Beyond Lactose: The Forgotten Disaccharidase
While lactose intolerance dominates public awareness, sucrose intolerance — caused by deficiency of the brush border enzyme sucrase-isomaltase (SI) — is an underdiagnosed cause of chronic gastrointestinal symptoms. Congenital sucrase-isomaltase deficiency (CSID) results from homozygous or compound heterozygous mutations in the SI gene on chromosome 3q26. At least 25 pathogenic variants have been identified, affecting enzyme folding, intracellular trafficking, or catalytic activity.
Prevalence varies dramatically by population. In European-descent populations, CSID affects roughly 1 in 5,000 live births. Among Indigenous peoples of Greenland and northern Canada, prevalence reaches 5 to 10 percent — one of the highest frequencies of any single-gene enzyme deficiency. Heterozygous carriers, who express reduced but not absent SI activity, may account for a meaningful proportion of patients with unexplained IBS-like symptoms.
Clinical Presentation
Symptoms typically begin when sucrose-containing foods are introduced to the infant diet — fruit juices, puréed fruits, sweetened cereals. Osmotic diarrhoea, abdominal distension, and failure to thrive are the classical presentation. In milder or heterozygous forms, symptoms may not become apparent until later childhood or adulthood, when they are often misdiagnosed as IBS-D.
Undigested sucrose draws water into the intestinal lumen (osmotic effect) and, upon reaching the colon, is fermented by bacteria into hydrogen, carbon dioxide, methane, and organic acids. The resulting combination of watery stool, bloating, and acidic pH (which can cause perianal excoriation in infants) is characteristic.
Diagnosis
The gold standard is duodenal biopsy with disaccharidase activity assay, which directly measures sucrase and isomaltase activity in mucosal homogenate. A sucrose hydrogen breath test (50g sucrose load in adults, 2g/kg in children) is a non-invasive alternative, though sensitivity is imperfect. The 4-4-4 rule for breath testing — a rise of 4 ppm hydrogen at 4 consecutive time points within 4 hours — has been proposed to improve specificity.
Genetic testing for SI mutations is increasingly available and can confirm CSID without invasive biopsy, though genotype-phenotype correlation is incomplete — environmental factors and microbiome composition modulate clinical expression.
Acquired Forms
Secondary sucrase deficiency can result from any condition causing villous atrophy or brush border damage: coeliac disease, Crohn's disease, tropical sprue, giardiasis, or post-infectious enteritis. In these cases, the enzyme deficiency is reversible once the mucosa heals. Distinguishing acquired from congenital forms matters for prognosis and treatment planning.
Treatment
Sacrosidase (Sucraid) — a yeast-derived liquid enzyme replacement — is taken with meals to pre-digest sucrose in the stomach. It restores normal sucrose digestion in most patients. Dietary modification involves limiting sucrose and starch, though the degree of starch restriction depends on residual isomaltase activity. Interestingly, the colonic microbiome may partially compensate: bacteria expressing invertase (sucrose-cleaving enzymes) can salvage some energy from undigested sucrose, and this microbial adaptation may explain why some CSID patients tolerate moderate starch loads better than predicted from enzyme assays alone.