Hi,

I know that the vitamin biotin (B7) improves hair growth. What I wonder is why is this? One website states the following: ""The biotin you consume in foods helps your body to turn the food you eat (protein, carbohydrates, and fat) into energy.". How does it make the hair grow faster then?

Actually there is little evidence that biotin improves hair growth, unless a person has biotin deficiency. Various forms of hair loss and skin conditions are associated with biotin deficiency and for the most part the cause is that carboxylases require biotin as a cofactor. I.e. hair loss and other conditions are the result of certain carboxylases not functioning any more. 

Otoh, there is little to no evidence that in patients with sufficient biotin intake will improve hair growth.

Biotin is a required cofactor for carboxylase enzymes that become activated once they are joined together by holocarboxylase synthase

 

But some people also have a enzyme deficiency, which generally a couple months of Biotin supplementation can correct, of course requiring the user to continue supplementation.

 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5582478/