I am a huge proponent of eating vitamins rather than applying them to your hair. I may however stand slightly corrected on panthenol. This post comes courtesy of India who emailed to ask, ' I would like to know the benefits of panthenol for the hair because I see this ingredient in many hair care products.'
Well the research on panthenol is not entirely clear. The hair bible - (Chemical and Physical Behaviour of Human Hair by Clarence Robbins) refers to panthenol as a hype ingredient.
Panthenol is converted to pantothenic acid (B vitamin) on hair. Consuming vitamin B is thought to help with growth however when applied externally to hair there is no conclusive evidence to show that it acts in this way.
There is however evidence that hair could possibly absorb pantothenic acid and this action could help to mend damaged hair. (Hair and Hair Care by Dale H Johnson quoting a paper by Driscoll pg 82). I tried to find more information on what 'mend damaged hair' meant but unfortunately the paper by Driscoll does not seem to be available digitally.
The same book (Dale Johnson) also says that pantothenic acid can be absorbed into the cortex of the hair and help to repair damage from perming and colouring. I therefore think that 'mend damaged hair' probably means pantothenic acid may be able to patch up areas of protein damage. I am not however convinced that this would be permanent nor do I think it would be better than hydrolysed protein.