NORD gratefully acknowledges Audrey M. Schlachter, MMSc, NORD Editorial Intern from the Emory University Genetic Counseling Training Program and Cecelia A. Bellcross, PhD, MS, CGC, Associate Professor, Director, Genetic Counseling Training Program, Emory University School of Medicine, for assistance in the preparation of this report.
Glucose-galactose malabsorption (GGM) is an inherited metabolic disorder. It is caused by the small intestines not being able to absorb and use glucose and galactose (simple sugars). Glucose and galactose have very similar chemical structures. The same protein carries both sugars into the intestines. The intestines absorb the simple sugars which are used throughout the body. The gene for GGM makes this enzyme work properly. When this gene is changed (mutated), the enzyme cannot bring the simple sugars into the small intestines, causing GGM.
Symptoms usually begin in the first days of life, when a newborn drinks milk. Milk is broken down into simple sugars. Symptoms include severe diarrhea which can lead to life-threatening dehydration and difficulties gaining weight. The diarrhea and dehydration are fatal if left untreated. Once symptoms are managed, affected children typically have a normal lifespan. The symptoms may become less severe as patients age, which can allow for some sugars to be introduced back into their diet.
Children with GGM can have infrequent diarrhea when eating high sugar foods. Mild amounts of sugar in the urine (glucosuria) of an affected child may be a warning that kidney stones are developing.
In adults, symptoms of GGM may include bloating, nausea, diarrhea, abdominal cramps, rumbling sounds caused by gas in the intestine and frequent urination.
Normally, the SCL5A1 gene creates a protein that transports glucose and galactose into the intestines and kidneys. These simple sugars are used throughout the body and removed through urination.
When the gene is not working, the sugars are not transported and absorbed properly. There is nowhere for the glucose and galactose to go except into the stool. These sugars draw large amounts of water out of the body and into the stool, leading to watery (osmotic) diarrhea.
GGM is a recessive genetic condition. Recessive genetic disorders occur when an individual inherits a non-working gene from each parent. If an individual receives one working gene and one non-working gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. The risk for two carrier parents to both pass on the non-working gene and, therefore, have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier, like the parents, is 50% with each pregnancy. The chance for a child to receive working genes from both parents is 25%. The risk is the same for males and females.
GGM is an extremely rare disorder. There are around 300 cases worldwide. It appears to be more common in females. Most cases are seen in families where the parents are related by blood because they are more likely to carry the same harmful gene change.
GGM can be diagnosed by testing the SLC5A1 gene to look for harmful changes. Diagnosis may also be confirmed through restricting dietary sugars (glucose, galactose, sucrose and lactose) to see if the symptoms stop. Genetic testing is replacing the “glucose hydrogen breath test” which is now less frequently used.
Treatment involves avoiding milk, milk products, and foods with glucose and galactose. Patients will need to follow a low glucose and low galactose diet. Fructose, a different simple sugar, can be used as a substitute for glucose and galactose. Some individuals may eventually be able to introduce these foods back into their diet, however, others may need to follow these restrictions for life. It is recommended that patients with GGM speak to a nutritionist familiar with the condition to identify a low or glucose-galactose-free diet and to see if gradual reintroduction is tolerated.
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