Facts about vitamins A, D, E and K
Vitamins A, D, E and K are fat-soluble.
Vitamin A
Vitamin A, which is actually a group of interrelated substances (retinol, retinal and retinoic acid), can be synthesised in the body from β-carotene, found in dark-green leafy vegetables such as cabbage, sprouts, broccoli and spinach, and in carrots. Cooking the vegetables does not damage the β-carotene molecules and in fact β-carotene is more easily absorbed into the body from cooked carrots.
The speed at which the human eye adapts to seeing in the dark depends on the amount of vitamin A available in the body, known as the vitamin A status. Xerophthalmia or dry-eye is a classic sign of vitamin A deficiency.
Vitamin A is involved in normal growth and bone formation and it plays a part in the production of red blood cells and therefore the prevention of anaemia.
Vitamin D
The main role of vitamin D is to facilitate the uptake of calcium from food, through the lining of the small intestine into the blood. It also controls the deposition of calcium in the bones during growth and maintains adult bone structure.
In fact, the beneficial effect of sunlight, in playing a part in vitamin D synthesis, has to be balanced with the detrimental effect, its role in causing skin cancers. It appears that an exposure to sunlight of about 30 minutes per day (avoiding the part of the day when sunlight is strongest) is an appropriate balance between the harmful and beneficial effects.
Vitamin E
Some chemical reactions in the body produce harmful substances called free radicals that contain single electrons and become involved in chain reactions in the cells, which can be damaging to the body. Antioxidants like vitamins A and E neutralise the harmful substances and prevent further damage.
Vitamin K
Vitamin K is widely distributed in the diet and it is absorbed from the small intestine with the assistance of bile acids.
The main role of vitamin K is in blood clotting. This process requires the presence of a number of different chemicals, called clotting factors, in the blood. A number of these (including prothrombin and Factors VII, IX and X) require vitamin K in their synthesis.