Sucrose intolerance or genetic sucrase-isomaltase deficiency (GSID) is the condition in which sucrase-isomaltase, an enzyme needed for proper metabolism of sucrose (sugar) and starch (e.g., grains), is not produced or the enzyme produced is either partially functional or non-functional in the small intestine. All GSID patients lack fully functional sucrase, while the isomaltase activity can vary from minimal functionality to almost normal activity. The presence of residual isomaltase activity may explain why some GSID patients are better able to tolerate starch in their diet than others with GSID.