Isomaltase is a digestive enzyme found within the small intestine. Its primary role involves the breakdown of complex sugars and starches. This enzyme helps prepare these carbohydrates for absorption into the bloodstream, where they can then be used by the body for energy. Without proper isomaltase activity, certain dietary sugars cannot be processed efficiently.
The Function of Isomaltase in Digestion
Isomaltase functions as part of the sucrase-isomaltase enzyme complex. This complex is located on the brush border of the small intestine, a surface composed of millions of tiny, finger-like projections called microvilli. This extensive surface area allows for effective nutrient absorption. The sucrase-isomaltase complex, anchored to the intestinal cell membrane, is important for the final stages of carbohydrate digestion.
The isomaltase component breaks down α-1,6 glycosidic bonds, a type of chemical linkage found in certain carbohydrates. Its primary substrates include isomaltose and alpha-limit dextrins, which are branched fragments of starch that remain after initial digestion by other enzymes like amylase. By cleaving these specific bonds, isomaltase converts these larger carbohydrate molecules into simpler glucose units, making them available for absorption.
Congenital Sucrase-Isomaltase Deficiency
Congenital Sucrase-Isomaltase Deficiency (CSID) is a genetic disorder stemming from mutations in the SI gene, which provides instructions for producing the sucrase-isomaltase enzyme. This condition is inherited in an autosomal recessive pattern, meaning an individual must inherit two altered copies of the SI gene, one from each parent, to develop the disorder. Parents who carry one altered gene usually do not show symptoms themselves.
Genetic variations in the SI gene can disrupt the enzyme’s structure, impair its production, or prevent its proper localization on the intestinal cell surface. When the sucrase-isomaltase enzyme is absent or has significantly reduced activity, certain sugars like sucrose (table sugar) and maltose (found in grains) cannot be broken down efficiently into their simpler components. Instead of being absorbed in the small intestine, these undigested sugars pass into the large intestine. In the large intestine, normal bacteria ferment these sugars, leading to various gastrointestinal issues.
Symptoms and Diagnostic Methods
When undigested sugars reach the large intestine in individuals with CSID, they draw water into the bowel and are fermented by gut bacteria, leading to a range of gastrointestinal symptoms. Common manifestations include chronic, watery diarrhea, abdominal bloating, excessive gas production, and abdominal pain. In infants, symptoms often appear after weaning when foods containing sucrose or starch are introduced, and can include poor weight gain or failure to thrive. Symptom severity can vary based on residual enzyme activity and the amount of sugar and starch consumed.
Diagnosis of CSID involves specific tests. The carbon-13 (13C-sucrose) breath test is a non-invasive method that measures carbon dioxide in breath samples after ingesting a labeled sucrose solution. An abnormally low level of labeled carbon dioxide indicates impaired sucrose digestion. The definitive diagnostic standard is an upper endoscopy with a biopsy of the small intestine. This procedure allows for direct measurement of sucrase and isomaltase enzyme activity in the tissue samples. Genetic testing for known mutations in the SI gene also offers a less invasive diagnostic alternative.
Treatment and Dietary Management
Managing Congenital Sucrase-Isomaltase Deficiency primarily involves dietary adjustments to limit the intake of sugars that cannot be properly digested. This involves eliminating or strictly limiting sucrose (table sugar) from the diet, including sweetened beverages, many fruits, and processed foods containing added sugars. Depending on the individual’s tolerance, restriction of starches, such as those found in grains and potatoes, may also be necessary, especially since isomaltase activity can vary.
Enzyme replacement therapy offers a medical treatment option to help digest sucrose. Sacrosidase, marketed under the brand name Sucraid, is an FDA-approved oral solution that provides a functional sucrase enzyme. This therapy breaks down sucrose into glucose and fructose, allowing these simpler sugars to be absorbed. While sacrosidase effectively addresses the sucrase deficiency, it does not provide specific replacement for deficient isomaltase activity. Therefore, individuals using sacrosidase may still need to manage their starch intake based on their symptoms.