What Is Congenital Sucrase-Isomaltase Deficiency?

Congenital Sucrase-Isomaltase Deficiency (CSID) is a rare disorder that affects a person’s ability to digest certain sugars due to absent or low levels of two digestive enzymes, sucrase and isomaltase. Sucrase and isomaltase are involved in the digestion of sugar and starch. Sucrase is the intestinal enzyme that aids in the breakdown of sucrose (table sugar) into glucose and fructose, which are used by the body as fuel. Isomaltase is one of several enzymes that help digest starches.


CSID is also known as genetic sucrase deficiency, SI Deficiency, Disaccharide Intolerance I, and Congenital Sucrose Intolerance. This disorder occurs equally as frequent in males and females.






Sucrose (a sugar found in fruits, and also known as table sugar) and maltose (a sugar found in grains) are called disaccharides because they are made of two simple sugars. Disaccharides are broken down into simple sugars during digestion by intestinal enzymes. Sucrose is broken down into glucose and fructose, and maltose is broken down into two glucose molecules. A person with CSID has an impaired ability to break down these disaccharides (sucrose and maltose) into simpler forms of sugar called monosaccharides (glucose and fructose) that the body uses as fuel. People with CSID may also have difficulty breaking down compounds made from these sugar molecules (carbohydrates or starches). Typically, a patient with CSID has decreased (often absent) sucrase activity, decreased to normal isomaltase activity, and reduced maltase activity.


Although simple sugars are the main carbohydrate absorbed in the digestive tract, monosaccharides (glucose, fructose, and galactose) are rarely found in human diets. Instead, disaccharides (sucrose, lactose, and maltose) are more common in the human diet and must be broken down into monosaccharides by small intestinal enzymes before absorption into the bloodstream for use by the body. Without this breakdown, none of these disaccharides can be used for energy because they are too big and are not in a simple sugar form to be absorbed by the intestinal wall. The small intestine is lined with finger-like projections called microvilli. The microvilli are known collectively as the brush border, based on their appearance. The microvilli absorb nutrients from food as it passes through the small intestine.



  • Enzymes are proteins that catalyze (speed up) chemical reactions in the body.

  • The molecules that are affected by enzymes are called substrates.

  • Substrate molecules temporarily bond with the enzyme at the active site to form an enzyme/substrate complex.

  • The enzyme bends the substrate in a way that breaks the molecule apart.

  • The result of an enzymatic reaction is a product.




Patients with CSID have varying amounts of the enzymes required for starch digestion (isomaltase, palatinase, maltase), so these patients may also experience gastrointestinal symptoms from starch consumption.


Carbohydrates are classified as simple or complex. Classification depends on the chemical structure of the food, and how quickly the sugar is digested and absorbed. Simple carbohydrates have one sugar (monosaccharides) or two sugars (disaccharides). Complex carbohydrates have three or more sugars linked together. Many complex carbohydrates are starchy carbohydrates, which require ample time and adequate enzymatic activity in the digestive tract to be digested properly. They are known as polysaccharides, because they contain many sugars linked together. Starch digestion is dependent on the combination of salivary, pancreatic, and intestinal brush border enzyme activities.


The digestion of starches begins in the mouth with an enzyme in saliva called salivary amylase. Chewing grinds the food and breaks it apart allowing salivary amylase to work more effectively. Salivary amylase is sometimes called ptyalin. After swallowing, the starchy carbohydrates reach the stomach where gastric enzymes begin to break down food. The stomach plays a major role in digestion by mechanically mixing and crushing the food and enzymatically by further breaking down food. The stomach also acts as a reservoir for emptying out smaller amounts of food into the small intestine. The pancreas helps control glucose metabolism and secretes digestive/pancreatic juice that further digests starches.


In the small intestine, starch is processed by an enzyme called pancreatic amylase and converted into maltose and sucrose. Maltose and sucrose must be broken down into simple sugars for the body to use as fuel. A person with CSID may have difficulty breaking down maltose since he/she lacks some of the digestive properties of the brush border enzymes. Throughout the lining of the small intestine there are normally numerous brush border enzymes (sucrase, lactase, maltase) whose function is to further break down digested food into small, more absorbable particles.


Maltase-glucoamylase (MGAM) is a brush border enzyme that plays a role in the final steps of small intestinal digestion of starch to glucose. Isomaltase is an enzyme that breaks the bonds linking sugars, which cannot be broken by amylase or maltase. Glucose is the end product of all starch digestion. After the starch is broken down into glucose, the small intestine transports the glucose into the bloodstream, which is transported throughout the body to provide energy to all cells, particularly the brain. If the glucose is not immediately used, it could be stored in the liver for later use.


Many patients with CSID have the necessary components needed for starch digestion which includes functional salivary amylase and pancreatic amylase. Problems digesting starch arise with the decreased sucrase-isomaltase and maltase-glucoamylase (brush border enzymes) in the final digestive stages. Approximately 60-80% of starch digestion in the small intestine is accounted for by sucrase-isomaltase and 20-40% is accounted for by maltase-glucoamylase. Due to maltase-glucoamylase activity combined with the ability of bacteria in the colon to break down starches, patients with CSID may be able to gradually tolerate starches much better as they get older. Patients should consult their healthcare provider(s) to help determine how much starch they can tolerate.