Starch, a complex carbohydrate found abundantly in plant-based foods like grains, potatoes, and legumes, serves as a primary source of energy for the human body. This large molecule is essentially a long chain of glucose units linked together, representing how plants store their energy. Humans can break down this complex structure. Complete digestion requires a coordinated sequence of mechanical actions and specific enzymatic steps to break the starch down into its simple sugar components.
Starch Breakdown Begins in the Mouth
Starch digestion begins immediately upon eating, involving both mechanical and chemical actions in the oral cavity. Chewing breaks large food particles into smaller pieces and mixes them with saliva. This mixing introduces the first digestive enzyme, salivary alpha-amylase, which starts the chemical breakdown of the starch molecule. Salivary amylase begins cleaving the long starch chains into smaller fragments, primarily producing the disaccharide maltose and various shorter chains known as dextrins. This enzyme functions optimally at the neutral pH of saliva. However, its action is short-lived; once the food bolus is swallowed and enters the stomach, the highly acidic environment quickly denatures the enzyme. This inactivation halts the initial starch digestion, meaning only a small fraction of the total starch is broken down before the mixture moves into the small intestine.
Enzymatic Action and Absorption in the Small Intestine
The most complete phase of starch digestion occurs in the duodenum, the first part of the small intestine. Once the acidic contents from the stomach enter this area, the pancreas releases pancreatic juice containing bicarbonate to neutralize the acid, along with pancreatic amylase. Pancreatic amylase continues the work of its salivary counterpart, efficiently breaking down the remaining starch into maltose, maltotriose, and short oligosaccharides, including limit dextrins. The final step of hydrolysis takes place directly on the surface of the small intestinal cells, which are lined with microscopic projections called the brush border. Enzymes embedded in this brush border, such as maltase, sucrase-isomaltase, and glucoamylase, are responsible for cutting the final molecular bonds. Maltase splits the maltose molecules into two individual glucose molecules. Once the starch is fully converted into single glucose units, these simple sugars are absorbed through the intestinal wall and transported into the bloodstream. This glucose then circulates to fuel the body’s cells, making the small intestine the site where the body gains the energy stored in starch.
Starch That Resists Digestion
Not all starch consumed is broken down and absorbed in the small intestine; a portion, termed resistant starch (RS), bypasses this process entirely. RS resists enzymatic digestion due to structural factors, such as being physically trapped within food cell walls or having a crystalline structure that is inaccessible to amylase. This undigested carbohydrate continues its journey to the large intestine, where it acts similarly to dietary fiber. Upon reaching the large intestine, resistant starch is fermented by the resident gut microbiota. These beneficial bacteria break down the RS, using it as a food source. A significant outcome of this fermentation is the production of short-chain fatty acids (SCFAs), predominantly acetate, propionate, and butyrate. Butyrate serves as a primary energy source for the cells lining the colon, helping to maintain a healthy gut barrier.
External Factors Affecting Digestibility
The extent of starch digestion is influenced by how food is prepared and individual genetic differences. Cooking starch through heat and moisture, a process known as gelatinization, significantly increases its digestibility. Gelatinization causes the starch granules to swell and rupture, making the structure more open and easily accessible to amylase enzymes. Conversely, cooling cooked starchy foods, such as rice or potatoes, can increase the amount of resistant starch through retrogradation. During cooling, the starch molecules realign themselves into a structure that is less susceptible to breakdown. Individual efficiency in starch digestion also varies due to genetics, specifically the copy number of the AMY1 gene, which codes for salivary amylase. People with a higher number of AMY1 gene copies tend to produce more salivary amylase, which may improve the initial digestion of starchy foods.