Biotin is a water-soluble vitamin that is generally classified as a B-complex vitamin. After the initial discovery of biotin, nearly 40 years of research were required to establish it as a vitamin (1). Biotin is required by all organisms but can be synthesized only by bacteria, yeasts, molds, algae, and some plant species (2).
Biotin is attached at the active site of five mammalian enzymes known as carboxylases (3). The attachment of biotin to another molecule, such as a protein, is known as "biotinylation." Holocarboxylase synthetase (HCS) catalyzes the biotinylation of apocarboxylases (i.e., the catalytically inactive form of the enzyme) and of histones (See below). Biotinidase catalyzes the release of biotin from histones and from the peptide products of carboxylase breakdown.
Each carboxylase catalyzes an essential metabolic reaction:
- Acetyl-CoA carboxylase I and II catalyze the binding of bicarbonate to acetyl-CoA to form malonyl-CoA. Malonyl-CoA is required for the synthesis of fatty acids. The former is crucial in cytosolic fatty acid synthesis, and the latter functions in regulating mitochondrial fatty acid oxidation.
- Pyruvate carboxylase is a critical enzyme in gluconeogenesis—the formation of glucose from sources other than carbohydrates, for example, amino acids.
- Methylcrotonyl-CoA carboxylase catalyzes an essential step in the catabolism of leucine, an essential amino acid.
- Propionyl-CoA carboxylase catalyzes essential steps in the metabolism of certain amino acids, cholesterol, and odd chain fatty acids (fatty acids with an odd number of carbon molecules) (4).
Histones are proteins that bind to DNA and package it into compact structures to form nucleosomes—integral structural components of chromosomes. The compact packaging of DNA must be relaxed somewhat for DNA replication and transcription to occur. Modification of histones through the attachment of acetyl or methyl groups (acetylation or methylation) has been shown to affect the structure of histones, thereby affecting replication and transcription of DNA. Mounting evidence indicates that biotinylation of histones plays a role in regulating DNA replication and transcription as well as cellular proliferation and other cellular responses (5-7).
Although overt biotin deficiency is very rare, the human requirement for dietary biotin has been demonstrated in two different situations: prolonged intravenous feeding (parenteral) without biotin supplementation and consumption of raw egg white for a prolonged period (many weeks to years). Avidin is an antimicrobial protein found in egg white that binds biotin and prevents its absorption. Cooking egg white denatures avidin, rendering it susceptible to digestion and therefore unable to prevent the absorption of dietary biotin (8).
Three measures of biotin status have been validated as indicators of biotin status: (1) high excretion of an organic acid (3-hydroxyisovaleric acid) that reflects decreased activity of the biotin-dependent enzyme, methylcrotonyl-CoA carboxylase; (2) reduced urinary excretion of biotin; and (3) propionyl-CoA carboxylase activity in peripheral blood lymphocytes (4, 9-11).
Signs and symptoms
Signs of overt biotin deficiency include hair loss and a scaly red rash around the eyes, nose, mouth, and genital area. Neurologic symptoms in adults have included depression, lethargy, hallucination, and numbness and tingling of the extremities. The characteristic facial rash, together with unusual facial fat distribution, has been termed the "biotin deficient facies" by some investigators (8). Individuals with hereditary disorders of biotin metabolism resulting in functional biotin deficiency often have similar physical findings as well as evidence of impaired immune system function and increased susceptibility to bacterial and fungal infections (12).
There are several ways in which the hereditary disorder, biotinidase deficiency, leads to biotin deficiency. Intestinal absorption is decreased because a lack of biotinidase inhibits the release of biotin from dietary protein. Recycling of one's own biotin bound to protein is impaired, and urinary loss of biotin is increased because the kidneys more rapidly excrete biotin that is not bound to biotinidase (5, 8). Biotinidase deficiency uniformly responds to moderate biotin supplementation. Oral supplementation with as much as 5 to 10 milligrams (mg) of biotin daily is sometimes required, although smaller doses are often sufficient. Some forms of holocarboxylase synthetase (HCS) deficiency respond to biotin supplementation with large doses. HCS deficiency results in an enzyme that catalyzes the attachment of biotin to all four carboxylase enzymes (see Function). HCS deficiency results in decreased formation of all holocarboxylases at normal blood levels of biotin; thus, high-dose supplementation (40 mg to 100 mg of biotin/day) is required. The inborn error, biotin transporter deficiency, also responds to high-dose biotin supplementation (13). The prognosis of all three of these disorders is often, but not always, good if biotin therapy is introduced early (infancy or childhood) and continued for life (12).
Aside from prolonged consumption of raw egg white or total intravenous nutritional support lacking biotin, other conditions may increase the risk of biotin depletion. The rapidly dividing cells of the developing fetus require biotin for histone biotinylation and synthesis of essential carboxylases; hence, the biotin requirement is likely increased during pregnancy. Research suggests that a substantial number of women develop marginal or subclinical biotin deficiency during normal pregnancy (6, 14). However, the recommended adequate intake does not change for pregnancy (See below). Additionally, some types of liver disease may decrease biotinidase activity and theoretically increase the requirement for biotin. A study of 62 children with chronic liver disease and 27 healthy controls found serum biotinidase activity to be abnormally low in those with severely impaired liver function due to cirrhosis (15). However, this study did not provide evidence of biotin deficiency. Further, anticonvulsant medications, used to prevent seizures in individuals with epilepsy, increase the risk of biotin depletion (16, 17). See Safety for more information on biotin and anticonvulsants.
