Introduction
Biotin (vitamin H) is a water-soluble vitamin that is usually classified
as a B vitamin. It is required by all living organisms, but can be synthesized
by plants and microorganisms. It plays multiple important roles in the body.
Biotin is the coenzyme for four carboxylases (a category of enzymes) – acetyl-CoA
carboxylase, pyruvate carboxylase, propionyl-CoA carboxylase, and methylcrotonyl-CoA
carboxylase. These enzymes respectively play important roles in fatty acid
synthesis; gluconeogenesis; amino acid, cholesterol, and fatty acid metabolism;
and leucine metabolism. Biotin also plays a role in gene expression and glucokinase
synthesis, among other things.
Organ meats such as liver and kidney are the best dietary sources of biotin.
Some other foods, such as wheat, contain considerable amounts of biotin but
with very low bioavailability. Other meats, vegetables, and fruits usually
contain low amounts of biotin.
Deficiency
Deficiency of biotin is associated with hair loss, skin irritation, impaired
immune function, increased risk of sudden infant death syndrome, fatty liver
and kidney, abnormal bone growth, and of course, death [1-4]. A common misconception
is that biotin is not needed from external sources because it is synthesized
by bacteria; multiple studies have shown that the amount provided by bacteria
is insufficient for various reasons [1]. Still, biotin deficiency is very
rare, but has been found in some circumstances. Multiple carboxylase deficiency,
a hereditary disorder, results in increased biotin needs. Sometimes deficiency
is seen in patients on long-term parenteral nutrition. Consuming uncooked
egg whites for long periods can also result in biotin deficiency, because
a protein found in egg whites binds to biotin and prevents absorption. Chronic
alcohol ingestion may also increase biotin requirements.
Although clinical deficiency is rare, subclinical biotin deficiency may be
quite common. Studies in animals are finding that the optimal amount for
health may be well above that needed to just alleviate classical deficiency
symptoms [1]. Suboptimal biotin intake or a marginal deficiency may impair
leucine metabolism, affect insulin sensitivity, and increase the risk of
birth defects [1, 2, 5-6]. It is also important to note that two common supplements,
alpha lipoic acid (ALA) and pantothenic acid (vitamin B5), may both compete
with biotin. Specifically, ALA administration to rats results in decreased
activity of the biotin-dependent carboxylases, presumably due to structural
similarity [7].
Insulin sensitivity
In theory, high doses of biotin may improve insulin sensitivity. This is
because biotin induces the synthesis of glucokinase, which leads to increased
glucose uptake [2]. However, the data is presently very limited. It is quite
likely that biotin improves insulin sensitivity in an insulin resistant state
[8-9]; while the data on this is limited, there is almost no data in normoglycemic
individuals. One study in normoglycemic hemodialysis patients found biotin
to have a beneficial effect on glucose metabolism [10], but it is not unlikely
that the biotin status of these patients was suboptimal to begin with. It
will take further research to establish whether or not biotin can have a
beneficial effect in this regard.
Dosage and toxicity
Studies in animals have found no toxicity from acute, short-term, and subchronic
administration of large doses of biotin, and no adverse events from biotin
supplementation (even from commonly utilized dosages of 5-20 mg) have been
reported [11]. Levels as high as 200 mg daily have been reported without
any side effects. The adequate intake for biotin has been set at 35-60 micrograms
daily, but this is based solely on the average biotin intake of the population
(which is presumed to be adequate, since deficiency is uncommon) so it has
limited utility in defining optimal levels.
As a supplement, biotin even in large doses is ~100 percent bioavailable
[12]. Taking a multivitamin that provides biotin should provide one with
enough to avoid suboptimal status. The general recommendation for biotin
supplementation to prevent deficiency due to ALA is 1 mg or more per 100
mg of ALA. If one is taking more than a gram of pantothenic acid daily, a
biotin supplement providing at least 1 mg is warranted, and 3-5 mg may be
best to stay on the safe side (it should also be noted that biotin and pantothenic
acid exhibit a synergistic relationship under some circumstances). Because
large amounts adminstered at a time might increase renal excretion, biotin
is more effective when taken as two or three smaller doses throughout the
day [12].
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