Evidence for the existence of accessory food factors, later to be known as vitamins was obtained from the study of deficiency diseases such as scurvy, beriberi, pellagra, and rickets, which were suspected to be of dietary origin, and also from experiments in which highly purified diets were fed to rats and mice. From these two independent lines of investigation, the idea emerged that natural foodstuffs contain small quantities of substances, other than proteins, carbohydrates, fats, and minerals that are essential for normal growth and health. This chapter describes vitamin as an organic substance present in natural foodstuffs that the body requires in very small amounts but is not able to make for itself in sufficient quantity. This rather clumsy definition is necessary to cover what is now known about this very varied collection of nutrients. From the dietary point of view, the vitamins may be divided into major and minor vitamins. Lack of any of the major vitamins, which include vitamins A, D, and C and three or four members of the vitamin B complex, leads to a recognizable deficiency disease. The requirement for the remaining vitamins is much less; they are very widely distributed, and many of them are synthesized by microorganisms that inhabit the gut so that deficiency rarely occurs. A vitamin may be an essential dietary constituent for one animal species but unnecessary for another. In certain instances, the body may have a limited ability to synthesize a particular vitamin but be unable to meet its requirements in all circumstances.
Evidence for the existence of ‘accessory food factors’, later to be known as ‘vitamins’, was obtained from the study of deficiency diseases such as scurvy, beriberi, pellagra and rickets, which were suspected to be of dietary origin, and also from experiments in which highly purified diets were fed to rats and mice. From these two independent lines of investigation the idea emerged that natural foodstuffs contain small quantities of substances, other than proteins, carbohydrates, fats and minerals, that are essential for normal growth and health. The nature of these substances was unknown, but that there was more than one was soon recognized because both fat-soluble and water-soluble factors were required. Initially the vitamins were designated by letters of the alphabet but as their structure and functions were determined most of them were given specific names. However, since in various instances several closely related compounds have similar biological effects, it is convenient to use a generic term for a group of substances, e.g. vitamin D to include both cholecalciferol and ergocalciferol (page 156).
A vitamin is an organic substance present in natural foodstuffs which the body requires in very small amounts but is not able to make for itself in sufficient quantity. This rather clumsy definition is necessary to cover what is now known about this very varied collection of nutrients.
From the dietary point of view the vitamins may be divided into ‘major’ and ‘minor’ vitamins. Lack of any of the so-called major vitamins, which include vitamins A, D and C and three or four members of the vitamin B complex, leads to a recognizable deficiency disease. The requirement for the remaining vitamins is much less; they are very widely distributed and many of them are synthesized by microorganisms that inhabit the gut so that deficiency rarely occurs.
Other important points are that a vitamin may be an essential dietary constituent for one animal species but unnecessary for another, and that in certain instances the body may have a limited ability to synthesize a particular vitamin but be unable to meet its requirements in all circumstances. Furthermore, even though the diet may contain what is normally regarded as an adequate supply of a vitamin for one reason or another the body may not be able to use it.
The fat-soluble vitamins include vitamins A, D, E and K and their distribution is as varied as their functions. Their absorption is closely tied to that of fat and, if for any reason the intake or absorption of fat is inadequate, a secondary deficiency may occur. Furthermore, they tend to be stored in body fats, notably in the liver and an excessive intake of vitamin A or D produces toxic effects.
Vitamin A is a colourless substance that is present in animal fats, e.g. liver, milk, butter and eggs, although the richest sources are fish liver oils. Margarines are fortified to bring their content up to that of butter. Because of its specific effects in the visual process and the fact that it is an alcohol, the alternative name for vitamin A is retinol.
Plants do not contain vitamin A as such, although many of them include a variety of highly pigmented carotenoids. Some of these, namely the α-, β– and γ-carotenes, can be converted into vitamin A in the body and hence are provitamins, β-carotene is the most active of these. It is a symmetrical compound and can be converted into two molecules of vitamin A in the intestinal wall and in the liver, but since β-carotene is poorly absorbed and only partially converted, its utilization efficiency is only about one-sixth. Unchanged carotene is found in the plasma and in the body fat. Vitamin A and carotene are both fairly stable and not much affected by cooking, although some activity may be lost if fats become rancid. The recommended daily intake for adults is 750 μg (5000 International Units) of retinol, 6 × 750 μg of β-carotene or 12 × 750 μg of α- and γ-carotene.
Apart from its specific function in the visual process (page 155), vitamin A is needed for growth, reproduction and the maintenance of the integrity of epithelial cells. Since retinol is stored in the liver, signs of its deficiency develop very slowly. In Man the earliest noticeable effect is night-blindness, i.e. the inability to see in dim light, and this is readily cured by administration of the vitamin. Later, xerophthalmia develops, the eyes become dry and susceptible to infection and the conjunctiva and cornea become keratinized. This is followed by keratomalacia or softening of the cornea which leads to permanent blindness. Even today vitamin A deficiency is the commonest cause of blindness in Africa, India and the Far East in spite of the fact that carotene can be obtained from green leaves, maize and other vegetables. Carotene is poorly absorbed even in normal subjects and, where nutrition is inadequate, not only may the diet be deficient in fat but the gastrointestinal tract may be abnormal so that the fat-soluble vitamins are very poorly absorbed.
