When ingested, 70-90% of preformed vitamin A is absorbed and used.

Absorption and storage in the liver of preformed vitamin A occur very efficiently until a pathologic condition develops.

The life expectancy of patients with homocystinuria is reduced only if untreated. It is known that before the age of 30, almost one quarter of patients die as a result of thrombotic complications (e.g., heart attack).

There are three main mechanisms involved in hypercarotenemia: excessive dietary intake of carotenoids, increased serum lipids, and decreased metabolism of carotenoids. The most common reported cause of hypercarotenemia (and thus carotenoderma) is increased intake, either through increased dietary foods or nutritional supplements. This change takes approximately 4 to 7 weeks to be recognized clinically. Numerous ingested substances are rich in carotenoids. Increased serum lipids also cause hypercarotenemia because there are increased circulating lipoproteins that contain bound carotenoids. Finally, in certain disease states, the metabolism and conversion of carotenoids to retinol is slowed, which can lead to decreased clearance and increased plasma levels. Elevated serum beta-carotene does not necessarily result in carotenosis, but the latter is likely to show up when intake is more than 20 mg/day. Average adult intake in the U.S. around 2.3 mg/day. One medium-sized carrot has about 4.0 mg.

Carotenoderma can be divided into two major types, primary and secondary. Primary carotenoderma is that developing from increased oral ingestion of carotenoids, whereas secondary carotenoderma is caused from underlying disease states that increase serum carotenoids with normal oral intake of these compounds. Primary and secondary carotenoderma can coexist in the same patient.

Foods associated with high levels of carotenoids include:

No sexual predilection is observed because the deficiency of glycogen synthetase activity is inherited as an autosomal recessive trait.

The major morbidity is a risk of fasting hypoglycemia, which can vary in severity and frequency. Major long-term concerns include growth delay, osteopenia, and neurologic damage resulting in developmental delay, intellectual deficits, and personality changes.

Disease states associated with carotenoderma include hypothyroidism, diabetes mellitus, anorexia nervosa, nephrotic syndrome, and liver disease. In hypothyroidism and diabetes mellitus, the underlying mechanism of hypercarotenemia is thought to be both impaired conversion of beta-carotene into retinol and the associated increased serum lipids. Diabetes mellitus has also been associated with carotenoderma through disease-specific diets that are rich in vegetables. In the nephrotic syndrome, the hypercarotenemia is related to the associated increased serum lipids, similar to the above entities.

It is of note that kidney dysfunction in general is associated with hypercarotenemia as a result of decreased excretion of carotenoids. Liver dysfunction, regardless of origin, causes hypercarotenemia as a result of the impaired conversion of carotenoids to retinol. This is of particular interest because jaundice and carotenoderma can coexist in the same patient. Anorexia nervosa causes carotenoderma mainly through diets that are rich in carotenoids and the associated hypothyroidism. It tends to be more common in the restricting subtype of this disease, and is associated with numerous other dermatologic manifestations, such as brittle hair and nails, lanugo-like body hair, and xerosis. Although Alzheimer's disease has been associated with carotenoderma in some reports, most studies on serum carotenoids in these patients show that their levels of carotenoids and retinol are depressed, and may be associated with the development of dementia. A true association between Alzheimer's disease and carotenoderma is unclear at this time. There have been case reports in the literature of increased serum carotenoids and carotenoderma that is unresponsive to dietary measures, with a genetic defect in carotenoid metabolic enzymes proposed. Canthaxanthin and astaxanthin are naturally occurring carotenoids that are used in the British and US food industry to add color to foods such as sausage and fish. Canthaxanthin has been used in over-the-counter “tanning pills” in the United States and Europe, but is not currently Food and Drug Administration (FDA)-approved for this purpose in the United States because of its adverse effects. These include hepatitis, urticaria, aplastic anemia, and a retinopathy characterized by yellow deposits and subsequent visual field defects.

Infants and small children are especially prone to carotenoderma because of the cooked, mashed, and pureed vegetables that they eat. Processing and homogenizing causes carotene to become more available for absorption. A small 2.5 ounce jar of baby food sweet potatoes or carrots contains about 400-500% of an infant's recommended daily value of carotene. In addition to that source of carotene, infants are usually prescribed a liquid vitamin supplement, such as Tri-Vi-Sol, which contains vitamin A.

Classical homocystinuria, also known as cystathionine beta synthase deficiency or CBS deficiency, is an inherited disorder of the metabolism of the amino acid methionine, often involving cystathionine beta synthase. It is an inherited autosomal recessive trait, which means a child needs to inherit a copy of the defective gene from both parents to be affected.

The addition of SPCD to newborn screening panels has offered insight into the incidence of the disorder around the world. In Taiwan, the incidence of SPCD in newborns was estimated to be approximately 1:67,000, while maternal cases were identified at a higher frequency of approximately 1:33,000. The increased incidence of SPCD in mothers compared to newborns is not completely understood. Estimates of SPCD in Japan have shown a similar incidence of 1:40,000. Worldwide, SPCD has the highest incidence in the relatively genetically isolated Faroe Islands, where an extensive screening program was instituted after the sudden death of two teenagers. The incidence in the Faroe Islands is approximately 1:200.

Heterozygous protein C deficiency occurs in 0.14–0.50% of the general population. Based on an estimated carrier rate of 0.2%, a homozygous or compound heterozygous protein C deficiency incidence of 1 per 4 million births could be predicted, although far fewer living patients have been identified. This low prevalence of patients with severe genetic protein C deficiency may be explained by excessive fetal demise, early postnatal deaths before diagnosis, heterogeneity in the cause of low concentrations of protein C among healthy individuals and under-reporting.

The incidence of protein C deficiency in individuals who present with clinical symptoms has been reported to be estimated at 1 in 20,000.

According to Clinicaltrials.gov, there are no current studies on hyperglycerolemia.

Clinicaltrials.gov is a service of the U.S. National Institutes of Health. Recent research shows patients with high concentrations of blood triglycerides have an increased risk of coronary heart disease. Normally, a blood glycerol test is not ordered. The research was about a child having elevated levels of triglycerides when in fact the child had glycerol kinase deficiency. This condition is known as pseudo-hypertriglyceridemia, a falsely elevated condition of triglycerides. Another group treated patients with elevated concentrations of blood triglycerides with little or no effect on reducing the triglycerides. A few laboratories can test for high concentrations of glycerol, and some laboratories can compare a glycerol-blanked triglycerides assay with the routine non-blanked method. Both cases show how the human body may exhibit features suggestive of a medical disorder when in fact it is another medical condition causing the issue.

Systemic primary carnitine deficiency (SPCD), also known as carnitine uptake defect, carnitine transporter deficiency (CTD) or systemic carnitine deficiency is an inborn error of fatty acid transport caused by a defect in the transporter responsible for moving carnitine across the plasma membrane. Carnitine is an important amino acid for fatty acid metabolism. When carnitine cannot be transported into tissues, fatty acid oxidation is impaired, leading to a variety of symptoms such as chronic muscle weakness, cardiomyopathy, hypoglycemia and liver dysfunction. The specific transporter involved with SPCD is OCTN2, coded for by the "SLC22A5" gene located on chromosome 5. SPCD is inherited in an autosomal recessive manner, with mutated alleles coming from both parents.

Acute episodes due to SPCD are often preceded by metabolic stress such as extended fasting, infections or vomiting. Cardiomyopathy can develop in the absence of an acute episode, and can result in death. SPCD leads to increased carnitine excretion in the urine and low levels in plasma. In most locations with expanded newborn screening, SPCD can be identified and treated shortly after birth. Treatment with high doses of carnitine supplementation is effective, but needs to be rigorously maintained for life.

SPCD is more common in the Faroe Islands than in other countries, at least one out of every 1000 inhabitants of the Faroes has the illness, while the numbers for other countries are one in every 100,000. Around 100 persons in the islands have been diagnosed, around one third of the whole population of 48,000 people have been screened for SPCD. Several young Faroese people and children have died a sudden death with cardiac arrest because of SPCD. Scientists believe that around 10% of the Faroese population are carriers of the gene for SPCD. These people are not ill, but may have a lower amount of carnitine in their blood than non-carriers.

Protein C deficiency is a rare genetic trait that predisposes to thrombotic disease. It was first described in 1981. The disease belongs to a group of genetic disorders known as thrombophilias. Protein C deficiency is associated with an increased incidence of venous thromboembolism (relative risk 8–10), whereas no association with arterial thrombotic disease has been found.

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.

In adults, fibrates and statins have been prescribed to treat hyperglycerolemia by lowering blood glycerol levels. Fibrates are a class of drugs that are known as amphipathic carboxylic acids that are often used in combination with Statins. Fibrates work by lowering blood triglyceride concentrations. When combined with statins, the combination will lower LDL cholesterol, lower blood triglycerides and increase HDL cholesterol levels.

If hyperglycerolemia is found in a young child without any family history of this condition, then it may be difficult to know whether the young child has the symptomatic or benign form of the disorder. Common treatments include: a low-fat diet, IV glucose if necessary, monitor for insulin resistance and diabetes, evaluate for Duchenne muscular dystrophy, adrenal insufficiency & developmental delay.

The Genetic and Rare Diseases Information Center (GARD) does not list any treatments at this time.

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:

Lipoprotein lipase deficiency (also known as "familial chylomicronemia syndrome", "chylomicronemia", "chylomicronemia syndrome" and "hyperlipoproteinemia type Ia") is a rare autosomal recessive lipid disorder caused by a mutation in the gene which codes lipoprotein lipase. As a result, afflicted individuals lack the ability to produce lipoprotein lipase enzymes necessary for effective breakdown of triglycerides.

The disorder affects about 1 out of 1,000,000 people, however epidemiological data are limited and there are regional differences due to cofounder effect (e.g. in Canada) or intermarriage.

This disorder is caused by an abnormality of the TBCE gene, the locus for which is on Chromosome 1q42.3. The locus is a 230 kb region of gene with identified deletions and mutations in affected individuals. There are rare cases of the disorder not being due to a TBCE gene abnormality.

Rabson–Mendenhall syndrome is a rare autosomal recessive disorder characterized by severe insulin resistance. The disorder is caused by mutations in the insulin receptor gene. Symptoms include growth abnormalities of the head, face and nails, along with the development of acanthosis nigricans. Treatment involves controlling blood glucose levels by using insulin and incorporating a strategically planned, controlled diet. Also, direct actions against other symptoms may be taken (e.g. surgery for facial abnormalities) This syndrome usually affects children and has a prognosis of 1–2 years.

There is no information on birth ratios/rates, but "X-Linked SCID is the most common form of SCID and it has been estimated to account for 46% to 70% of all SCID cases."

The condition is transmitted as an autosomal recessive trait, and often affects children of consanguineous parents. The physical findings and symptoms vary greatly among each individual.

Genetic diseases are determined by two genes, one from the mother and one from the father. Recessive genetic disorders occur when an individual inherits the same abnormal gene for the same trait from each parent. If one of the inherited genes is normal, while the other is for the disease, the person will only be a carrier and will not display any symptoms.

The risk for two carrier parents to both pass the defective gene and, therefore, have an affected child is 25 percent with each pregnancy. The risk to have a child who is a carrier like the parents is 50 percent with each pregnancy. The chance for a child to receive normal genes from both parents and be genetically normal for that particular trait is 25 percent.

Researchers have determined that the Rabson–Mendenhall syndrome is caused by mutations of the insulin receptor gene. The insulin receptor gene is located on the short arm (p) of chromosome 19. Mutations of the insulin-receptor gene lead to an alteration of structure or reduced number of insulin receptors. This results in reduced binding of insulin, and may also lead to abnormalities in the post-receptor pathway.

Individuals with Rabson-Mendenall syndrome will need ways to compensate for their insulin resistance, and may do this by increasing insulin secretion. This can lead to excessive insulin levels in the blood (hyperinsulinemia), which can be responsible for multiple symptoms. Definitive genotype–phenotype correlation for insulin receptor defects is difficult to establish primarily due to the rarity of these syndromes. However, researchers believe more severe phenotype changes are due to a mutation in the alpha subunit of the receptor.

It was first described in 1920 by German doctors, Fritz Rabe and Eugene Salomon, studying a bleeding disorder presenting itself in a child from birth. This disorder may also be simply called afibrinogenemia or familial afibrinogenemia. About 1 in 1 million individuals are diagnosed with the disease; typically at birth. Both males and females seem to be affected equally, but it has a higher occurrence in regions where consanguinity is prevalent.

Although genetic deficiencies are currently considered rare, variations in these genes may predispose to common obesity. Many candidate genes are highly expressed in the central nervous system.

Several additional loci have been identified. Also, several quantitative trait loci for BMI have been identified.

Confirmed and hypothesized associations include:

Some studies have focused upon inheritance patterns without focusing upon specific genes. One study found that 80% of the offspring of two obese parents were obese, in contrast to less than 10% of the offspring of two parents who were of normal weight.

The thrifty gene hypothesis postulates that due to dietary scarcity during human evolution people are prone to obesity. Their ability to take advantage of rare periods of abundance by storing energy as fat would be advantageous during times of varying food availability, and individuals with greater adipose reserves would more likely survive famine. This tendency to store fat, however, would be maladaptive in societies with stable food supplies. This is the presumed reason that Pima Indians, who evolved in a desert ecosystem, developed some of the highest rates of obesity when exposed to a Western lifestyle.

Numerous studies of laboratory rodents provide strong evidence that genetics plays an important role in obesity.

The risk of obesity is determined by not only specific genotypes but also gene-gene interactions. However, there are still challenges associated with detecting gene-gene interactions for obesity.

The vWF gene is located on the short arm "p" of chromosome 12 (12p13.2). It has 52 exons spanning 178kbp. Types 1 and 2 are inherited as autosomal dominant traits and type 3 is inherited as autosomal recessive. Occasionally, type 2 also inherits recessively. vWD occurs in approximately 1% of the population and affects men and women equally.