Inborn errors of carbohydrate metabolism
Inborn errors of carbohydrate metabolism are inborn error of metabolism that affect the catabolism and anabolism of carbohydrates. An example is lactose intolerance. Carbohydrates account for a major portion of the human diet and are metabolized into three principal monosaccharides: glucose, galactose and fructose.   The failure to effectively use this sugars accounts for the majority of the inborn errors of human carbohydrates metabolism. 

# Galactose
The most common monogenic disorder of carbohydrate metabolism, transferase deficiency galactosemia (classical galactosemia)  affects i in every 55,000 newborns.  Caused by mutations in the gene encoding galactose-1-phospate urydil transferase (GAL-1-P urydil transferase).  Approximately 70% of galactosemia causing alleles have a single missense mutation in exon 6.  As a result, diminished GAL-1-P urydil transferase activity, affected individual cannot effectively convert galactose to glucose.  Consequently, galactose is alternatively metabolized to galactitol and galactonate.  Galactose and its metabolites accumulates in tissues.
The most common signs are: 
- failure to thrive
- hepatic insufficiency
- cataract
- developmental delay
Long term disabilities include
- poor growth
- mental retardation
- ovarian failure in females.
Screening is performed by measuring GAL-1-P urydil transferase activity.  Early identification affords prompt treatment, which consists largely of eliminating dietary galactose. 
Galactosemia can also be caused by mutations in the genes encoding galactokinase or uridine diphosphate galactose-4-epimerase.  Deficiency of galactokinase is associated with cataracts, but does not cause growth failure, mental retardation, or hepatic disease.  Dietary reduction of galactose is also the treatment but not as severe as in patients with classical galactosemia. This deficiency can be systemic or limited to red blood cells and leukocytes.

# Fructose
Three autosomal recessive disorders have been described.  The most common is caused by mutations in the gene encoding hepatic fructokinase.  This enzymes catalyzes the first step in the metabolism of dietary fructose.  Inactivation of the hepatic fructokinase results in asymptomatic fructosuria. 
Hereditary fructose intolerance (HFI) results in poor feeding, failure to thrive, hepatic and renal insufficiency, and death.  HFI is caused by a deficiency pf fructose 1,6-biphosphate aldolase in the liver, kidney cortex and small intestine. Infants and adults are asymptomatic unless they ingest fructose or sucrose. 
Deficiency of hepatic fructose 1,6-biphosphate(FBPase) causes impaired gluconeogenesis, hypoglycemia and severe metabolic acidemia.  If patients are adequately supported beyond childhood, growth and development appear to be normal.

# Glucose
Abnormalities of glucose metabolism are the most common errors of carbohydrate metabolism.  The causes are heterogeneous and include both environmental and genetic factors.  Disorders associated with  elevated levels of plasma glucose have been classified into three categories : diabetes mellitus (DM) type 1, DM type 2 and maturity onset diabetes of youth (MODY), a subtype of DM 2.
DM 1 is associated with reduced or absent levels of plasma insulin and usually presents in childhood.  DM type 2 is characterized by insulin resistance and typically adult onset. 

# Lactose
The ability to metabolize glucose depends, in part, on the activity of an intestinal brush border enzyme called lactase-phlorizin hydrolase (LPH).  In most mammals, LPH activity diminishes after infants are weaned from maternal milk.  However, the persistence of intestinal LPH activity is common on autosomal recessive trait in human populations with a frequency of 5% to 90%.  The geographic distribution of lactase persistence is concordant with areas of high milk intake.  The persistent ability for adult to use dairy products as a source of vitamin D may have had a selective advantage in this populations. 
Lactase non persistence is common in tropical and subtropical countries.  Individuals with lactase non persistency may experience nausea, bloating and diharrea after ingesting lactose.
LPH is encoded by the lactase gene on chromosome 2. 

# Glycogen
Carbohydrates are most commonly stored as glycogen in humans. Consequently, enzyme deficiencies that leads to impaired synthesis or degradation of glycogen are also considered disorders of carbohydrates metabolism. The two organs most commonly affected are the liver and the skeletal muscle.  Glycogen storage disorders that affect the liver typically cause hepatomegaly and hypoglycemia. Those that affect skeletal muscle cause exercise intolerance, progressive weakness and cramping.