It is a component of a pigment present in the retina of the eye. Vitamin A is essential for the proper functioning of most body organs and also affects the functioning of the immune system. It promotes bone growth, teeth development and reproduction. It helps form and maintain healthy skin, hair, mucous membranes.

Deficiency in Vitamin A - a powerful antioxidant - results in various disorders that most commonly involve the eye and the epithelial tissues--the skin and the mucous membranes lining the internal body surfaces. An early symptom of vitamin A deficiency is the development of night blindness, and continued deficiency eventually results in loss of sight. If deficiency is prolonged, the skin may become dry and rough and eventually lead to defective bone and teeth formation.

Excessive intake of vitamin A causes a toxic condition. The symptoms may include nausea, coarsening and loss of hair, drying and scaling of the skin, bone pain, fatigue, and drowsiness. There may also be blurred vision and headache in adults, and growth failure, enlargement of the liver, and nervous irritability in children.

Deficiency is rare but may impair neuromuscular and circulatory functions. Symptoms of deficiency include lack of vitality, apathy, inability to concentrate, irritability, and muscular weakness.

Deficiency of this vitamin causes edema and hemolytic Anemia in children. Although serious toxicity has not been attributed to large doses of vitamin E, adverse effects have been reported.

Deficiency is marked by joint pains, tooth/gum defects, irritability, growth retardation, anemia, shortness of breath, and increased susceptibility to infection (slow wound healing). Scurvy is the classic disease related to deficiency. Symptoms peculiar to infantile scurvy include swelling of the lower extremities, pain upon flexing them, and bone lesions.

Excessive ascorbic-acid intake can cause kidney stones, gastrointestinal disturbances, and red-blood-cell destruction.

Reddening of the lips with cracks at the corners of the mouth, inflammation of the tongue, and a greasy, eyes sensitive to light, dizziness, trembling, insomnia, scaly inflammation of the skin are common symptoms of deficiency. Low riboflavin (B2) levels have been reported (a cause or result) in 81% of patients with cataracts.

Dark urine, nausea and vomiting are signs of overdose.

Deficiency most commonly results in pernicious anemia, weakness, fatigue, red sore tongue, nerve degeneration, nausea, appetite (and weight) loss, bleeding gums, confusion and dementia, headache, yellow eyes and skin, and shortness of breath.

Large doses of vitamin B12 may cause nosebleed, ear bleeding, and dry mouth.