Experimental animals which are deficient in vitamin A fail to breed. Spermatogenesis is affected and female rats, if they conceive, resorb the fetuses or produce malformed offspring. In both animals and Man there are widespread generalized changes in the epithelial tissues which, whatever their normal type, become increasingly stratified, squamous and keratinized. Glandular epithelium is affected before the acinal cells so that the ducts become blocked by desquamating cells, and secretion, e.g. of the lachrymal and sweat glands, is impaired. This results in dryness of the eyes and hyperkeratosis of the skin. Nevertheless the basal cells appear not to be affected since, when the vitamin is administered, the epithelium promptly reverts to the normal type. The epithelial changes which result from deficiency reduce the local resistance to infection, and death often results from secondary infections. The general defence mechanisms of the body appear to be impaired.
Retinol is nearly always present in the food in the form of esters which are hydrolysed in the lumen of the intestine. The retinol released is quite readily absorbed into the mucosal cells where it is re-esterified, chiefly with palmitic acid. The retinyl esters are then transported via the lymphatic system into the portal circulation from which they are removed and stored in the liver. Release of the vitamin from the liver depends on the production by the liver of a special retinol-binding protein (RBP). Production of the retinol-binding protein may be disturbed in diseases of the liver or kidneys or in protein/energy malnutrition. In such circumstances retinol cannot be mobilized from the stores and a secondary deficiency may result. Thus it can be seen that the level of retinol in the general circulation is normally highly regulated and is more or less independent of the body’s reserves.
In the liver a small proportion of the retinol is oxidized to retinoic acid for which the liver has no storage capacity so that it is released into the bloodstream, where it is carried directly by serum albumin with which the retinol–RBP complex also combines. Some retinoic acid may be taken up by the tissues but it is constantly eliminated through the bile as glucuronide so that no accumulation occurs.
The biochemical significance of retinoic acid is under debate. Rats deficient in vitamin A, when given retinoic acid as the sole source of vitamin A activity, grew normally and showed no epithelial irregularities but were both blind and sterile, whereas, if given the corresponding aldehyde, retinal, they were normal in all respects. Retinoic acid cannot, however, be dismissed as a mere by-product because there are intracellular binding proteins for both retinol and retinoic acid. The existence of the latter suggests that retinoic acid may have a specific role in metabolism.
From the effects of its deficiency it appears that vitamin A is required for the controlled division and differentiation of cells. Retinol is delivered to specific sites on the cell surface that recognize the RBP and cause it to release the retinol. Once inside the cell the retinol combines with the cellular retinol-binding protein and retinoic acid with cellular retinoic acid-binding protein and each is delivered to nuclear binding sites which may be important in gene expression. Thus their mode of action resembles that of the steroid hormones (page 349).
Apart from effects on the nucleus, retinol has been found to have a stabilizing effect on cell membranes. More specifically retinol seems to take part in glycosyl-transfer reactions and the synthesis of various glycoconjugates (page 407). Effects on cell surface components may mediate changes in intracellular recognition and interactions and also in the process of cell adhesion and aggregation.
Normally after exposure to a bright light it takes several minutes before a person is able to perceive dimly lit objects. Vision then slowly improves over the next 30 min or so. This process of dark adaptation is associated with the regeneration in the rod cells of the retina of a pigment known as visual purple or rhodopsin which is bleached by strong light. Rhodopsin consists of a protein opsin combined with a stereoisomer of retinal, the aldehyde of vitamin A, Δ11–cis-retinal. During the visual process, i.e. the conversion of the energy of the light falling on the
retina to nerve impulses, the cis-retinal undergoes a molecular rearrangement to form all-trans-retinal which is more stable. This change in configuration causes the rhodopsin to dissociate into its constituents, opsin and all-trans-retinal. Rhodopsin can then only be regenerated after the retinal has been rearranged to the 11-cis form and this can only occur in dim light. Reutilization of the retinal is never complete since an appreciable fraction of the all-trans-retinal is reduced to retinol by alcohol dehydro-genase and NADH and some may be lost into the blood. The visual cycle is shown in Figure 12.1.
It is not clear why scotopic vision is so much more readily affected by vitamin A deficiency than photopic vision since retinal is a constituent of the three cone pigments as well as of rhodopsin. Normal vision requires the presence of 11-cis-retinal and the synthesis of four different opsins.
