Digesting Sucrose and Starch
Individuals with Sucrose Intolerance Have Varying Amounts of the Enzymes Required for the Digestion of Sucrose and Starch
Sucrose is a relatively large sugar molecule made by plants by combining two smaller sugars called glucose and fructose. Sucrose is harvested from sugar cane or sugar beets and is commonly referred to as white sugar, brown sugar, table sugar or just “sugar.” Your body cannot absorb sucrose directly because it is too large to pass into your cells without first being digested by sucrase-isomaltase. The term “saccharide” means sugar, and sucrose is a disaccharide because it has two sugar molecules – glucose and fructose.
Once sugar reaches the small intestine, digestion begins. The small intestine is lined with finger-like projections called “microvilli” that are known collectively as the “brush border.” The microvilli digest and absorb nutrients from food as it passes through the small intestine. Your body cannot absorb disaccharides or polysaccharides, so it must first break sucrose and starch down into its smaller monosaccharide components. This breakdown process is called “hydrolysis” because water is used to assist in the breakdown of the glycosidic bonds linking the glucose and fructose molecules in sucrose. One molecule of water is needed to break down one molecule of sucrose. Sucrase, an enzyme in the small intestine, accelerates this reaction, which will occur very slowly without this enzyme.
Sucrase-isomaltase works in combination with other enzymes called alpha-amylase and maltase-glucoamylase to breakdown large, complex starch polysaccharides into single molecules of glucose for absorption. Depending on the type of starch and how complex it is, sucrase-isomaltase may or may not be necessary for digestion. This is why individuals with CSID may or may not have symptoms after eating starch.
As single monosaccharides, or one-molecule sugars, glucose and fructose are free to be absorbed into the bloodstream. Once absorbed, both glucose and fructose enter blood vessels called the hepatic portal system — the system responsible for directing blood from parts of the gastrointestinal tract to the liver — by permeating the lining of the intestine via transporters. The hepatic portal system is unique because rather than returning blood directly to the heart like most venous systems, it carries absorbed nutrients in its blood, including glucose and fructose, directly to the liver where these nutrients can be immediately processed for use. This special venous system is one of only three in the entire body that do not return blood directly to the heart.
Glucose is a primary source of energy. After leaving the liver, glucose travels to nearly every cell in the body, where the hormone insulin assists in its uptake into cells. Next, a process called “glycolysis” occurs. Glycolysis is the main process that works to make energy from glucose for your cells to perform their everyday functions.
Complications of Sucrose Digestion
A deficiency in the sucrase enzyme, called Congenital Sucrase-Isomaltase Deficiency (CSID), also called Sucrose Intolerance, can impede sucrose digestion and absorption. Individuals who have this genetic disorder produce little to no sucrase to hydrolyze (breakdown) sucrose into glucose and fructose. This deficiency allows sucrose to pass undigested through the small intestine, serving as fuel for the naturally occurring bacteria in the large intestine. It is this bacterial metabolism, by fermentation, that results in excessive gas, bloating, cramping, abdominal pain, and diarrhea. This bacterial metabolism also results in a lack of glucose absorption, decreasing energy production and disrupting thousands of daily biochemical processes, which can affect physical growth and development.
In addition to lacking the enzyme sucrase, many individuals with Sucrose Intolerance are also deficient in the enzymes necessary for starch digestion. The sucrase-isomaltase enzymes account for 60% to 80% of starch digestion in the small intestine, while maltase-glucoamylase enzymes, coupled with colonic bacteria, account for only 20% to 40% of starch breakdown.
Starch is an even larger molecule than sucrose, made in plants by combining complex arrangements of glucose molecules in linear and branched structures, much like a very small tree with many limbs and branches at the microscopic level. Starch is a polysaccharide because it contains many molecules (up to tens of thousands) of glucose. Common starches include rice, pasta, grains, bread, potatoes, and baked goods. Many vegetables and legumes (beans) contain starch.
While sugar (sucrose) digestion is simple (sucrose is digested by sucrase), starch digestion is very complex and involves multiple enzymes and even “good” bacteria in the colon.
In the small intestine, starch is processed by the pancreatic enzyme amylase into maltose. To be used by the body as fuel, maltose must be further broken down into simple sugars. Persons affected with Sucrose Intolerance may not be able to efficiently break down maltose since they lack some of the digestive enzymes normally present in the brush border, due to an enzyme deficiency. Normally, brush border enzymes such as sucrase, lactase, and maltase-glucoamylase help break down digested food into smaller particles that are easier to absorb.
Maltase-glucoamylase and isomaltase are brush border enzymes responsible for the final steps of starch digestion. Glucose is the end product of all starch digestion. Once starch is broken down into glucose, it is transported through the small intestine into the bloodstream, which then delivers it to cells throughout the body, particularly the brain. Glucose may be stored in the liver if it is not immediately used.
Tolerance of Starches
In most individuals with Sucrose Intolerance, the enzymes salivary amylase and pancreatic amylase function normally. These enzymes are necessary for the initial phase of starch digestion at the beginning of the transit through the gastrointestinal tract. For people with Sucrose Intolerance, problems digesting starch begin in the small intestine where the sucrase-isomaltase digestive enzymes are normally present. Some people with Sucrose Intolerance may be able to gradually tolerate more starches as they get older for various reasons. However, they should consult their healthcare providers to help determine how much starch they can tolerate.