Dry, brittle and damaged hair lacks sheen and often appears unhealthy. People with dry hair also experience extra hair breakage, making their hair unruly at times. This makes it tricky to attain a lustrous and sophisticated hairstyle. Keeping your hair hydrated is a good way to make sure your hair becomes glossy, strong and more manageable.
Dry or damaged hair means lack of enough moisture and oil in the hair to sustain its normal shine and texture. Hair can get damaged when the scalp is not able to make sufficient oil to moisturize it, or when the hair lets moisture escape. Insufficient oil production makes the hair look dull and lifeless due to lack of natural lubrication. Shine and moisture of the hair rely on oil made in the root of hair. As you age, your hair makes a smaller amount of natural oil. Since the hair roots under your skin get dry, your scalp also becomes dry .
Malnutrition: It is a condition caused due to a lack of proper nutrition resulting from insufficient dietary intake or from a lack of balanced diet. Problems with digestion, an inadequate diet and medical problems can all play a role. Malnutrition is also known to cause dry hair due to nutritional deficiency.
Overdoing Hair Care: Habits like too much shampooing can strip away essential components that help your hair retain moisture. Hair perm treatments and hair dyes that expose hair to harsh chemicals have terrible effects on your locks. Do not experiment excessively as this may cause dry hair.
Excessive blow-drying: The excess use of hair dryers can damage hair and result in dry hair. Overuse of electric curlers and curling irons also contributes to dry hair.
Anorexia nervosa: It is a disorder in which a person starts to skip meals, eats little or exercises extensively to lose as much weight as possible. When it leads to more weight loss than is considered healthy, it usually causes deficiency of vital nutrients in our body. Dry and damaged hair can be the initial symptom of anorexia.
Intensive conditioning can help treat dry and damaged hair. Conditioning shampoos for dry and damaged hair contain large molecules that stick to the damaged scales of the cuticle (causing dry hair). They then fill in the fractures and fissures, make hair smooth and impart softness and lustre to dry hair. They help make hair manageable, repairing damaged hair and improving texture.
The molecules present in conditioners that help deal with dry hair include hydrolyzed proteins, dimethicone and cationic polymers. 
Experts recommends foods rich in protein (beans and pulses), carbohydrates (brown rice, wholegrain bread and wholemeal pasta and oats), vitamin and minerals (carrots, peppers, sweet potatoes and green vegetables) and biotin (eggs, cheese, yoghurt, breakfast cereals and chicken) to ensure complete hair health.
You can see a dermatologist if:
 Dry hair: MedlinePlus Medical Encyclopedia. [ONLINE] Available at: https://www.nlm.nih.gov/medlineplus/ency/article/003245.htm. [Accessed 29 October 2015].
 Pharmacologic interventions in aging hair. [ONLINE] Available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2695167/. [Accessed 29 October 2015].
 Hair Shaft Damage from Heat and Drying Time of Hair Dryer. [ONLINE] Available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3229938/. [Accessed 29 October 2015].