If the rate of uptake of retinol from the intestine consistently exceeds the capacity of the liver to dispose of it, significant amounts of retinol, mainly in the form of retinyl palmitate, appear in the general circulation and may give rise to toxic effects. The effects of hypervitaminosis A are many and varied. They include increased intracranial pressure, severe headache, hyperirritability, vomiting, diarrhoea, bone decalcification and skin lesions. The condition can be fatal. It has in the past been caused by over-zealous administration of concentrated sources of the vitamin such as halibut liver oil. This may contain several hundred times as much vitamin A and 40 times as much vitamin D as cod liver oil. It has also occurred in people who have eaten polar bear or husky dog liver which contain massive amounts of vitamin A.
Recent work has undermined the status of vitamin D as a true vitamin (which was always dubious) but certainly not its biochemical significance. That it is not an essential nutrient for people whose skin is sufficiently exposed to sunlight has been known for many years, but it has now been shown that vitamin D can claim to be a hormone precursor.
The deficiency disease that results from a lack of vitamin D is rickets, a condition that used to be common in the smoky cities of the Northern hemisphere and for which even in the nineteenth century cod liver oil and sunshine were recognized treatments.
Like the other fat-soluble vitamins, vitamin D is found in more than one form and of these the two most important are cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2). They occur largely as inactive provitamins which are converted into the active forms on exposure to ultraviolet light. The provitamins are both sterols and the effect of irradiation is to open the B ring; this means that the vitamins themselves are not sterols. Provitamin D3 or 7-dehydrocholesterol is present in the unsaponifiable fraction of animal fats. It is always present in the skin and is converted to vitamin D3 on exposure to sunlight.
Vitamin D, like vitamin A, is stored in the body, chiefly in the liver. It is not readily metabolized or excreted and excessive amounts have a toxic effect. Hypervitaminosis D produces a variety of symptoms including increased resorption of the bones. In children there is a loss of appetite, nausea, vomiting, diarrhoea and eventually stupor; fatal cases have shown calcification of the arteries, renal tubules, heart and lungs. Since the consequences of an overdose are so damaging it is important that the body should not produce too much vitamin D and the need for protection against overproduction may have been a major factor in determining the selection and distribution of races of different skin colour. Penetration of the solar rays to the site of provitamin activation is determined by the pigmentation and degree of keratinization of the stratum corneum.
Alternatively vitamin D may be obtained from the diet. The only rich sources are the liver oils of fish; few other foods contain significant amounts. In recognition of the risk of deficiency in Britain, where sunshine cannot be relied upon, margarine, dried milk and infant foods are fortified by the addition of vitamin D, while in the USA foods such as milk and yeast are irradiated in order to increase their content. A dietary intake of 10 μg cholecalciferol per day is sufficient to protect a child against rickets without fear of causing hypervitaminosis, while the adult requirement is 2·5 μg.
This is responsible for the clinical condition of rickets which in infants and children is characterized by abnormal endochondral calcification. The degenerating hypertrophic cellular zone in the epiphyseal cartilaginous plate fails to calcify. The epiphyseal plate increases in width and deforming cartilaginous swellings form at the ends of the long bones and at the junctions of the ribs with the sternum, giving them a knobbly appearance. The bones are soft because they are hypocalcified and consequently the legs are unable to support the weight of the body and become bowed. In rickets there is usually a fall in the levels of both calcium and inorganic phosphate in the serum, although the most consistent feature is an increase in the serum alkaline phosphatase.
Rickets does not often occur in adults although an adult counterpart, osteomalacia, now mercifully rare, may occur in women living mostly indoors and consuming a diet containing inadequate amounts of calcium and vitamin D, and subjected to frequent childbearing. Demineralization of the bones, resulting from these circumstances, may cause gross deformities.
Dental changes in vitamin D deficiency During tooth development in vitamin D-deficient animals, the enamel and dentine become hypoplastic and, since the ameloblasts are unable to function properly, the enamel calcifies poorly. The dentine matrix may also remain uncalcified with the result that interglobular spaces are formed.
No clear-cut relationship has been established between vitamin D intake and the incidence of dental caries. If ameloblast formation is interrupted and the enamel matrix is defective it will become inadequately mineralized and the enamel will be pitted. While the enamel itself does not appear to be more susceptible to caries than usual, the rough surface may favour plaque formation.
Vitamin E was discovered in 1922 by Evans and Bishop who found that a fat-soluble factor present in vegetable oils prevented the abnormalities in reproduction that occurred in rats fed on a diet containing rancid lard. This led to the discovery of a group of alcohols known as tocopherols which differ from each other in the number and position of methyl groups attached to the tocol ring. The most active of these is α-tocopherol whose structure is shown. Tocopherols are present in almost every food, and particularly in vegetable fats and the germ of cereal grains, so that a dietary deficiency of vitamin E is rare in normal subjects. Deficiency may, however, result from conditions in which there is prolonged malabsorption of fat. These include the rare inherited condition of abetaliproproteinaemia in which chylomicron formation is prevented by
the absence of apoprotein B. Under such circumstances the serum tocopherol level falls and the fragility of the red blood corpuscle membrane is increased. Vitamin E deficiency also occurs in premature and low-birth-weight babies in which it may be responsible for haemolytic normocytic anaemia, blindness and intraventricular haemorrhage.