In undiagnosed and untreated children, the accumulation of precursor metabolites due to the deficient activity of galactose 1-phosphate uridylyltransferase (GALT) can lead to feeding problems, failure to thrive, liver damage, bleeding, and infections. The first presenting symptom in an infant is often prolonged jaundice. Without intervention in the form of galactose restriction, infants can develop hyperammonemia and sepsis, possibly leading to shock. The accumulation of galactitol and subsequent osmotic swelling can lead to cataracts which are similar to those seen in galactokinase deficiency. Long-term consequences of continued galactose intake can include developmental delay, developmental verbal dyspraxia, and motor abnormalities. Galactosemic females frequently suffer from ovarian failure, regardless of treatment in the form of galactose restriction.

The term fatty acid oxidation disorder (FAOD) is sometimes used, especially when there is an emphasis on the oxidation of the fatty acid.

In addition to the fetal complications, they can also cause complications for the mother during pregnancy.

Examples include:

- trifunctional protein deficiency

- MCADD, LCHADD, and VLCADD

Symptoms of enolase deficiency include exercise-induced myalgia and generalized muscle weakness and fatigability, both with onset in adulthood. Symptoms also include muscle pain without cramps, and decreased ability to sustain long term exercise.

Type A, which has been identified mostly in people from North America, has moderately severe symptoms that begin in infancy. Characteristic features include developmental delay and a buildup of lactic acid in the blood (lactic acidosis). Increased acidity in the blood can lead to vomiting, abdominal pain, extreme tiredness (fatigue), muscle weakness, and difficulty breathing. In some cases, episodes of lactic acidosis are triggered by an illness or periods without food. Children with pyruvate carboxylase deficiency type A typically survive only into early childhood.

The low incidence of this syndrome is often related to aldolase A's essential glycolytic role along with its exclusive expression in blood and skeletal muscle. Early developmental reliance and constitutive function prevents severe mutation in successful embryos. Infrequent documentation thus prevents clear generalisation of symptoms and causes. However five cases have been well described. ALDOA deficiency is diagnosed through reduced aldoA enzymatic activity, however, both physiological response and fundamental causes vary.

Pyruvate carboxylase deficiency type B has life-threatening signs and symptoms that become apparent shortly after birth. This form of the condition has been reported mostly in Europe, particularly France. Affected infants have severe lactic acidosis, a buildup of ammonia in the blood (hyperammonemia), and liver failure. They experience neurological problems including weak muscle tone (hypotonia), abnormal movements, seizures, and coma. Infants with this form of the condition usually survive for less than 3 months after birth.

Enolase Deficiency is a rare genetic disorder of glucose metabolism. Partial deficiencies have been observed in several caucasian families. The deficiency is transmitted through an autosomal dominant inheritance pattern. The gene for Enolase 1 has been localized to Chromosome 1 in humans. Enolase deficiency, like other glycolytic enzyme deficiences, usually manifests in red blood cells as they rely entirely on anaerobic glycolysis. Enolase deficiency is associated with a spherocytic phenotype and can result in hemolytic anemia, which is responsible for the clinical signs of Enolase deficiency.

Medium-chain acyl-CoA dehydrogenase deficiency, often known as MCAD deficiency or MCADD, is a disorder of fatty acid oxidation that impairs the body's ability to break down medium-chain fatty acids into acetyl-CoA. The disorder is characterized by hypoglycemia and sudden death without timely intervention, most often brought on by periods of fasting or vomiting.

Prior to expanded newborn screening, MCADD was an underdiagnosed cause of sudden death in infants. Individuals who have been identified prior to the onset of symptoms have an excellent prognosis.

MCADD is most prevalent in individuals of Northern European Caucasian descent, with an incidence of 1:4000 to 1:17,000 depending on the population. Treatment of MCADD is mainly preventative, by avoiding fasting and other situations where the body relies on fatty acid oxidation to supply energy.

