What is biotin?

In one line

Biotin is a water-soluble B vitamin that helps enzymes so the body can use the fat, sugar and amino acids in food.

What is biotin?
A name on hair and nail supplement bottles
You often see 'biotin' on supplements sold for hair and nails and on multivitamin labels. This post explains what kind of vitamin biotin is, what it does in the body, and what problems large supplement doses cause for blood tests.
In one line
A B vitamin that helps the body use nutrients
Vitamins are nutrients the body needs in small amounts to grow and work properly. Biotin is one of the B vitamins and is also called 'vitamin B7'. It helps the body use nutrients in food such as fat and sugar.
A water-soluble vitamin
Leftovers leave in urine
Biotin is a 'water-soluble vitamin'. The body does not store water-soluble vitamins; it sends leftovers out in urine. That is why they need to come in steadily from food.
What it does
It attaches to enzymes and helps them work
An 'enzyme' is a protein that speeds up chemical reactions in the body. Biotin attaches to a few enzymes and works with them. These enzymes process fat, sugar and 'amino acids', the small pieces that make up protein, so the body can use them.
Where it is found
Egg yolk, liver, milk
Biotin is found in tiny amounts in every living cell. Among foods, it is plentiful in egg yolk, organ meats such as beef liver and kidney, yeast, and milk.
How much a day
The adult reference amount is 30 micrograms
The adult daily amount used on US nutrition labels is 30 micrograms. A microgram is one-thousandth of a milligram, a very small amount. The US Food and Drug Administration says amounts at this level do not usually cause major interference with blood tests.
When you run low
Hair loss and a skin rash
When biotin runs low, hair falls out. A red, scaly rash appears around the eyes, nose and mouth. These signs appear in both children and adults.
How much is in supplements
Hundreds of times the daily amount
Supplements sold for hair, skin and nails contain up to 20 mg of biotin. 20 mg is 20,000 micrograms, and the US Food and Drug Administration says this is up to 650 times the daily amount.
It distorts blood tests
Results come out higher or lower than real
Many blood tests are built on biotin's ability to bind tightly to certain proteins. So when blood from someone taking a lot of biotin is tested, results come out higher or lower than the true value, depending on the test. Tests for 'hormones', substances that carry signals in the body, and heart tests are typical examples.
What actually happened
A death after a falsely low heart attack test
'Troponin' is a protein that leaks into the blood when heart muscle is damaged, and it is used to detect heart attacks. Biotin makes this level look lower than it really is. The US Food and Drug Administration received 1 report of a person taking high doses of biotin who died after a falsely low troponin result.
A common myth
Does biotin make hair grow better?
In the best-designed trial that looked at hair growth, there was no difference between biotin and a dummy pill (a pill with no active ingredient). The people in whom a benefit was reported were short of biotin or already had a hair or nail disorder.
Summary
A water-soluble B vitamin that helps enzymes
Biotin is a water-soluble B vitamin that helps enzymes so the body can use the fat, sugar and amino acids in food. Better hair growth has been reported only in people who were short of biotin or already had a disorder. Taking large supplement doses makes some blood test results come out wrong. Related terms: water-soluble vitamin, enzyme, troponin.