| Vitamin A is a fat soluble vitamin that is essential for an efficient
immune system and is important in the eyes, skin, teeth, bones, and
mucous membranes. Vitamin A plays a number of vital roles in the human
body, including assistance in the synthesis of mucopolysaccharides,
maintaining the stability and integrity of lysozomes and cell membranes,
and as a precursor of retinene in the retina. The physiological roles
of vitamin A include: Maintaining glucocorticoid production, supporting
epithelial regeneration, improving the antibody response to T-cell
antigens, increasing lymphocyte proliferation and cytokine production,
proper mucosal function, proper cellular differentiation, as well
as playing roles in maintaining visual and reproductive system integrity.
Vitamin A Storage
and Target Tissues
While all tissues contain trace amounts of vitamin A, more than
90% of the body’s vitamin A is stored in the liver (and the
kidneys to a lesser extent). Consequently, individuals with poor
liver or kidney function are particularly at risk for vitamin A
deficiency. Other predisposing factors include alcoholism, pancreatic,
gallbladder or respiratory disease, steatorrhea, measles, acute
protein deficiency, intestinal parasites, prolonged use of neomycin
sulfate and/or cortisone. Children under the age of 6 are also prone
to vitamin A deficiency.
Target tissues
for vitamin A include the retina, skin, bone, liver, adrenals, germinal
epithelium, intestines and salivary glands.
Signs and Symptoms
of Vitamin A Deficiency
Vitamin A is involved in a large number of systems in the human
body. Consequently, a large number of systems can be negatively
affected by vitamin A deficiency. Poor epithelial regeneration can
result in skin hyperkeratinization, problems with the genitourinary
reproductive system (poor fertility), the gastroenterological/biliary
system, the pulmonary system or the digestive tract. The effects
of vitamin A deficiency on the retina can result in night blindness
and/or epithelial degeneration in the eye. The effect of vitamin
A deficiency on the immune system can result in low lymphocyte count
and impaired immune response, as well as poor antioxidant function.
The developing skeletal system also requires vitamin A, and a deficiency
in this nutrient can result in growth retardation as well as abnormal
bone development.
Vitamin A Requirements
and Replenishment
The recommended daily allowance (RDA) for vitamin A has been based
on the amount needed to correct night blindness in people with vitamin
A deficiency, and the amount needed to raise plasma vitamin A levels
to normal levels in vitamin A deficient individuals. The highest
dietary sources of vitamin A are in fish liver oil and animal livers
and kidneys. Carrots, tomatoes, orange fruits and green vegetables,
palm oil, and dairy products (with the exception of cottage cheese)
are also good dietary sources of vitamin A.
Vitamin A Toxicity
Vitamin A is toxic if taken in excess over an extended period of
time. Taking more than 30,000 ug RE/day of vitamin A over a prolonged
period is generally considered toxic. Pregnant individuals should
be particularly careful to avoid excess vitamin A intake (more than
800 ug RE/day), as it is a known morphogen, and can cause abnormal
fetal development at dosages above 4800 ug RE/day.
Signs of vitamin
A toxicity include peeling/itching of the skin, brittle nails, yellowish
skin coloration (carotene only), hair loss (alopecia), anorexia,
gingivitis, delayed blood clotting and decreased vitamin K absorption,
elevated serum alkaline phosphatase, hypercalcemia, bone/joint pain,
hyperostosis, premature epiphysis closure (during growth), osteoporosis
and increased risk of hip fractures. Other symptoms include unusual
blood pressure in the eyes (papilledema), blurred vision, light
sensitivity, headaches, irritability, fatigue, and insomnia.
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