VITAMINS

 

Vitamins are dietary substances required by the body in very small amounts for normal biochemical function. There are two major groups: water-soluble vitamins (the B vitamins and vitamin C), and the fat-soluble vitamins A, D, E and K. Intensive research into their sources, their function and our requirements for them has been going on over the last sixty years or so and we now know a great deal about them.

Vitamin A. The term vitamin A covers several fat-soluble compounds including retinal, which is the most important component, and two lesser components, retinal and retinoic acid. Vitamin A is only found in animal produce but various vitamin-A-type compounds, the most important of which is beta-carotene, are found in vegetables, and these are water-soluble.

All of them are fat-soluble, are sensitive to light and are easily oxidized.

Though the deficiency signs of vitamin A were recognized in ancient Egyptian times, it was only in the century that its chemical nature was elucidated. Two researchers recognized a substance described as “fat-soluble A” as being a growth-promoting factor in animals. Later it was shown that vitamin A activity was present in plants in the yellow pigments known as carotenes.

What it does.

The most important and well-known role of vitamin A is in relation to eye function. Vitamin A is necessary to prevent drying of the eye (xerophthalmia) and corneal changes; also, the normal function of the retina, the part of the eye involved with vision, and particularly the function of the light-sensitive areas of the eye, are dependent upon there being sufficient vitamin A. Vitamin A is involved in a number of other bodily functions. It is important in maintaining the stability of cell membranes and this may be clinically important. Furthermore, beta-carotene is the most effective receptor of free radical oxygen and this may be relevant in situations involving cancer, inflammatory disease and atherosclerosis, in which free radical mechanisms are thought to play a part.

A connection between vitamin A and zinc metabolism has been described by several researchers. It appears that in severe zinc deficiency the formation of the protein which carries vitamin A, retinolbinding protein, is described. Zinc is an important constituent of many enzymes, including one found in the retina that is involved in vitamin A metabolism. Night blindness which does not improve with vitamin A supplements has been described in zinc deficient individuals, particularly in alcoholics with liver disease who are often vitamin A and zinc deficient.

In diseases such as alcoholic cirrhosis, pancreatic disease and cystic fibrosis, zinc and vitamin A and other deficiencies also often occur together.

Food sources:

The major dietory sources are from animal produce, which provides vitamin A-retinol – usually combined with a fatty acid. As vitamin A is stored in animal and fish livers, these provide the most concentrated sources. Others include kidneys, eggs, milk and butter. Margarine is, by law, fortified with vitamin A. Good vegetable sources include any green, yellow or orange-pigmented produce; the darker the colour, the higher the content of beta-carotine. Carrots, spinach, cabbage and orange and yellow fruits are the best sources.

Deficiency symptoms and signs.

Vitamin A deficiency, one of the commonest and most serious world-wide nutritional deficiencies, occurs in people who have malabsorption states. In particular, anyone with fatty stools caused by pancreatic disease, biliary obstruction, or small bowel disease, is at risk.

The earliest symptoms of vitamin A deficiency include night blindness (difficulty in seeing well in dim light) and dryness of the eyes.

Skin signs of vitamin A deficiency include follicular hyperkeratosis, a condition in which the whole hair follicle and its adjacent skin is raised by a plug of horny keratin. Dryness of the skin can be a feature of vitamin A deficiency but this may be due to an associate essential fatty acid deficiency or a deficiency of other micronutrients such as vitamin B complex.

Vitamin C and zinc are involved in fatty acid metabolism. Deficiency can occur without skin signs.

In general vitamin A deficiency is associated with poor growth and development, and impaired resistance to infection.