Metabolism: Functional Diversity and Integration; J. Ramsey Bronk
- Human Biochemistry; Charles Dreiling
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.