Certain mutations in the sucrase-isomaltase (SI) gene cause CSID/genetic sucrase deficiency. The SI gene provides instructions for producing the sucrase-isomaltase enzyme. This enzyme is found in the brush border of the small intestine and is responsible for breaking down sucrose and maltose into their simple sugar components. Once these simple sugars are broken down, they are absorbed by the small intestine. Genetic mutations that cause CSID alter the structure, disrupt the production, or impair the function of sucrase-isomaltase. These changes prevent the enzyme from breaking down sucrose and maltose, causing the gastrointestinal symptoms.
CSID is a rare, inherited disorder that is not outgrown. Congenital means it was “present at birth,” and the deficiency is not acquired due to infection or other external influence. Both parents must carry the mutated recessive gene for a child to be born with the disease. Proper sucrose digestion requires production of new sucrase-isomaltase enzymes each time food is consumed. CSID patients lack the ability to produce sucrase-isomaltase and, thus, CSID is a chronic disorder that will never be outgrown.