1.2 Vitamin A
Look back at Table 1 and identify the foods that contain vitamin A. On the basis of this information, try to predict where vitamin A is stored in the human body.
Vitamin A is found in liver. If it is stored in the liver of other animals that we eat, then it would be reasonable to suggest that it might be stored in human liver, which indeed it is. Since vitamin A is a fat-soluble vitamin, you might also predict that it would be stored in adipose tissue, which, to a lesser extent, it is too.
Vitamin A is toxic if taken in very large quantities and poisoning has occurred in Arctic explorers who have eaten polar bear liver, which is particularly rich in vitamin A. The concentration of vitamin A in lamb and calf liver has increased substantially in the last 20 years due to supplements to their feed. Pregnant women are advised to restrict their intake of liver and pâté made from liver, since there is some evidence that high doses of vitamin A can cause birth defects. However, vitamin A is an essential part of the human diet and severe health problems occur if there is a deficiency. Since dairy products, such as butter, are a good source of vitamin A, all types of margarine and similar spreads are now required by law to have vitamin A added to them, as you will see on their labels. 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 structure of β-carotene, retinol and retinal are shown in Figure 1.
Look carefully at the three molecules and identify the relationship between retinol and β-carotene and the difference between retinal and retinol.
If a β-carotene molecule (a) is split in the middle and a molecule of water (H2 O) added to each cut end, then two molecules of retinol (b) are produced. Retinol is the same as retinal (c) except that two hydrogen atoms have been removed from the end of the molecule to give a —CHO group, rather than a —CH2 OH group
Retinal is found in the cells of the eye where it plays a vital part in the perception of light and this is the reason why the ‘old wives’ tale’ that carrots help you to see in the dark is, in fact, true. 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. A ‘dark adaptation test’ can be used as a measure of vitamin A status. Vitamin A deficiency is a major public health problem in the developing world, causing blindness in a quarter of a million people each year. Vitamin A supplements are successful in preventing blindness from this cause.
Vitamin A also assists in keeping the epithelial cells of the body moist and healthy. As well as lining the whole of the digestive tract, epithelial cells cover the surfaces of the glands around the eyes and line the lungs (and are found elsewhere too). Xerophthalmia or dry-eye is a classic sign of vitamin A deficiency. Tear production is reduced and the eyes become susceptible to infections such as conjunctivitis. Children who are vitamin A-deficient are more susceptible to respiratory infections and measles. 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.
In order to understand another important role of vitamin A, and other vitamins, as antioxidants, you need to know a little more about the internal structure of atoms. Atoms sometimes carry a positive or a negative (+ or−) charge. These charges arise because atoms are made up of positively charged particles called protons and negatively charged particles called electrons. The protons, along with uncharged particles called neutrons, reside in the core of the atom as part of the atomic nucleus. For the purpose of this simple description, you can think of the electrons as tiny spheres that are in orbit around the nucleus. Normally, the numbers of protons and electrons in any particular atom are the same, so the positive and negative charges are balanced, and overall the atom has no charge. However, if an atom gains an electron, it has one extra negative charge (since it now has one more electron than it has protons), and so we would write a − sign beside it, and if it loses an electron, it has a positive charge (since it now has one more proton than it has electrons), and we would write + beside it. When atoms bond together, they can share pairs of electrons, one from each atom, so when we have been talking about a bond between two atoms (the atoms ‘holding hands’), we have actually been referring to these electron pairs.
However, sometimes a molecule can be formed in which there is an atom with a single free electron, and this type of molecule is called a free radical. Free radicals are extremely reactive and the problem is that as they react, they create more and more free radicals in a runaway chain reaction. This process happening within cells involves atoms in molecules such as DNA that are vital to the cell's functioning, and it can have serious health consequences. The precise way in which free radicals cause the damage attributed to them is not fully understood, but they are implicated in many human diseases and disorders. Many pollutants generate free radicals, as does smoking, and free radicals are probably the link between exposure to toxins and the development of cancer.
Certain molecules have the ability to donate electrons to free radicals, while not themselves being destroyed or becoming free radicals. Thus they can safely interact with free radicals and terminate the chain reactions before vital cell components are damaged or destroyed. Such molecules are known as antioxidants, and vitamin A is one of the important antioxidants in the body.