In 1998, the Food and Nutrition Board of the Institute of Medicine felt the existing scientific evidence was insufficient to calculate a RDA for biotin, so they set an Adequate Intake level (AI). The AI for biotin assumes that current average intakes of biotin (35 mcg to 60 mcg/day) meet the dietary requirement (1).Adequate Intake (AI) for Biotin Life Stage Age Males (mcg/day) Females (mcg/day) Infants 0-6 months 5 5 Infants 7-12 months 6 6 Children 1-3 years 8 8 Children 4-8 years 12 12 Children 9-13 years 20 20 Adolescents 14-18 years 25 25 Adults 19 years and older 30 30 Pregnancy all ages - 30 Breast-feeding all ages - 35
Research indicates that biotin is broken down more rapidly during pregnancy and that biotin nutritional status declines during the course of pregnancy (6). One study reported that biotin excretion dropped below the normal range during late pregnancy in six out of 13 women, suggesting that their biotin status was abnormally low. Over half of pregnant women have abnormally high excretion of a metabolite (3-hydroxyisovaleric acid) thought to reflect decreased activity of a biotin-dependent enzyme. A study of 26 pregnant women found that biotin supplementation decreased the excretion of this metabolite compared to placebo, suggesting that marginal biotin deficiency may be relatively common in pregnancy (14). In one study, the incidence of decreased lymphocyte propionyl-CoA carboxylase activity (a marker of biotin deficiency) in pregnancy was greater than 75% (18). Although the level of biotin depletion is not severe enough to cause diagnostic signs or symptoms, such observations are sources of concern because subclinical biotin deficiency has been shown to cause birth defects in several animal species (16). Currently, it is estimated that at least one third of women develop marginal biotin deficiency during pregnancy (8). Indirect evidence also suggests that marginal biotin deficiency causes birth defects in humans. On balance, the potential risk for teratogenesis (abnormal development of the embryo or fetus) from biotin deficiency makes it prudent to ensure adequate biotin intake throughout pregnancy. Since pregnant women are advised to consume supplemental folic acid prior to and during pregnancy (see Folic Acid) to prevent neural tube defects, it would be easy to consume supplemental biotin (at least 30 mcg/day) in the form of a multivitamin that also contains at least 400 mcg of folic acid. Toxicity at this level of biotin intake has never been reported (See Safety).
It has been known for many years that overt biotin deficiency impairs glucose utilization in rats (19). In one human study, blood biotin levels were significantly lower in 43 patients with non-insulin dependent diabetes mellitus (NIDDM; type 2 diabetes) than in non-diabetic control subjects, and lower fasting blood glucose levels were associated with higher blood biotin levels. After one month of biotin supplementation (9,000 mcg/day), fasting blood glucose levels decreased by an average of 45% (20). In contrast, a study in ten type 2 diabetics and seven nondiabetic controls reported that biotin supplementation (15,000 mcg/day) for 28 days did not decrease fasting blood glucose levels in either group (21). A more recent double-blind, placebo-controlled study by the same group of investigators found that the same biotin treatment protocol lowered plasma triglyceride levels in both diabetic and nondiabetic patients with hypertriglyceridemia (22). In this study, biotin administration did not affect blood glucose concentrations in either diabetic or nondiabetic subjects. Additionally, a few studies have shown that co-supplementation with biotin and chromium picolinate may be a beneficial adjunct therapy in patients with type 2 diabetes (23-26). However, several studies have reported that administration of chromium picolinate alone improves glycemic control in diabetic subjects (27). See the separate article on chromium.
Reductions in blood glucose levels were found in seven insulin-dependent (type 1) diabetics after one week of supplementation with 16,000 mcg of biotin daily (28). Several mechanisms could explain a possible blood glucose-lowering effect of biotin. As a cofactor of enzymes required for fatty acid synthesis, biotin may increase the utilization of glucose for fat synthesis. Biotin has been found to stimulate glucokinase, a liver enzyme that increases synthesis of glycogen, the storage form of glucose. Biotin has also been found to stimulate the secretion of insulin in the pancreas of rats, which also has the effect of lowering blood glucose (29). An effect on cellular glucose transporters (GLUT) is currently under investigation. Presently, studies of the effect of supplemental biotin on blood glucose levels in humans are extremely limited, but they highlight the need for further research.