A broad classification for genetic disorders that result from an inability of the body to produce or utilize one enzyme that is required to oxidize fatty acids. The enzyme can be missing or improperly constructed, resulting in it not working. This leaves the body unable to produce energy within the liver and muscles from fatty acid sources.

The body's primary source of energy is glucose; however, when all the glucose in the body has been expended, a normal body digests fats. Individuals with a fatty-acid metabolism disorder are unable to metabolize this fat source for energy, halting bodily processes. Most individuals with a fatty-acid metabolism disorder are able to live a normal active life with simple adjustments to diet and medications.

If left undiagnosed many complications can arise. When in need of glucose the body of a person with a fatty-acid metabolism disorder will still send fats to the liver. The fats are broken down to fatty acids. The fatty acids are then transported to the target cells but are unable to be broken down, resulting in a build-up of fatty acids in the liver and other internal organs.

Fatty-acid metabolism disorders are sometimes classified with the lipid metabolism disorders, but in other contexts they are considered a distinct category.

The presentation of mitochondrial trifunctional protein deficiency may begin during infancy, features that occur are: low blood sugar, weak muscle tone, and liver problems. Infants with this disorder are at risk for heart problems, breathing difficulties, and pigmentary retinopathy. Signs and symptoms of mitochondrial trifunctional protein deficiency that may begin "after" infancy include hypotonia, muscle pain, a breakdown of muscle tissue, and a loss of sensation in the extremities called peripheral neuropathy. Some who have MTP deficiency show a progressive course associated with myopathy, and recurrent rhabdomyolysis.

Galactose-1-phosphate uridylyltransferase deficiency, also called galactosemia type 1, classic galactosemia or GALT deficiency, is the most common type of galactosemia, an inborn error of galactose metabolism, caused by a deficiency of the enzyme galactose-1-phosphate uridylyltransferase. It is an autosomal recessive metabolic disorder that can cause liver disease and death if untreated. Treatment of galactosemia is most successful if initiated early and includes dietary restriction of lactose intake. Because early intervention is key, galactosemia is included in newborn screening programs in many areas. On initial screening, which often involves measuring the concentration of galactose in blood, classic galactosemia may be indistinguishable from other inborn errors of galactose metabolism, including galactokinase deficiency and galactose epimerase deficiency. Further analysis of metabolites and enzyme activities are needed to identify the specific metabolic error.

Glycerol Kinase Deficiency (GKD) is an X-linked recessive enzyme defect that is heterozygous in nature. Three clinically distinct forms of this deficiency have been proposed, namely infantile, juvenile, and adult. National Institutes of Health and its Office of Rare Diseases Research (ORDR) branch classifies GKD as a rare disease, known to affect fewer than 200,000 individuals in the United States. The responsible gene lies in a region containing genes in which deletions can cause Duchenne muscular dystrophy and adrenal hypoplasia congenita. Combinations of these three genetic defects including GKD are addressed medically as Complex GKD.

The defining characteristic of this form of the disorder is hemolytic anemia, in which red blood cells break down prematurely. Muscle weakness and pain are not as common in patients with hemolytic PFK deficiency.

Glycerol Kinase Deficiency causes the condition known as hyperglycerolemia, an accumulation of glycerol in the blood and urine. This excess of glycerol in bodily fluids can lead to many more potentially dangerous symptoms. Common symptoms include vomiting and lethargy. These tend to be the only symptoms, if any, present in adult GKD which has been found to present with fewer symptoms than infant or juvenile GKD. When GKD is accompanied by Duchenne Muscular Dystrophy and Adrenal Hypoplasia Congenita, also caused by mutations on the Xp21 chromosome, the symptoms can become much more severe. Symptoms visible at or shortly after birth include:

- cryptorchidism

- strabismus

- seizures

Some other symptoms that become more noticeable with time would be:

- metabolic acidosis

- hypoglycemia

- adrenal cortex insufficiency

- learning disabilities

- osteoporosis

- myopathy

Many of the physically visible symptoms, such as cryptorchidism, strabismus, learning disabilities, and myopathy, tend to have an added psychological effect on the subject due to the fact that they can set him or her apart from those without GKD. Cryptorchidism, the failure of one or both of the testes to descend to the scrotum, has been known to lead to sexual identity confusion amongst young boys because it is such a major physiological anomaly. Strabismus is the misalignment of one’s eyes. Typically, one is focused but the other is “lazy” and is directed inward or out ward (up and down is less common but does occur).

