A 2011 review of 176 cases found that diagnoses made early in life (within a few days of birth) were associated with more severe disease and a mortality of 33%. Children diagnosed later, and who had milder symptoms, showed a lower mortality rate of ~3%.

Based on the results of worldwide screening of biotinidase deficiency in 1991, the incidence of the disorder is:

5 in 137,401 for profound biotinidase deficiency

- One in 109,921 for partial biotinidase deficiency

- One in 61,067 for the combined incidence of profound and partial biotinidase deficiency

- Carrier frequency in the general population is approximately one in 120.

Histidinemia is a rare autosomal recessive disorder. However, histidinemia is considered the most prevalent inborn error of metabolism with a reported incidence of 1:8600 (Quebec); 1:180,000 (New York) and 1:9600 (Japan); and an average of 1:12,000 observed in the neonatal screening of over 20 million newborns.

That MMA can have disastrous effects on the nervous system has been long reported; however, the mechanism by which this occurs has never been determined. Published on June 15th 2015, research performed on the effects of methylmalonic acid on neurons isolated from fetal rats in an in vitro setting using a control group of neurons treated with an alternate acid of similar pH. These tests have suggested that methylmalonic acid causes decreases in cellular size and increase in the rate of cellular apoptosis in a concentration dependent manner with more extreme effects being seen at higher concentrations. Furthermore, micro-array analysis of these treated neurons have also suggested that on a epigenetic-level methylmalonic acid alters the transcription rate of 564 genes, notably including those involved in the apoptosis, p53, and MAPK signaling pathways.

Infant mortality is high for patients diagnosed with early onset; mortality can occur within less than 2 months, while children diagnosed with late-onset syndrome seem to have higher rates of survival. Patients suffering from a complete lesion of mut0 have not only the poorest outcome of those suffering from methylaonyl-CoA mutase deficiency, but also of all individuals suffering from any form of methylmalonic acidemia.

Isovaleric acidemia is estimated to affect at least 1 in 250,000 births in the United States.

In a study in British Columbia, the overall incidence of the inborn errors of metabolism were estimated to be 40 per 100,000 live births or 1 in 1,400 births, overall representing more than approximately 15% of single gene disorders in the population.

Propionic acidemia is inherited in an autosomal recessive pattern and is found in about 1 in 35,000 live births in the United States. The condition appears to be more common in Saudi Arabia, with a frequency of about 1 in 3,000. The condition also appears to be common in Amish, Mennonite and other populations where inbreeding is common.

Recent case studies in several patients presenting nonresponsive mut0 MMA with a specific mutation designated p.P86L have suggest the possibility of further subdivision in mut type MMA might exist. Though currently unclear if this is due to the specific mutation or early detection and treatment, despite complete nonresponse to cobalamin supplements, these individuals appeared to develop a largely benign and near completely asymptomatic version of MMA. Despite consistently showing elevated methylmalonic acid in the blood and urine, these individuals appeared for the large part developmentally normal.

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

Raw eggs should be avoided in those with biotin deficiency, because egg whites contain high levels of the anti-nutrient avidin. The name avidin literally means that this protein has an "avidity" (Latin: "to eagerly long for") for biotin. Avidin binds irreversibly to biotin and this compound is then excreted in the urine.

Overall, according to a study in British Columbia, approximately 2.3 children per 100,000 births (1 in 43,000) have some form of glycogen storage disease. In the United States, they are estimated to occur in 1 per 20,000–25,000 births. Dutch incidence rate is estimated to be 1 per 40,000 births.

If a metabolic crisis is not treated, a child with VLCADD can develop: breathing problems, seizures, coma, sometimes leading to death.

At present, no specific enzyme deficiency nor genetic mutation has been implicated as the cause of hypertryptophanemia. Several known factors regarding tryptophan metabolism and kynurenines, however, may explain the presence of behavioral abnormalities seen with the disorder.

Tryptophan is an essential amino acid, and is required for protein synthesis. Aside from this crucial role, the remainder of tryptophan is primarily metabolized along the kynurenine pathway in most tissues, including those of the brain and central nervous system.

As the main defect behind hypertryptophanemia is suspected to alter and disrupt the metabolic pathway from tryptophan to kynurenine, a possible correlation between hypertryptophanemia and the known effects of kynurenines on neuronal function, physiology and behavior may be of interest.

One of these kynurenines, aptly named kynurenic acid, serves as a neuroprotectant through its function as an antagonist at both nicotinic and glutamate receptors (responsive to the neurotransmitters nicotine and glutamate, respectively). This action is in opposition to the agonist quinolinic acid, another kynurenine, noted for its potential as a neurotoxin. Quinolinic acid activity has been associated with neurodegenerative disorders such as Huntington's disease, the neuroprective abilities of kynurenic acid forming a counterbalance against this process, and the related excitotoxicity and similar damaging effects on neurons.

Indoleic acid excretion is another indicator of hypertryptophanemia. Indirectly related to kynurenine metabolism, indole modifies neural function and human behavior by interacting with voltage-dependent sodium channels (integral membrane proteins that form ion channels, allowing vital synaptic action potentials).

The human GALK1 gene contains 8 exons and spans approximately 7.3 kb of genomic DNA. The GALK1 promoter was found to have many features in common with other housekeeping genes, including high GC content, several copies of the binding site for the Sp1 transcription factor and the absence of TATA-box and CCAAT-box motifs typically present in eukaryotic polymerase II promoters. Analysis by 5-prime-RACE PCR indicated that the GALK1 mRNA is heterogeneous at the 5-prime end, with transcription sites occurring at many locations between 21 and 61 bp upstream of the ATG start site of the coding region. In vitro translation experiments of the GALK1 cDNA indicated that the protein is cytosolic and not associated with endoplasmic reticulum membrane.