The finding that biotin supplements were effective in treating hoof abnormalities in horses and swine led to speculation that biotin supplements might also be helpful in strengthening brittle fingernails in humans. Three uncontrolled trials examining the effects of biotin supplementation (2.5 mg/day for up to six months) in women with brittle fingernails have been published (29-31). In two of the trials, subjective evidence of clinical improvement was reported in 67-91% of the participants available for follow-up at the end of the treatment period (29, 30). One trial that used scanning electron microscopy to assess fingernail thickness and splitting found that fingernail thickness increased by 25% and splitting decreased after biotin supplementation (31). Although the results of these small uncontrolled trials suggest that biotin supplements may be helpful in strengthening brittle nails, larger placebo-controlled trials are needed to assess the efficacy of high-dose biotin supplementation for the treatment of brittle fingernails.
Although hair loss is a symptom of severe biotin deficiency (see Deficiency), there are no published scientific studies that support the claim that high-dose biotin supplements are effective in preventing or treating hair loss in men or women.
Biotin is found in many foods, but generally in lower amounts than other water-soluble vitamins. Egg yolk, liver, and yeast are rich sources of biotin. Large national nutritional surveys in the U.S. were unable to estimate biotin intake due to the scarcity of data regarding biotin content of food. Smaller studies estimate average daily intakes of biotin to be from 40 to 60 mcg/day in adults (1). The table below lists some rich sources of biotin along with their content in micrograms (mcg) (32). However, a recent publication that employed chemical rather than microbial assays reported quite different content for some common foods (33).Food Serving Biotin (mcg) (32, 33) Yeast 1 packet (7 grams) 1.4-14 Bread, whole-wheat 1 slice 0.02-6 Egg, cooked 1 large 13-25 Cheese, cheddar 1 ounce 0.4-2 Liver, cooked 3 ounces* 27-35 Pork, cooked 3 ounces* 2-4 Salmon, cooked 3 ounces* 4-5 Avocado 1 whole 2-6 Raspberries 1 cup 0.2-2 Cauliflower, raw 1 cup 0.2-4
*A 3-ounce serving of meat is about the size of a deck of cards.
Most bacteria that normally colonize the small and large intestine (colon) synthesize biotin. Whether the biotin is released and absorbed by humans in meaningful amounts remains unknown. However, a specialized process for the uptake of biotin has been identified in cultured cells derived from the lining of the small bowel and colon (34), suggesting that humans may be able to absorb biotin produced by enteric bacteria—a phenomenon documented in swine.
Biotin is not known to be toxic. Oral biotin supplementation has been well-tolerated in doses up to 200,000 mcg/day in people with hereditary disorders of biotin metabolism (1). In people without disorders of biotin metabolism, doses of up to 5,000 mcg/day for two years were not associated with adverse effects (35). However, there is one case report of life-threatening eosinophilic pleuropericardial effusion in an elderly woman who took a combination of 10,000 mcg/day of biotin and 300 mg/day of pantothenic acid for two months (36). Due to the lack of reports of adverse effects when the Dietary Reference Intakes (DRI) were established for biotin in 1998, the Institute of Medicine did not establish a tolerable upper level of intake (UL) for biotin (1). Note: 1 mg = 1,000 mcg.
Large doses of pantothenic acid (vitamin B5) have the potential to compete with biotin for intestinal and cellular uptake due to their similar structures (37). In addition, very high (pharmacologic) doses of lipoic acid have been found to decrease the activity of biotin-dependent carboxylases in rats, but such an effect has not been demonstrated in humans (4, 38).
Individuals on long-term anticonvulsant (anti-seizure) therapy reportedly have reduced blood levels of biotin as well as increased urinary excretion of organic acids that indicated decreased carboxylase activity (39). The anticonvulsants primidone and carbamazepine inhibit biotin absorption in the small intestine. Chronic therapy with phenobarbital, phenytoin, or carbamazepine appears to increase urinary excretion of 3-hydroxyisovaleric acid. Use of the anticonvulsant valproic acid has been associated with decreased biotinidase activity in children (17). Long-term treatment with sulfa drugs or other antibiotics may decrease bacterial synthesis of biotin, theoretically increasing the requirement for dietary biotin.
Little is known regarding the amount of dietary biotin required to promote optimal health or prevent chronic disease. The Linus Pauling Institute supports the recommendation by the Food and Nutrition Board of 30 micrograms (mcg) of biotin/day for adults. A varied diet should provide enough biotin for most people. However, following the Linus Pauling Institute recommendation to take a daily multivitamin-mineral supplement will generally provide an intake of at least 30 mcg of biotin/day.
Older adults (65 years and older)
Presently, there is no indication that older adults have an increased requirement for biotin. If dietary biotin intake is not sufficient, a daily multivitamin-mineral supplement will generally provide an intake of at least 30 mcg of biotin/day.
Written in June 2004 by:
Jane Higdon, Ph.D.
Linus Pauling Institute
Oregon State University
Updated in August 2008 by:
Victoria J. Drake, Ph.D.
Linus Pauling Institute
Oregon State University
Updated and Reviewed in August 2008 by:
Donald Mock, M.D., Ph.D.
Departments of Biochemistry and Molecular Biology and Pediatrics
University of Arkansas for Medical Sciences
Copyright 2000-2008 Linus Pauling Institute
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