The first scientific awareness of Vitamin A dates back to 1913 when it became known as a factor that was required to combat night blindness and some other eye troubles. Initially the deficiency of it was countered by eating cod liver oil or animal liver. Its function at the molecular level in vision has now been long known. The chemical reactions sequences in the retina make up what is called the ‘visual cycle’, an essential process in visual function. Severe deficiency of Vitamin A is a common cause of blindness, both through the retina effects and also through ‚Äúxerophthalmia‚Äù, a condition in which the cornea at the front of the eye becomes cornified and therefore opaque enough to block vision. It was soon also recognised that this was an anti-infection vitamin. Resistance to infections was adversely affected in deficiency. The other roles of Vitamin A in the cell were not known until much more recently but they are concerned with the differentiation of the various different cell types from less differentiated and therefore more generalised cells. Specific receptors have been found on the cell surface for Vitamin A. When the vitamin binds to these they stimulate gene expression (or in a few cases inhibit it), thus giving an instruction relating to the further development of that cell. This has explained the role of Vitamin A in embryonic development and in spermatogenesis. The same phenomenon explains the involvement of Vitamin A in the senses of taste and hearing, and in appetite and growth. The same basic role may also explain the vitamin‚Äôs effect upon immunity, because all the different types of immune cells must differentiate. Notwithstanding the essential role of Vitamin A, it has now been realised that very high intakes of Vitamin A can be dangerous during pregnancy and may lead to birth defects. Hence, there has been a move to adjust intakes downwards generally. The official estimates of daily requirement for this Vitamin have also been revised downwards to 600 micrograms of retinol for women and 700 for men. However, paradoxically, this needs to be raised for pregnant women to 700 micrograms to provide for the special needs of their condition and the needs of the foetus. Since it is very likely that this amount will be provided by the diet, it is now common to recommend that pregnant women should take no supplements of Vitamin A whatsoever unless they are being expertly supervised. Given the modest daily requirement for this Vitamin and its wide availability in foods, supplementation is now considered less necessary than formerly. There was a practice in the past to offer high intake Vitamin A (up to 7,500 mcg of retinol daily for non-pregnant women and 9,000 mcg for men) to help those with skin conditions, especially acne, or to help the body to fight infections. These levels do not necessarily cause any problems, so there is an element of personal choice. But some official bodies now advise even non-pregnant adults to limit their regular daily intakes of supplementary Vitamin A to only 1,500 – 2,300 micrograms of retinol daily for men or women for safety reasons.>>Source
(Vitamin A refers to a group of fat-soluble substances, including retinol, retinal and carotenoids, found only in animal produce. Another substance that converts into vitamin A in the body is beta-carotene, which is water-soluble and found in vegetables and fruits.)
Uses in the body: essential for the development of healthy bones, skin and eyesight, particularly night vision; strengthens the immune system, increasing resistance to infection.
Signs of deficiency: night blindness (difficulty seeing in poor light), dry skin, poor growth and development.
Therapeutic uses: skin conditions such as eczema, acne and psoriasis, night blindness, ulcers, respiratory, menstrual and urinary problems; may also play an important role in cancer prevention.
Dietary sources: liver, kidney, oily fish, cod liver oil, dairy produce, eggs and margarine; beta-carotene is found in orange, yellow and dark green vegetables and fruit, such as carrots, yellow bell peppers (capsicum), broccoli, spinach and watercress.
Recommended daily allowance: 800µg
Typical therapeutic daily dose: 1,500 to 2,250µg
Daily intake shouldn’t exceed: 2,300µg
Cautions: not to be taken if pregnant or planning a pregnancy. Very high doses (above 9,000mg) can cause toxicity, signs of which include dry, scaly skin, fatigue, bone and joint pains and headache and require immediate medical attention. Discontinuation normally brings relief without further treatment.
Best taken: with food at breakfast and at lunch/evening meal for split doses.