This is congenital and due to the absence of the enzyme sucrase iso-maltase present in the small bowel and responsible for digesting the sucrose which is present in refined sugar and also as a component of the sugars found in fruits.
It has variable expression in that children can be mildly affected or badly affected depending on the amount of enzyme left.
The condition manifests as gaseous distension and diarrhoea and gas production and explosive diarrhoea is often present. It can run in families and is helped by complete avoidance of sucrose with the help of a paediatric dietitian.
It can be diagnosed with a direct sample from small bowel biopsy or with a sucrose hydrogen breath test.