Vitamins in Endocrine Metabolism

I. W. Jennings

• Publisher: Butterworth-Heinemann
• Year: 2014
• ISBN:
  9781483192888
• Topic: Médecine
• Subtopic: Médecine clinique

Vitamins in Endocrine Metabolism covers the problems of nutritional diseases in the fields of endocrinology, pathology, enzymology, and vitamin research. This book is divided into 11 chapters that discuss the conditions affecting vitamin requirements. The introductory chapters deal with the intracellular localization, synthesis, molecular structure, and reaction rate of enzymes. The succeeding chapters examine the methods of analysis and mode of action of hormones; pathology of vitamin A deficiency in man and animals; description of vitamin B complex; and diseases of vitamin C deficiency. Other chapters explore the modifying effects of the diet and availability of enzymic activators, as well as the biochemical aspects of isoenzymes, pre-enzymes, coenzymes, and enzyme cofactors. A chapter highlights the characteristics of vitamin, while another chapter is devoted to the chemical structure of vitamin E and the essential fatty acids. The final chapters focus on the exogenous chemical carcinogen. The book can provide useful information to doctors, nutritionists, students, and researchers.

Chapter 1

The Vitamins

Publisher Summary

Vitamins are a heterogenous group of organic compounds required by most living organisms for normal growth and health. This chapter discusses only those vitamins that are required by the higher animals. Vitamins are essential food factors, of very high biological activity, and required in exceedingly small amounts. Nevertheless, these minute quantities are able to maintain the normal structure and function of all the tissues of the animal body. In the absence of vitamins, metabolic defects arise that may be expressed as anatomical lesions, as mental disturbances with personality changes, or as biochemical changes, which may prove difficult to detect, particularly in the numerous cases of marginal deficiency. Vitamins normally must be provided in the diet either in their active form or in the form of their precursors. The status of certain substances as vitamins may vary with the species of animal for which it is required. Great advances made in determining the chemical structures of the vitamins has led to a better appreciation of their mode of action and to a deeper understanding of the pathological changes that occur in their absence. Broadly, they may be classified into two types: (1) the fat-soluble vitamins and (2) the coenzymatic vitamins. The chapter discusses these two types of vitamins. It further discusses physical and mental stress factors of animals that have a common primary effect of depleting bodily stores of vitamins and a secondary effect of reducing the capacity of the endocrine organs to respond adequately to physiological requirements.

In animal tissues vitamins function, both in the structural parts of cells and in the fluid medium, in close collaboration with enzymes and hormones. Understanding of the complicated interactions of these substances is not yet at a very advanced stage, but an examination of our present knowledge of the subject may help to mark out the considerable areas in the field which are still available for exploration.

The vitamins are a heterogenous group of organic compounds required by most living organisms for normal growth and health. In this present context, however, only those required by the higher animals will be considered. Vitamins are essential food factors, of very high biological activity, and required in exceedingly small amounts. Nevertheless these minute quantities are able to maintain the normal structure and function of all the tissues of the animal body. In the absence of vitamins, metabolic defects arise which may be expressed as anatomical lesions, as mental disturbances with personality changes, or as biochemical changes, which may prove difficult to detect, particularly in the numerous cases of marginal deficiency. Vitamins normally must be provided in the diet, either in their active form, or in the form of their precursors. The status of certain substances as vitamins may vary with the species of animal for which it is required. For example vitamin C, although essential for domestic animals, is readily synthesized by them from carbohydrate sources, and need not be provided in the diet. For such animals, ascorbic acid is an endogenously synthezised essential metabolite, but is not strictly speaking, a vitamin. For man, subhuman primates and guinea pigs, ascorbic acid is a true vitamin.

Within the last twenty years, great advances have been made in determining the chemical structures of the vitamins. Inevitably this has led to a better appreciation of their mode of action, and to a deeper understanding of the pathological changes which occur in their absence. Broadly, they may be classified into two types, the fat-soluble vitamins and the coenzymatic vitamins. The fat-soluble group includes vitamins A, D, E and K. Of these, the first three are stored in the body in relatively large amounts. In contrast, the coenzymatic, water soluble vitamins B and C are not stored to any great extent in the body, and renewed supplies are required at fairly frequent intervals.

The fat soluble vitamins appear to function as integral parts of cell membranes and variations in availability of, for example vitamins A or E may have a profound effect on the structure and properties of the membranes of cells and their contained organelles. The biochemical make-up of these membranes varies not only from one tissue to another, but also from one intracellular organelle to another. As a result, pathological effects resulting from deficiency or excess of vitamins are highly selective in their situation, whether in the tissues or within the cells themselves. The fat soluble vitamins in general are more akin to the steroid hormones, than to the vitamins which function as enzymic cofactors.

Coenzymes are complicated organic molecules, which, by virtue of their chemical constitution and configuration are able to accelerate enzymatic reactions, often as carriers of some particular chemical grouping. Thiamine, in its active form of thiamine pyrophosphate provides an example of this type of coenzyme. One of its functions is to assist in the transfer of carboxyl groups, and it is therefore known as a cocarboxylase. Other examples are the adenylic acid-containing coenzymes, nicotinamide adenine dinucleotide (NAD) and its phosphate (NADP) and coenzyme A, all of which contain vitamins of the B complex as part of their molecule. In some cases vitamin-containing organic molecules are so firmly attached to enzymes, that their removal from the enzyme results in loss of activity. These integral parts of enzyme molecules are known as prosthetic groups. They act as carriers of chemical groups from one substrate to another. The flavoproteins, which contain vitamin B₂ are examples of enzymes with such prosthetic groups. Many essential enzyme systems within the animal body are completely dependent on the presence of the cofactoral vitamins which function as essential parts of coenzymes and prosthetic groups.

