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. Signs and symptoms
Abdominal cramps and bloating
Diarrhea and constipation
Vomiting
Hypoglycemia and headaches
Poor weight gain and growth
Upper respiratory tract and viral diseases
Anxiety and heart palpitations
Excess gas production

Cause
Sucrose intolerance can be caused by genetic mutations in which both parents must contain this gene for the child to carry the disease (so-called primary sucrose intolerance). Sucrose intolerance can also be caused by irritable bowel syndrome, aging, or small intestine disease (secondary sucrose intolerance). There are specific tests used to help determine if a person has sucrose intolerance. The most accurate test is the enzyme activity determination, which is done by biopsying the small intestine. This test is a diagnostic for GSID.
If the cumulative amount of 13CO2 exhaled is below the normal level that occurs when sucrose is digested by sucrase, the person may have CSID and the person should undergo further examination by their doctor.The findings from a 13C-breath test are believed to be more definitive for CSID than the hydrogen/methane breath test although neither test is validated to provide, by themselves, a diagnosis for CSID. In both breath tests, the consumption of sugar may cause severe gastrointestinal symptoms in those who have CSID. For this reason, these breath tests should be conducted under the supervision of a healthcare professional. A relatively new, noninvasive test that shows promise is a genetic test for the gene SI, which codes for the enzyme sucrase-isomaltase. This test requires a swab of tissue from the inside of the cheek. So far, 37 aberrant variations of the SI gene have been found in patients who have been diagnosed with CSID.A positive genetic test for one or more of the 37 SI gene variations known to cause CSID can help confirm a diagnosis of CSID. However, a negative genetic test cannot rule out a diagnosis of CSID. There are more than 2,000 different variations of the SI gene, and many of these variations have not yet been investigated for their ability to cause CSID.