Your Digestive System

Sugar (Sucrose) and Starch

Patients with GSID have varying amounts of the enzymes required for the digestion of sugar (sucrose) and starch

Sugar (Sucrose)

Sucrose, commonly known as sugar or table sugar, consists of one molecule of glucose and one molecule of fructose. Polysaccharides are sugars similar to sucrose that consist of three or more molecules, and are joined by a chemical link called a glycosidic bond, which binds carbohydrates together. Although glucose and fructose are digested, absorbed, and metabolized separately, they both result in the same end product which your body uses for energy.

Digestion

Once sugar reaches the small intestine, sucrose digestion begins. The small intestine is lined with finger-like projections called microvilli, and are known collectively as the brush border. The microvilli absorb nutrients from food as it passes through the small intestine. Your body cannot absorb polysaccharides as is, so it must first break sucrose down into its component parts. Through a process called hydrolysis, water assists in breaking the glycosidic bond to separate the glucose and fructose molecules. One molecule of water is needed for each molecule of sucrose. This reaction naturally occurs very slowly. Sucrase, an enzyme in the small intestine, helps accelerate this reaction.

Absorption

As separate monosaccharides, or one-molecule sugars, glucose and fructose are free for absorption. Both glucose and fructose enter the hepatic portal system by permeating the lining of the intestines via distinct complex transporters. The hepatic portal system is unique because rather than returning blood directly to the heart like most venous systems, it carries its blood and absorbed nutrients like glucose and fructose directly to the liver where they may be immediately processed and utilized. This special system is one of only three in the entire body that does not return blood directly to the heart. Rather, this system carries its blood that contains all absorbed nutrients to the liver where further processing occurs.

Metabolism

Glucose is the primary source of energy. After leaving the liver, glucose travels to nearly every cell in the body, where the hormone insulin assists in the process of assimilation in cells. Next, a process called glycolysis occurs. Glycolysis is the metabolic breakdown of glucose, which is converted to pyruvate, an acidic compound. Pyruvate then enters into either aerobic or anaerobic respiration to produce energy. The metabolism of fructose occurs through fructolysis, a process similar to, but more complex than, glycolysis. Fructolysis results in glucose-like products that can enter respiration for energy production. Fructolysis occurs primarily in the liver, whereas glycolysis can take place in nearly all tissues.

Storage

When glucose is not immediately used for energy, it undergoes a process called glycogenesis, which links individual glucose sub-units into long chains through a chemical bond. Glycogen is then stored in the liver and muscles, and then hydrolyzed (broken down by a chemical reaction with water) back into glucose as needed, usually between meals, and/or when you are sleeping and blood sugar levels are low. According to “Human Physiology,” after your body reaches its glycogen storage capacity, it converts all remaining glucose into fat. Fructose, however, is not stored long-term in the body because the liver metabolizes all fructose to glucose-like molecules.

Complications

A deficiency in the sucrase enzyme, called Genetic Sucrase-Isomaltase Deficiency, can impede sucrose digestion and absorption. Individuals who have this genetic disorder produce little to no sucrase to hydrolyze (break) sucrose into glucose and fructose. This deficiency allows sucrose to pass in an undigested form through the intestines, serving as fuel for the naturally occurring bacteria. The bacterial metabolism symptoms that result are excessive gas, bloating, cramping, abdominal pain, constipation, and diarrhea. The lack of glucose absorption decreases energy production and disrupts thousands of daily biochemical processes, which can affect physical growth and development.

Starch

Many patients with GSID don’t have the necessary components needed for starch digestion, which includes functional salivary amylase, and pancreatic amylase. Problems digesting starch begin in the final digestive stages with the decreased sucrase-isomaltase and maltase-glucoamylase enzymes (brush border enzymes). 60-80% of starch digestion in the small intestine is accounted for by sucrase-isomaltase, and 20-40% is accounted for by maltase-glucoamylase, coupled with the ability of bacteria in the colon to break down starches. Patients with GSID may be able to gradually tolerate more starches as they get older. Patients should consult their healthcare provider(s) to help determine how much starch they can tolerate.

Digestion

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

Absorption

In the small intestine, starch is processed by the enzyme pancreatic amylase, and converted into maltose. Maltose must be broken down into simple sugars for the body to use as fuel. A person suffering with GSID may not be able to efficiently break down maltose since he/she lacks some of the digestive properties of the brush border enzymes. There are normally numerous brush border enzymes (sucrase, lactase, maltase) found within the lining of the small intestine whose function is to further break down digested food into small, more absorbable particles.

Metabolism

Maltase-glucoamylase (MGAM) and isomaltase are brush border enzymes that work in the final steps of small intestinal digestion of starch to glucose. 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 then transports it throughout the body to provide energy to all cells, particularly the brain. Glucose may be stored in the liver if it is not immediately used.

Tolerance

Many patients with GSID have the necessary components needed for early starch digestion, which includes functional salivary amylase, and pancreatic amylase. Problems digesting starch begin in the final digestive stages with the decreased sucrase-isomaltase. Patients with GSID may be able to gradually tolerate more starches as they get older. Patients should consult their healthcare provider(s) to help determine how much starch they can tolerate.