Vitamin D refers to a group of fat-

soluble secosteroids responsible for enhancing

intestinal absorption

of calcium, iron, magnesium, phosphate and zinc.

In humans, the most important compounds in

this group are vitamin D3(also known

as cholecalciferol) and vitamin

D2

(ergocalciferol).Cholecalciferol and

ergocalciferol can be ingested from the diet and

from supplements. The body can also synthesize

vitamin D (specifically cholecalciferol) in the

skin, from cholesterol, when sun exposure is

adequate (hence its nickname, the "sunshine

vitamin").

Insufficient 50–100 nmol/L (20–40 ng/mL)

Mild 25–50 nmol/L (10–20 ng/mL)

Moderate 12.5–25.0 nmol/L (5–10 ng/mL)

Severe < 12.5 nmol/L (< 5 ng/mL)

Vitamin A deficiency (VAD) is a lack of vitamin A in humans. It is common in poorer countries but rarely seen in more developed countries. Nyctalopia(night blindness) is one of the first signs of VAD. Xerophthalmia, keratomalacia, and complete blindness can also occur since Vitamin A has a major role in phototransduction.Approximately 250,000 to 500,000 malnourished children in the developing world go blind each year from a deficiency of vitamin A, approximately half of whom die within a year of becoming blind. The United Nations Special Session on Children in 2002 set a goal of the elimination of VAD by 2010.The prevalence of night blindness due to VAD is also high among pregnant women in many developing countries. VAD also contributes to maternal mortality and other poor outcomes in pregnancy and lactation.

VAD also diminishes the ability to fight infections. In countries where children are not immunized, infectious diseases like measles have higher fatality rates. As elucidated by Alfred Sommer, even mild, subclinical deficiency can also be a problem, as it may increase children's risk of developing respiratory and diarrheal infections, decrease growth rate, slow bone development, and decrease likelihood of survival from serious illness.VAD is estimated to affect approximately one third of children under the age of five around the world. It is estimated to claim the lives of 670,000 children under five annually.Approximately 250,000–500,000 children in developing countries become blind each year owing to VAD, with the highest prevalence in Southeast Asia and Africa. According to the World Health Organization (WHO), VAD is under control in the United States, but, in developing countries, VAD is a significant concern.

Its a medical condition in which the eye fails to produce tears. It may be caused by a deficiency in vitamin A and is sometimes used to describe that lack, although there may be other causes.Xerophthalmia caused by a severe vitamin A deficiency is described by pathologic dryness of the conjunctiva and cornea. The conjunctiva becomes dry, thick and wrinkled. If untreated, it can lead to corneal ulceration and ultimately to blindness as a result of corneal damage.

The resulting cornea becomes totally opaque. Unfortunately, this is one of the most common reasons for blindness around the world, particularly in developing countries.

Keratomalacia is an eye disorder that results from Vitamin A deficiency.Vitamin A is required to maintain specialized epithelia (such as in the cornea and conjunctiva).