Question

Carbohydrate intolerance is a rare but very serious hereditary disorder. Cases due to lack of digestive enzyme and to impaired hexose transport have been reported. It is estimated that glucose-galactose malabsorption accounts for 2% of the patients with protracted diarrhea of infancy. In two reported cases of glucose-galactose malabsorption, the infants developed diarrhea soon after birth. Their stools contained reducing sugars and had an acidic pH. Analysis of tissue obtained by biopsy of the small intestine showed normal villi with normal disaccharidase values. Monosaccharide feeding (loading) tests showed a normal rise in blood glucose following a fructose load dose, which was tolerated well. This result was in contrast to a flat glucose curve in response to a glucose load, which was accompanied by marked abdominal distension, profuse diarrhea, and the presence of reducing sugars in the stools.

What is a reducing sugar? Why would it be increased in these patients?

Which monosaccharide transport protein is lacking in these patients? Give a rationale for your answer.

If the plasma glucose concentration had been monitored in these two patients following a load of sucrose, would a rise in plasma glucose have been observed? Give a rationale for your answer.

In terms of plasma glucose concentration, what would have been observed following a loading dose of lactose?

What would be an appropriate dietary treatment for these individuals?

Answer 1

1. Any sugar which is capable of acting as reducing agents are called reducing sugars. This property of a sugar comes from hemiacetal (free aldehyde or ketone group ). Ketones, as such cannot oxidize so they undergo tautomerization and can act as reducing sugar. All monosaccharides are reducing sugars. Mutations in slc5a1 genes cause malabsorption.
2. SGLT1(sodium-glucose transporters) is a transporter protein which belongs to slc5a family is responsible for the absorption of glucose/galactose through intestine. SGLT2 is responsible for glucose reabsorption through nephron. SGLT1 is expressed on luminal brush border of enterocytes, which are responsible for nutrient absorption. It also have role in incretin secretion through glucose sensing capacity.
3. Plasma glucose concentration will raise following a load of sucrose. Sucrose is a disaccharide and it is not a reducing sugar. If you observe the structure of sucrose, it only has acetal groups, as they don’t open up and not available for oxidation
4. Plasma glucose concentration will not raise following a load of lactose. Even though lactose is a disaccharide it contains hemiacetal, hence it acts as a reducing sugar. Raise in 20mg/dl is a normal response but in patients with malabsorption it remains unchanged.
5. Only fructose based formulations were recommended to these patients as they are intolerant to glucose and galactose. and remove glucose and lacctose from their diet