Carbohydrates are broken down by a series of enzymes that work in stages, starting in your mouth and finishing in your small intestine. The process begins with amylase, which chops long starch chains into smaller fragments, and ends with a group of enzymes lining the intestinal wall that split those fragments into single sugar molecules your body can absorb. Each enzyme has a specific job and a specific location in the digestive tract.
Salivary Amylase: Digestion Starts in Your Mouth
The moment you start chewing, glands in your mouth release salivary amylase into the food. This enzyme targets starch, the main complex carbohydrate in foods like bread, rice, potatoes, and pasta. Salivary amylase breaks the chemical links between glucose units in starch chains, producing smaller sugar fragments. This is why a piece of bread starts tasting slightly sweet if you chew it long enough.
Food doesn’t stay in your mouth for long, so salivary amylase only gets a head start on digestion. Once you swallow and the food reaches your stomach, the acidic environment deactivates the enzyme. The real heavy lifting happens further down.
Pancreatic Amylase: The Main Event
When partially digested food moves from the stomach into the first section of the small intestine (the duodenum), the pancreas releases a flood of pancreatic amylase. This enzyme does the same basic job as salivary amylase but on a much larger scale, breaking starch down into maltose and short chains of glucose called maltooligosaccharides. Pancreatic amylase is considered the key enzyme in starch digestion because it handles the bulk of the work.
The small intestine gives these enzymes plenty of contact time. The median transit time through the small bowel is about 4.6 hours, giving pancreatic amylase and the enzymes that follow ample opportunity to finish the job. But amylase alone can’t complete digestion. It leaves behind two- and three-sugar fragments that need one more round of processing before your body can absorb them.
Brush Border Enzymes: The Final Step
The inner surface of your small intestine is covered in tiny finger-like projections, and the outer membrane of those projections (called the brush border) is packed with enzymes that perform the last stage of carbohydrate digestion. These enzymes split the remaining sugar fragments into individual molecules small enough to cross the intestinal wall. There are four main players, and each one targets a different sugar.
Maltase-Glucoamylase
This enzyme has two active sites. One breaks maltose (a two-glucose fragment) into individual glucose molecules. The other handles slightly more complex fragments, including those with branching points where glucose units connect at different angles. Together, these two sites mop up the majority of what pancreatic amylase left behind.
Sucrase-Isomaltase
This is actually a dual-purpose enzyme complex. The sucrase site breaks table sugar (sucrose) into glucose and fructose. It can also break down maltose, providing backup for maltase. The isomaltase site, sometimes called alpha-dextrinase, handles a specific job that no other enzyme can do well: it cleaves the branching points in starch fragments called limit dextrins. Without isomaltase, those branched pieces would pass through undigested.
Lactase
Lactase breaks lactose, the sugar in milk and dairy products, into glucose and galactose. What makes lactase unique is that most people on Earth gradually stop producing it after childhood. Roughly 70% of the global population loses lactase activity in adulthood, which is why lactose intolerance is so common. People who continue producing lactase into adulthood carry a genetic trait called lactase persistence, most prevalent in populations with a long history of dairy farming.
What Happens After the Enzymes Finish
Once carbohydrates have been reduced to single sugar molecules (glucose, fructose, and galactose), they still need to cross the intestinal wall and enter your bloodstream. This isn’t passive. Your intestinal cells use specialized transport proteins to pull sugars in.
Glucose and galactose are actively pumped into intestinal cells by a transporter called SGLT1, which harnesses sodium to drag sugar molecules across the membrane. This is an energy-dependent process, meaning your body spends resources to absorb these sugars efficiently even at low concentrations. Fructose, on the other hand, enters through a different transporter called GLUT5, which works by simple diffusion and is specific to fructose. Once inside the cells, all three sugars exit through another transporter on the opposite side of the cell and enter the bloodstream.
This transport system is one reason glucose from starchy foods enters your blood relatively quickly. The combination of efficient enzymatic breakdown and active transport means that a meal heavy in refined starch can raise blood sugar faster than one containing sugars that require slower processing.
Fiber: The Carbohydrate Your Enzymes Can’t Touch
Not all carbohydrates are digestible. Dietary fiber, found in vegetables, whole grains, legumes, and fruits, is made of carbohydrate chains that human enzymes simply cannot break apart. The chemical bonds holding fiber together are different from those in starch, and we lack the enzymes needed to cut them.
Bacteria in your large intestine pick up where your own enzymes leave off. These microbes produce specialized enzymes called glycoside hydrolases and polysaccharide lyases, two classes of enzymes that don’t exist in the human body. Through fermentation, gut bacteria break fiber down into short-chain fatty acids, which nourish the cells lining your colon and contribute to overall gut health. This is why fiber, despite being “indigestible” by your own enzymes, still has significant nutritional value.
The Full Sequence at a Glance
- Mouth: Salivary amylase begins breaking starch into smaller fragments.
- Stomach: Acid stops amylase activity. No significant carbohydrate digestion occurs here.
- Duodenum: Pancreatic amylase breaks starch into maltose and short sugar chains.
- Small intestine lining: Maltase, sucrase, isomaltase, and lactase split remaining sugars into single molecules (glucose, fructose, galactose) for absorption.
- Large intestine: Bacterial enzymes ferment fiber that human enzymes couldn’t digest.
The entire process is sequential and location-specific. Each enzyme works on a particular type of chemical bond, and each one operates in a specific stretch of the digestive tract. When any one enzyme is missing or underproduced, as with lactase in lactose intolerance, the corresponding sugar passes through undigested and gets fermented by gut bacteria instead, producing the gas, bloating, and discomfort that follow.