Symptoms of congenital Type III Galactosemia are apparent from birth, but vary in severity depending on whether the peripheral or generalized disease form is present. Symptoms may include:

- Infantile jaundice

- Infantile hypotonia

- Dysmorphic features

- Sensorineural hearing loss

- Impaired growth

- Cognitive deficiencies

- Depletion of cerebellar Purkinje cells

- Ovarian failure (POI) and hypertrophic hypergonadism

- Liver failure

- Renal failure

- Splenomegaly

- Cataracts

Studies of Type III galactosemia symptoms are mostly descriptive, and precise pathogenic mechanisms remain unknown. This is largely due to a lack of functional animal models of classic galactosemia. The recent development of a "Drosophila melanogaster" GALE mutant exhibiting galactosemic symptoms may yield a promising future animal model.

Phosphofructokinase deficiency also presents in a rare infantile form. Infants with this deficiency often display floppy infant syndrome (hypotonia), arthrogryposis, encephalopathy and cardiomyopathy. The disorder can also manifest itself in the central nervous system, usually in the form of seizures. PFK deficient infants also often have some type of respiratory issue. Survival rate for the infantile form of PFK deficiency is low, and the cause of death is often due to respiratory failure.

Galactose epimerase deficiency, also known as GALE deficiency, Galactosemia III and UDP-galactose-4-epimerase deficiency, is a rare, autosomal recessive form of galactosemia associated with a deficiency of the enzyme "galactose epimerase".

MCADD presents in early childhood with hypoketotic hypoglycemia and liver dysfunction, often preceded by extended periods of fasting or an infection with vomiting. Infants who are exclusively breast-fed may present in this manner shortly after birth, due to poor feeding. In some individuals the first manifestation of MCADD may be sudden death following a minor illness. A number of individuals with MCADD may remain completely asymptomatic, provided they never encounter a situation that sufficiently stresses their metabolism. With the advent of expanded newborn screening, some mothers have been identified with MCADD after their infants had positive newborn screens for low carnitine levels.

The enzyme "MCAD" is responsible for the dehydrogenation step of fatty acids with chain lengths between 6 and 12 carbons as they undergo beta-oxidation in the mitochondria. Fatty acid beta-oxidation provides energy after the body has used up its stores of glucose and glycogen. This oxidation typically occurs during periods of extended fasting or illness when caloric intake is reduced, and energy needs are increased.

Transaldolase deficiency is a disease characterised by abnormally low levels of the Transaldolase enzyme. It is a metabolic enzyme involved in the pentose phosphate pathway. It is caused by mutation in the transaldolase gene (TALDO1). It was first described by Verhoeven et al. in 2001.

Mitochondrial trifunctional protein deficiency is an autosomal recessive fatty acid oxidation disorder that prevents the body from converting certain fats to energy, particularly during periods without food. People with this disorder have inadequate levels of an enzyme that breaks down a certain group of fats called long-chain fatty acids.

Aldolase A deficiency, also called ALDOA deficiency, red cell aldolase deficiency or glycogen storage disease type 12 (GSD XII) is an autosomal recessive metabolic disorder resulting in a deficiency of the enzyme aldolase A; the enzyme is found predominantly in red blood cells and muscle tissue. The deficiency may lead to hemolytic anaemia as well as myopathy associated with exercise intolerance and rhabdomyolysis in some cases.

