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.

Dietary supplements or medications may be taken as a substitute for the missing enzyme or to introduce healthy bacteria into the immune system.

[citation needed] Chronic gastrointestinal symptoms that are fairly common but difficult to diagnose may be caused by congenital sucrase-isomaltase deficiency (CSID).

[5][6] A deficiency or absence of sucrase-isomaltase function is likely to cause chronic gastrointestinal symptoms whenever a person eats food containing sucrose or starch sugars, which are very common in carbohydrates.

These symptoms can range from mild to severe and include chronic, watery, acidic diarrhea; intestinal gas and bloating; nausea; and abdominal pain.

Other symptoms include abdominal distention, gassiness, colic, irritability, excoriated buttocks, severe diaper rash due to acidic diarrhea, indigestion, and vomiting.

One diagnostic method, considered the gold standard for diagnosing CSID, measures the level of activity of four intestinal enzymes that digest compound sugars, also called disaccharides.

[6][8][9] Noninvasive diagnostic methods include two breath tests, which can be useful screening tools but are not specific enough for a confirmed diagnosis of CSID.

[6][10][11] The first of these is the hydrogen/methane breath test, which measures the amount of hydrogen and methane gases a person exhales after consuming sugary water.

The exhaled breath is collected in sealed test tubes at 30-minute intervals over a three-hour period after drinking the sugary water.

If a person has CSID and there are little or no working sucrase-isomaltase enzymes in the intestines, greater than normal levels of hydrogen and/or methane are generated and exhaled in the breath.

As with all diagnoses, a diagnosis of CSID depends on combining a doctor's clinical findings with results from objective tests.

[10] If it is clinically inappropriate or difficult to perform a biopsy or if a CSID diagnosis is in doubt, a physician may suggest a two-week therapeutic trial with an enzyme replacement.

A registered dietitian or nutritionist can also help the patient plan a diet that will meet the nutritional needs for normal growth and development.