Fat Soluble Vitamins - Vitamin A
Vitamin A has two major forms of biological significance – the animal based retinol, found in liver, dairy products and oily fish; and the plant derived ß-carotene, found in orange and dark green vegetables and palm oil (the latter have a lower potency than animal derived vitamin A).
Vitamin A is absorbed in the small intestine and mainly stored in the liver.
The best known role of vitamin A is in vision, particularly night vision, as a vitamin A derivative 11-cis retinal, in the sensory areas of the retina. It also plays a role in colour vision. Other major functions are in cell differentiation (i.e. control of gene expression in different cell types, be they a liver or brain cell) and the immune system’s responses to infection.
Deficiency is characterised by night blindness, particularly in the developing world. Dietary inadequacy also increases the risk of infections, especially in children. Treatment is via immediate oral dose of a vitamin A derivative in oil. Prevention is better than treatment and many countries administer large doses of vitamin A as a precaution.
Vitamin A excess is seen acutely, after large doses around 200 times the RNI. Symptoms include nausea, vomiting, headaches, muscular in coordination, blurred vision and vertigo. Drowsiness, lack of appetite, itching, peeling skin and vomiting may then follow. All these symptoms disappear within a week of an acute dose.
The more common chronic toxicity brought on by recurrent ingestion over weeks or years of over 10 times the recommended amount may result in headaches, cracking skin around the lips, dry itchy skin, liver toxicity and bone pain.
Most patients fully recover from toxicity, following reduction of vitamin A intake. These symptoms are particularly seen with the acne drug Roaccutane, which also brings about transient increases in serum triglycerides and cholesterol.
Excessive intakes of vitamin A also cause severe birth defects, so use of Roaccutane and excessive intakes of dietary vitamin A are contraindicated in pregnancy.
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