Vitamin B7

Biotin, needed for the enzymes that build fat and glucose. Widely marketed for hair and nails on thin evidence in people who are not actually deficient.

A coenzyme in fat and carbohydrate metabolism, and in gene regulation.

• Egg yolk
• Liver
• Nuts and seeds
• Salmon
• Sweet potato

Raw egg white blocks absorption; cooking destroys the avidin.

30 µg for most adults. The upper limit, past which harm becomes likely, is None established.

Reference intakes are population figures. Pregnancy, illness, age and medication all move an individual’s needs, which is one of several reasons this page is information rather than advice.

Most at risk: People eating raw egg white regularly — avidin in it binds biotin. Also pregnancy and some genetic disorders.

Rare. Long-term raw egg white consumption (avidin binds it), prolonged anticonvulsant treatment, some inherited enzyme disorders, and long-term parenteral nutrition without it.

Thinning hair, a scaly red rash around the eyes and mouth, and neurological symptoms. Rare.

Every sign here has other, often likelier, explanations. A list of symptoms is a reason to look into something, never a diagnosis.

Rarely measured. Its more important interaction with testing is different: high-dose biotin interferes with many laboratory immunoassays, including thyroid and troponin tests, and can produce badly misleading results.

Eggs (cooked), liver, nuts, seeds, salmon and sweet potato. Gut bacteria contribute some as well.

Doses in hair and nail products are often many times the reference intake. There is little evidence of benefit without a deficiency, and a real risk of distorting blood tests.

No toxicity, but a genuine practical hazard: high-dose biotin supplements interfere with many laboratory blood tests, including thyroid and troponin assays. That has caused missed heart attacks. Tell any doctor ordering blood tests if you take it.

Interactions: Distorts immunoassay blood tests — stop several days before testing.