Sucrose intolerance is a term describing the inability of individuals to process sucrose and the development of watery diarrhea and abdominal cramping after ingestion of sucrose-containing carbohydrates, eg. fruits. Symptoms appear after cessation of breastmilk feeding and the introduction of carbohydrates from food. Genetic mutations of intestinal glycoproteins are responsible for this clinical entity. Biopsy of the wall of the intestine is used to make the diagnosis, but several other methods have been described in the literature.
Sucrose intolerance (SI) is roughly defined as a disorder of the human gastrointestinal (GI) tract characterized by the reduced capacity of the small bowel membrane to process carbohydrates such as starch and sucrose . This phenomenon is seen in patients who suffer from an autosomal recessive disorder called congenital sucrase-isomaltase deficiency (CSID), in which genetic mutations of sucrase and isomaltase, vital enzymes and glycoprotein constituents of the intestinal cell membrane, lead to significantly lower capacities for processing sucrose    . Although rare, this condition has been reported throughout the world, but particularly higher prevalence rates are observed in Inuits and native populations of Alaska, Greenland, and Canada  . The clinical presentation stems from an increased amount of disaccharides in the lumen of the intestine that create an osmotic shift, leading to watery diarrhea that is often accompanied by abdominal cramping as the main symptom   . SI becomes evident when infants are switched from breastfeeding and milk formulas to regular food containing starches and sucrose (eg. fruits)  . Diarrhea is often described as "fermentative" due to the abundance of carbohydrates, whereas additional findings include failure to thrive, bloating, and abdominal distension   . Severe malnutrition has been reported in rare cases in whom an early presentation was observed .
A comprehensive clinical approach is necessary in order to make the diagnosis of SI. Firstly, physicians must evaluate the exact onset of symptoms, their progression, but also severity, and detect if their appearance coincided with the introduction of solid foods and carbohydrates, which may be one of the first and most important clues. Because of the autosomal recessive nature of the disease, a detailed family history is of critical importance, as familial cases are well-documented . The physical examination particularly focused on abdominal palpation, should be the next step in the workup. To solidify clinical findings obtained during the evaluation, various tests (both invasive and non-invasive) can be employed. The sucrose tolerance test (comprised of intravenous administration of sucrose and subsequent measurements of serum glucose levels, as well as detection of symptoms within several hours) and the 13C-sucrose breath hydrogen test (evaluation of carbohydrate levels in the patient's breath) are two non-invasive methods that have been recommended by several studies in the diagnostic assessment of sucrose intolerance  . The gold standard, however, is a biopsy of the wall of the small intestine through upper endoscopy, and subsequent confirmation of low sucrase and isomaltase activity can confirm this disease   .