Healthy looking hair is in general a sign of good health and good hair-care practices. Most healthy individuals have adequate nutrients in their diet; however some people do not have access to good nutrition, others have medical illnesses that predispose them to nutritional deficiency which influence scalp / body hair. ]

Nutrition is a complex subject - the effects of correct nutrition are indirect and often slow to appear. Hair in particular is slow to respond to any stimulus.Trials have indicated that correct nutrition is instrumental inhealthy hair growth, and conversely many deficiencies correlate with hair loss. 
Hair nutrition is therefore a vital part of any treatment regime. A truly systematic and rigorous approach must be taken when formulating a nutritional supplement for hair due the many factors that affect the eventual efficacy of the treatment. 
Malnutrition, congenital heart disease, neuromuscular disease, chronic illnesses, malignancy, alcoholism, and advanced age can cause hair to change colour, be weakened, or lost. 
Genetics and health are factors in hair wellbeing. Proper nutrition is important. The living part of hair is under the scalp skin where its root is housed within its follicle. It derives its nutrients from blood.  Health concerns e.G. Stress, trauma, medications, medical conditions, heavy metals, smoking etc. Can affect the hair.

Hair is the fastest growing natural tissue in the human body: the average rate of growth is 1 cm per month. Optimal growth occurs from age 15 - 30 and reduces from age 40 - 50. Hair products (shampoos or vitamin supplements) have not been shown to noticeably change this rate. The cycles of growth of each follicle consist of creation followed by self destruction. During each new cycle the follicle is built anew from raw materials.

The speed of hair growth varies based upon genetics, gender, age, hormones. It may be reduced by nutrient deficiency (i.E, anorexia, anemia, zinc deficiency) and hormonal fluctuations (i.E, menopause, polycystic ovaries, thyroid disease).

It is important to mention that many of the metabolic requirements of follicle cells (minerals and vitamins) must be satisfied for optimal hair growth (not always derived from fast foods and punishing work schedules). 
Nutritionists confirm that people with certain nutritional deficiencies tend to have dry, stringy and dull hair, and sometimes experience hair loss. Fortunately the latter can be restored once the deficiency is addressed.