Although the general division into fat soluble and cofactoral vitamins is valid in most cases, the distinction must not be taken as absolutely clear cut. Vitamin A, for example, in addition to its role in the activities of membranes, also functions, in the form of its aldehyde retinene, as a cofactor in the visual cycle.

Conditions affecting vitamin requirements

Requirements for vitamins vary from species to species, from one individual to another, and during the lifetime of an individual in accordance with

1. Age

2. Physiological status

3. Inherited enzyme pattern

4. Modifying effect of other food stuffs in the diet

5. Availability of activators

6. Pathological status

1 Age

During periods of rapid growth, requirements for vitamins rise steeply; at some periods an adequate supply of vitamins is literally vital. The most important of these periods occur during prenatal life at the various stages of organogenesis. As each organ is formed it has requirements for essential food factors which must be met during a restricted period of time if malformation or even agenesis is not to occur. The endocrine organs, unlike some other tissues such as the lung, start working before birth. As each endocrine organ swings into action during prenatal life, and starts producing its own hormones, its vitamin requirements for hormone synthesis are in all probability, similar in quality if not in quantity, to those of the post-natal organ. Requirements are drawn from maternal sources, which must be adequate to support two sets of organs as gestation proceeds.

Early post-natal life is another period of rapid growth during which depletion in vitamin status can have a lasting effect, not only in growth but also in the production of irreparable pathological lesions. Of these last, perhaps arterial lesions are the most important in the so-called ‘affluent’ societies, epithelial and bone lesions in the underprivileged societies. Artificially fed infants are greatly at risk at this period, partly because of their predisposition to gastrointestinal disturbances with resulting malabsorption, and partly because dietary formulas may not be adequate. It would be arrogant to assume that we know, at the present day, all the vitamins and trace elements required by man and domestic animals in their first few months of life. The recent discovery of the entirely new series of hormones, known as the prostaglandins, derived from essential food factors, should be sufficient to induce a proper state of humility in the nutritional scientist, and to raise doubts about his ability to imitate nature.

During adolescence there is another spurt in growth which requires dietary support if growth is not to be retarded. The widespread occurrence of deficiency diseases in old age is probably a reflection of inadequate intake, rather than of increased requirements for essential food factors.

2 Physiological status

It goes without saying that during periods of physiological stress such as pregnancy and lactation, vitamin requirements are raised to compensate for the resources drained by the infant. Training for athletic competition too imposes its own demands on the body for a more rapid turnover of metabolites. It is no coincidence that the barrier of the first four minute mile was broken by a man with good knowledge of human nutrition.

3 Inherited enzymic pattern

Veterinary scientists familiar with vitamin deficiency disease are well aware of the remarkable variation in requirements for vitamins throughout a herd composed of animals of equal age and size. Diets which prove quite adequate for some members of the herd may produce overt disease in others. To a certain extent this may be attributed to the genetic ability of some animals to produce the enzymes necessary for synthesis and metabolism of the active form of the vitamins concerned, and to the relative inability of others to produce the necessary enzymes. Species variations in inherited enzyme patterns are best illustrated by the inability of man and other primates to synthesize a particular enzyme which converts inactive vitamin C precursor to active vitamin C. Most other animals are able to undertake the conversion quite readily.

4 Modifying effects of the diet

Certain diets require the addition of larger than usual amounts of vitamins to replace loss due to oxidation of essential food factors. For example, loss of the antioxidant activity of vitamin E in diets high in polyunsaturated fats must be compensated for, by vitamin supplementation.

5 Availability of enzymic activators

The substances known as activators are usually simple electrolytes, which are essential for inducing a catalytically active state in some vitamin-assisted enzyme systems, but do not themselves take part in the reaction catalysed: examples are sodium, potassium, calcium, zinc, copper and cobalt. The lack of essential metallic activators is common enough in domestic animals grazing on soils deficient in the element in question. Supplementation of the defective diet with the related vitamin appears in some cases to improve the deficiency, perhaps by making the best use of the small amount of trace element available. However, complete cure of the deficiency depends on restoration of adequate amounts of the missing trace element.

6 Pathological status

In pathological states, requirements for some or all of the vitamins may be increased.

Hyperthyroidism, fevers and post-operative recuperation are examples of such states demanding supplementation. The diabetic patient is a special case. He is unable to convert vitamin A precursor into vitamin A, and must therefore get his quota of this vitamin from animal sources, as distinct from the vegetable sources which provide the precursor.

Stress

The stress of modern life is much discussed nowadays in a rather abstract way. But stress has clearly identifiable effects on the animal body, ranging all the way from the physiological results of a small output of adrenalin to rapid death from shock. Many stress factors have the common primary effect of dep...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Introduction
• Chapter 1: The Vitamins
• Chapter 2: Enzymes
• Chapter 3: Hormones
• Chapter 4: Vitamin A
• Chapter 5: The Vitamin B Complex
• Chapter 6: Vitamin C
• Chapter 7: Vitamin D
• Chapter 8: Vitamin E
• Chapter 9: The Essential Fatty Acids
• Chapter 10: Vitamins and Hormones in Carcinogenesis
• Chapter 11: Hormones and Vitamins in Prenatal Life
• Index