Premature greying of hair can be due to stress, smoking, nutritional deficiency, thyroid disorder, chemical use over hair etc. but most important cause is hereditary factor.
As such nothing much can be done for premature greying esp if it is hereditary or familial. The hair that has turned white would remain so but its progression can be prevented to some extent by taking hair supplements containing calcium pantothenate and PABA. For the same you may take tablet follihair one daily.
Food rich in proteins (like sprouts, lentils, soy products), vitamin A and B like whole wheat products, green leafy vegetables, carrots, yellow and orange fruits like mangoes, papayas, nuts like sunflower seeds, walnuts, sesame seeds provide minerals like copper which are essential for maintaining hair pigment.
Hope that answers your query. Feel free to write back if having any doubts.
Regards
Shield360 
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