The key identifying feature of HFI is the appearance of symptoms with the introduction of fructose to the diet. Affected individuals are asymptomatic and healthy, provided they do not ingest foods containing fructose or any of its common precursors, sucrose and sorbitol. In the past, infants often became symptomatic when they were introduced to formulas that were sweetened with fructose or sucrose. These sweeteners are not common in formulas used today. Symptoms such as vomiting, nausea, restlessness, pallor, sweating, trembling and lethargy can also first present in infants when they are introduced to fruits and vegetables. These can progress to apathy, coma and convulsions if the source is not recognized early.

When patients are diagnosed with HFI, a dietary history will often reveal an aversion to fruit and other foods that contain large amounts of fructose. Most adult patients do not have any dental caries.

Most individuals with G6PD deficiency are asymptomatic.

Symptomatic patients are almost exclusively male, due to the X-linked pattern of inheritance, but female carriers can be clinically affected due to unfavorable lyonization, where random inactivation of an X-chromosome in certain cells creates a population of G6PD-deficient red blood cells coexisting with unaffected red blood cells. A female with one affected X chromosome will show the deficiency in approximately half of her red blood cells. However, in rare cases, including double X-deficiency, the ratio can be much more than half, making the individual almost as sensitive as males.

Red blood cell breakdown (also known as hemolysis) in G6PD deficiency can manifest in a number of ways, including the following:

- Prolonged neonatal jaundice, possibly leading to kernicterus (arguably the most serious complication of G6PD deficiency)

- Hemolytic crises in response to:

- Illness (especially infections)

- Certain drugs (see below)

- Certain foods, most notably broad beans from which the word favism derives

- Certain chemicals

- Diabetic ketoacidosis

- Very severe crises can cause acute kidney failure

Favism may be formally defined as a hemolytic response to the consumption of fava beans, also known as broad beans. Important to note is that all individuals with favism show G6PD deficiency, but not all individuals with G6PD deficiency show favism. The condition is known to be more prevalent in infants and children, and G6PD genetic variant can influence chemical sensitivity. Other than this, the specifics of the chemical relationship between favism and G6PD are not well understood.

Hereditary fructose intolerance (HFI) is an inborn error of fructose metabolism caused by a deficiency of the enzyme aldolase B. Individuals affected with HFI are asymptomatic until they ingest fructose, sucrose, or sorbitol. If fructose is ingested, the enzymatic block at aldolase B causes an accumulation of fructose-1-phosphate. This accumulation has downstream effects on gluconeogenesis and regeneration of adenosine triphosphate (ATP). Symptoms of HFI include vomiting, hypoglycemia, jaundice, hemorrhage, hepatomegaly, hyperuricemia and potentially kidney failure. While HFI is not clinically a devastating condition, there are reported deaths in infants and children as a result of the metabolic consequences of HFI. Death in HFI is always associated with problems in diagnosis.

HFI is an autosomal recessive condition caused by mutations in the "ALDOB" gene, located at 9q22.3. HFI is typically suspected based on dietary history, especially in infants who become symptomatic after breast feeding. This suspicion is typically confirmed by molecular analysis. Treatment of HFI involves strict avoidance of fructose in the diet. Older patients with HFI typically self-select a diet low in fructose, even before being diagnosed.

The most common clinical history in patients with glycogen-storage disease type 0 (GSD-0) is that of an infant or child with symptomatic hypoglycemia or seizures that occur before breakfast or after an inadvertent fast. In affected infants, this event typically begins after they outgrow their nighttime feeds. In children, this event may occur during acute GI illness or periods of poor enteral intake.

Mild hypoglycemic episodes may be clinically unrecognized, or they may cause symptoms such as drowsiness, sweating, lack of attention, or pallor. Uncoordinated eye movements, disorientation, seizures, and coma may accompany severe episodes.

Glycogen-storage disease type 0 affects only the liver. Growth delay may be evident with height and weight percentiles below average. Abdominal examination findings may be normal or reveal only mild hepatomegaly.Signs of acute hypoglycemia may be present, including the following: