7.3F: Vitamin B₇ (Biotin)
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Biotin is a water-soluble B-vitamin, also called vitamin B7 and formerly known as vitamin H or coenzyme R. It is composed of a ureido ring fused with a tetrahydrothiophene ring. A valeric acid substituent is attached to one of the carbon atoms of the tetrahydrothiophene ring. Biotin is a coenzyme for carboxylase enzymes, involved in the synthesis of fatty acids, isoleucine, and valine, and in gluconeogenesis. Biotin deficiency can be caused by inadequate dietary intake or inheritance of one or more inborn genetic disorders that affect biotin metabolism. Subclinical deficiency can cause mild symptoms, such as hair thinning or skin rash typically on the face. Neonatal screening for biotinidase deficiency began in the United States in 1984 and today many countries test for this disorder at birth. Individuals born prior to 1984 are unlikely to have been screened, thus the true prevalence of the disorder is unknown.
Skeletal formula of biotin
Biotin is necessary for cell growth, the production of fatty acids, and the metabolism of fats and amino acids. Biotin assists in various metabolic reactions involving the transfer of carbon dioxide. It may also be helpful in maintaining a steady blood sugar level. Biotin is often recommended as a dietary supplement for strengthening hair and nails, though scientific data supporting this outcome are weak. Nevertheless, biotin is found in many cosmetics and health products for the hair and skin.
Dietary Reference Intake
The Food and Nutrition Board of the U.S. Institute of Medicine updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for B vitamins in 1998. At that time there was not sufficient information to establish EARs and RDAs for biotin. In instances such as this, the Board sets Adequate Intakes (AIs), with the understanding that at some later date, AIs will be replaced by more exact information. The current AI for adults ages 19 and up is 30 μg/day. AI for pregnancy is 30 μg/day. AI for lactation is 35 μg/day. For infants up to 12 months the AI is 5-6 μg/day For children ages 1–18 years the AI increases with age from 8 to 25 μg/day.
As for safety, the FNB sets Tolerable Upper Intake Levels (known as ULs) for vitamins and minerals when evidence is sufficient. In the case of biotin there is no UL, as there is insufficient human data to identify adverse effects from high doses. The European Food Safety Authority reviewed the same safety question and also reached the conclusion that there was not sufficient evidence to set a UL for biotin.
For U.S. food and dietary supplement labeling purposes the amount in a serving is expressed as a percent of Daily Value (%DV). For biotin labeling purposes 100% of the Daily Value was 300 μg, but as of May 2016 it has been revised to 30 μg to bring it into agreement with the AI. Food and supplement companies have until July 2018 to comply with the change.
Biotin is synthesized by intestinal bacteria, but there is a lack of good quality studies about how much biotin they provide. Biotin is stable at room temperature and isn’t destroyed by cooking. Some of the best sources are (content per 100 grams):
- Brewer's yeast: 188.8 mcg
- Soybeans: 179.4 mcg
- Beef liver: 113.3 mcg
- Butter: 94.3 mcg
- Split peas: 77.7 mcg
- Sunflower seeds: 66 mcg
- Green peas/lentils: 40 mcg
- Peanuts/walnuts: 37.5 mcg
- Pecans: 27.75 mcg
- Eggs: 18.9 mcg
Egg whites contain a protein (avidin) that blocks the absorption of biotin, so people who regularly consume a large number of eggs may become biotin-deficient. The dietary biotin intake in Western populations has been estimated to be 35 to 70 micrograms per day (143–287 nmol per day). Biotin is also available in dietary supplements in which a dose of 30 micrograms meets 100% of the Daily Value for adults.
Biotin deficiency is rare. The amounts needed are small, a wide range of foods contain biotin, and intestinal bacteria synthesize biotin, which is then absorbed by the host animal. For that reason, statutory agencies in many countries, for example the USA and Australia, have not formally established a recommended daily intake of biotin. Instead, an Adequate Intake (AI) is identified based on the theory that average intake meets needs. A number of rare metabolic disorders exist in which an individual's metabolism of biotin is abnormal.
Biotin deficiency typically occurs from dietary absence of the vitamin. Consuming raw egg whites over months may result in biotin deficiency. Deficiency can be addressed with nutritional supplementation. Deficiency symptoms include:
- Brittle and thin fingernails
- Hair loss (alopecia)
- Dermatitis in the form of a scaly, red rash around the eyes, nose, mouth, and genital area.
- Neurological symptoms in adults, such as depression, lethargy, hallucination, and numbness and tingling of the extremities
The neurological and psychological symptoms can occur with only mild deficiencies. Dermatitis, conjunctivitis, and hair loss will generally occur only when deficiency becomes more severe. Individuals with hereditary disorders of biotin deficiency have evidence of impaired immune system function, including increased susceptibility to bacterial and fungal infections.
Pregnant women tend to have a high risk of biotin deficiency. Nearly half of pregnant women have abnormal increases of 3-hydroxyisovaleric acid, which reflects reduced status of biotin. Several studies have reported this possible biotin deficiency during the pregnancy may cause infants' congenital malformations, such as cleft palate. Mice fed with dried raw egg to induce biotin deficiency during the gestation resulted in up to 100% incidence of the infants' malnourishment. Infants and embryos are more sensitive to the biotin deficiency. Therefore, even a mild level of the mother's biotin deficiency that does not reach the appearance of physiological deficiency signs may cause a serious consequence in the infants.
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