Hi, Thanks for your query. White Hair or greying hair mainly occurs when your hair follicles stop producing melanin, the pigment that gives hair its color. Though, ageing is the primary reason of grey hair  but premature greying of hair in early 20’s or 30’s is now common. Genetic factors, stress, lack of nutrition, chemicals, smoking or any underlying disease like anemia can also lead to premature greying of hair. All these factors lead to underproduction melanin that leads to grey or white hair. Lack of minerals like zinc, copper is one of the mos underestimated yet one of the most analysed reasons for grey hair. Turning already grey hair into black is not possible after a particular age, but you can always slow down further greying of hair. A balanced diet is the most effective solution to control greying of hair. Include lots of fresh fruits, vegetables, protein rich sources of food in your diet. Load up on food items containing antioxidant and vitamin C like gooseberry, oranges, lemon etc.