I think the quality of your water has a huge affect on the quality of your hair, my water is horrendous, it stinks of chlorine, shampoos struggle to lather up and my hair looks like wire, regardless of what I use.
There are lots of good brands available, but I am a firm believer in buying the brand you can afford to continue with. In the past I have paid a lot on money for a brand of vitamin, but sadly couldn’t afford to maintain that brand. Pukka do a great range of organic supplements, but their range is not large, and they are quite specific. Solgar are another good brand with an extensive range, and cytoplan are another good supplier of vitamins.
I would also suggest that where possible incorporate foods than are known to be beneficial for hair, like Brazil nuts, with their selenium content (only eat a maximum of 3 per day) when getting out vitamins and minerals from foods it has better bioavailability, which means we absorb more or the nutrients, than we would if we consumed it in man made form.
In addition I read that when you extract a vitamin from a food source and give it a value, it can look quite small, however, when eaten as a whole within the context of that food source, that nutrient is increased exponentially due to the other nutrients within the food they act synergistically.
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Changing the way you live your life
Healthy hair does seem to be genetic as my friend eats all the right things and she has terrible thin hair and so does her daughter but I do think you can help a little by taking a vitamin supplement like B6
Rising your hair with apple cider vinegar is said to help add a shine to your hair, also rinse with cool water.
There is more research to suggest genes do not influence in the ways we were originally meant to believe they did. It may be that your friend has a problem absorbing sufficient nutrition to keep her hair healthy. Lots of factors can effect our bodies ability to absorb, illness, medication, poor gut health. It may be worth considering improving the gut health with probiotics.