Vitamin A is generic or family name which covers the active vitamin, retinol and the provitamin, carotene. Carotene is found in animal products and carotene is found in fruit and vegetables.
Vitamin A is stored here before it is transported to other parts of the body. Being fat soluble it cannot mix directly with the watery medium of the blood, but the liver forms special protein carriers for the vitamin so that it cab be transported into the main bloodstream. The particular need for protein in the functioning of vitamin A is one of the underlying causes of malnutrition. Vitamin A deficiency is common plight among under-nourished children in the world. Lack of fat and protein in the diet means that any available vitamin A cannot be digested and absorbed, and the little which does get to the bloodstream cannot be transported efficiently.
It is essential for the growth of teeth and bones and for the replacement of the surface layers of skin, both externally and internally. Internal surfaces include the linings of the respiratory passages, the mucous lining of the gut and the tracts of the urinary and genital organs. All these continually renew their fine layers of skin cells. Dead cells are worn away or sloughed off, as new cells replace them.
Vitamin A is found in the retina of the eye. It forms part of a substance called rhodopsin which enables us to distinguish black and white objects in low light. There are two types of receptors at the back of the eye called rods and cones. Night blindness can be a symptom of early stages of vitamin A deficiency.
The recommended daily amount of retinol for health is 750 micrograms. A good mixed diet will usually provide more than this. Excess vitamin A can be stored in the liver, and most of us have sufficient supplies to last for many months even if the diet became inadequate or if we were ill and could not eat properly. The pure vitamin, retinol is toxic at high concentrations and causes sickness, dizziness and dermatitis. Vitamin supplements are common practice for tiny babies and these are given as drops or cod liver oil preparations. It is very important to adhere strictly to the dosage and never be tempted to give more than is needed.