Incomplete list of various fatty-acid metabolism disorders.

- Carnitine Transport Defect

- Carnitine-Acylcarnitine Translocase (CACT) Deficiency

- Carnitine Palmitoyl Transferase I & II (CPT I & II) Deficiency

- 2,4 Dienoyl-CoA Reductase Deficiency

- Electron Transfer Flavoprotein (ETF) Dehydrogenase Deficiency (GAII & MADD)

- 3-Hydroxy-3 Methylglutaryl-CoA Lyase (HMG) Deficiency

- Very long-chain acyl-coenzyme A dehydrogenase deficiency (VLCAD deficiency)

- Long-chain 3-hydroxyacyl-coenzyme A dehydrogenase deficiency (LCHAD deficiency)

- Medium-chain acyl-coenzyme A dehydrogenase deficiency (MCAD deficiency)

- Short-chain acyl-coenzyme A dehydrogenase deficiency (SCAD deficiency)

- 3-hydroxyacyl-coenzyme A dehydrogenase deficiency (M/SCHAD deficiency)

Most individuals with SBCADD are identified through newborn screening, where they present with an elevation of a five carbon acylcarnitine species. Confirmatory testing includes plasma and urine analysis to identify the carnitine and glycine conjugates of 2-methylbutyryl-CoA.

It has been suggested that a possible method of treatment for histidinemia is through the adoption of a diet that is low in histidine intake. However, the requirement for such dietary restrictions is typically unnecessary for 99% of all cases of histidinemia.

In the middle of the 20th century the principal treatment for some of the amino acid disorders was restriction of dietary protein and all other care was simply management of complications. In the past twenty years, enzyme replacement, gene therapy, and organ transplantation have become available and beneficial for many previously untreatable disorders. Some of the more common or promising therapies are listed:

Propionic acidemia, also known as propionic aciduria, propionyl-CoA carboxylase deficiency and ketotic glycinemia, is an autosomal recessive metabolic disorder, classified as a branched-chain organic acidemia.

The disorder presents in the early neonatal period with progressive encephalopathy. Death can occur quickly, due to secondary hyperammonemia, infection, cardiomyopathy, or basal ganglial stroke.

Propionic acidemia is a rare disorder that is inherited from both parents. Being autosomal recessive, neither parent shows symptoms, but both carry a defective gene responsible for this disease. It takes two faulty genes to cause PA, so there is a 1 in 4 chance for these parents to have a child with PA.

The disorder is caused by a mutation in the "ACADSB" gene, located on the long arm of human chromosome 10 (10q25-q26). It is inherited in an autosomal recessive manner, which means an affected individual must inherit one copy of the mutation from each parent.

Galactokinase deficiency is an autosomal recessive disorder, which means the defective gene responsible for the disorder is located on an autosome (chromosome 17 is an autosome), and two copies of the defective gene (one inherited from each parent) are required in order to be born with the disorder. The parents of an individual with an autosomal recessive disorder both carry one copy of the defective gene, but usually do not experience any signs or symptoms of the disorder.

Unlike galactose-1-phosphate uridyltransferase deficiency, the symptoms of galactokinase deficiency are relatively mild. The only known symptom in affected children is the formation of cataracts, due to production of galactitol in the lens of the eye. Cataracts can present as a failure to develop a social smile and failure to visually track moving objects.

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".

Maple syrup urine disease (MSUD) is a rare, inherited metabolic disorder. Its prevalence in the United States population is approximately 1 newborn out of 180,000 live births. However, in populations where there is a higher frequency of consanguinity, such as the Mennonites in Pennsylvania or the Amish, the frequency of MSUD is significantly higher at 1 newborn out of 176 live births. In Austria, 1 newborn out of 250,000 live births inherits MSUD. It also is believed to have a higher prevalence in certain populations due in part to the founder effect since MSUD has a much higher prevalence in children of Amish, Mennonite, and Jewish descent.

Hawkinsinuria, also called 4-Alpha-hydroxyphenylpyruvate hydroxylase deficiency, is an autosomal dominant metabolic disorder affecting the metabolism of tyrosine. Normally, the breakdown of the amino acid tyrosine involves the conversion of 4-hydroxyphenylpyruvate to homogentisate by 4-Hydroxyphenylpyruvate dioxygenase. Complete deficiency of this enzyme would lead to tyrosinemia III. In rare cases, however, the enzyme is still able to produce the reactive intermediate 1,2-epoxyphenyl acetic acid, but is unable to convert this intermediate to homogentisate. The intermediate then spontaneously reacts with glutathione to form 2-L-cystein-S-yl-1,4-dihydroxy-cyclohex-5-en-1-yl acetic acid (hawkinsin).

Patients present with metabolic acidosis during the first year of life, which should be treated by a phenylalanine- and tyrosine-restricted diet. The tolerance toward these amino acids normalizes as the patients get older. Then only a chlorine-like smell of the urine indicates the presence of the condition, patients have a normal life and do not require treatment or a special diet.

The production of hawkinsin is the result of a gain-of-function mutation, inheritance of hawkinsinuria is therefore autosomal dominant (presence of a single mutated copy of the gene causes the condition). Most other inborn errors of metabolism are caused by loss-of-function mutations, and hence have recessive inheritance (condition occurs only if both copies are mutated).