What Is a Vitamin?

Chapter 1

What Is a Vitamin? Chapter Outline 1. Thinking About Vitamins 2. Vitamin: A Revolutionary Concept 3. An Operating Definition of a Vitamin

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Anchoring Concepts 1. Certain factors, called nutrients, are necessary for normal physiological function of animals, including humans. Some nutrients cannot be synthesized adequately by the host and must therefore be obtained from the external chemical environment; these are referred to as dietary essential nutrients. 2.  Diseases involving physiological dysfunction, often accompanied by morphological changes, can result from insufficient intakes of dietary essential nutrients.

Imagination is more important than knowledge. A. Einstein

LEARNING OBJECTIVES 1. To understand the classic meaning of the term vitamin as it is used in the field of nutrition. 2. To understand that the term vitamin describes both a concept of fundamental importance in nutrition as well as any member of a rather heterogeneous array of nutrients, any one of which may not fully satisfy the classic definition. 3. To understand that some compounds are vitamins for one species and not another, and that some are vitamins only under specific dietary or environmental conditions. 4. To understand the concepts vitamer and provitamin.

VOCABULARY Vitamer Vitamin Provitamin The Vitamins. http://dx.doi.org/10.1016/B978-0-12-802965-7.00001-0 Copyright © 2017 Elsevier Inc. All rights reserved.

4. The Recognized Vitamins 5. Study Questions and Exercises

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1. THINKING ABOUT VITAMINS Among the nutrients required for the many physiologic functions essential to life are the vitamins. Unlike other nutrients, the vitamins do not serve structural functions, nor does their catabolism provide significant energy. Instead, the physiologic functions of vitamins are highly specific, and, for that reason, they are required in only small amounts in the diet. The common food forms of most vitamins require some metabolic activation to their functional forms. Although the vitamins share these general characteristics, they show few close chemical or functional similarities; their categorization as vitamins is strictly empirical. Consider also that, whereas several vitamins function as enzyme cofactors (vitamins A, K, and C; thiamin; niacin; riboflavin; vitamin B6; biotin; pantothenic acid; folate; and vitamin B12), not all enzyme cofactors are vitamins.1 Some vitamins function as biological antioxidants (vitamins E and C), and several function as cofactors in metabolic ­oxidation–reduction reactions (vitamins E, K, and C; niacin; riboflavin; and pantothenic acid). Two vitamins (vitamins A and D) function as hormones; one of them (vitamin A) also serves as a photoreceptive cofactor in vision.

2. VITAMIN: A REVOLUTIONARY CONCEPT Everyday Word or Revolutionary Idea? The term vitamin, today a common word in everyday language, was born of a revolution in thinking about the interrelationships of diet and health that occurred at the 1. Other enzyme cofactors are biosynthesized, e.g., heme, coenzyme Q, and lipoic acid.

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4  PART | I  Perspectives on the Vitamins in Nutrition

beginning of the 20th century. That revolution involved the growing realization of two phenomena that are now taken for granted, even by the nonscientist:

in understanding human physiology and nutrition, the actual definition of a vitamin has evolved in consequence of that understanding.

1. Diets are sources of many important nutrients. 2. Insufficient intakes of specific nutrients can cause certain diseases.

3. AN OPERATING DEFINITION OF A VITAMIN

In today’s world each of these concepts may seem selfevident, but in a world still responding to and greatly influenced by the important discoveries in microbiology made in the 19th century, each represented a major departure from contemporaneous thinking in the area of health. Nineteenthcentury physiologists perceived foods and diets as sources of only four types of nutrients: protein, fat, carbohydrate, ash,2 and water. After all, these accounted for very nearly 100% of the mass of most foods. With this view, it is understandable that, at the turn of the century, experimental findings that now can be seen as indicating the presence of hitherto unrecognized nutrients were interpreted instead as substantiating the presence of natural antidotes to unidentified disease-causing microbes. Important discoveries in science have ways of directing, even entrapping, one’s view of the world; resisting this tendency depends on critical and constantly questioning minds. That such minds were involved in early nutrition research is evidenced by the spirited debates and frequent polemics that ensued over discoveries of apparently beneficial new dietary factors. Still, the systematic development of what emerged as nutritional science depended on a new intellectual construct for interpreting such experimental observations.

Vitamin or Vitamine? The elucidation of the nature of what was later to be called thiamin occasioned the proposition of just such a new construct in physiology.3 Aware of the impact of what was a departure from prevailing thought, its author, the Polish biochemist Casimir Funk, chose to generalize from his findings on the chemical nature of that “vital amine” to suggest the term vitamine as a generic descriptor for many such accessory factors associated with diets. That the factors soon to be elucidated comprised a somewhat chemically heterogeneous group, not all of which were nitrogenous, does not diminish the importance of the introduction of what was first presented as the vitamine theory, later to become a key concept in nutrition: the vitamin. The term vitamin has been defined in various ways. While the very concept of a vitamin was crucial to progress 2. The residue from combustion, i.e., minerals. 3. This is a clear example of what T.H. Kuhn called a “scientific revolution” (Kuhn, T.H., 1968. The Structure of Scientific Revolutions. University of Chicago Press, Chicago, IL.), i.e., the discarding of an old paradigm with the invention of a new one.

A vitamin is defined as follows (Fig. 1.1). A vitamin is an organic compound distinct from fats, carbohydrates, and proteins l is a natural component of foods in which it is usually present in minute amounts l is essential, also usually in minute amounts, for normal physiological function (i.e., maintenance, growth, development, and/or production) l prevents a specific deficiency syndrome, which occurs when it is absent or underutilized l is not synthesized by the host in amounts adequate to meet normal physiological needs. l

This definition will be useful in the study of vitamins, as it effectively distinguishes this class of nutrients from others (e.g., proteins and amino acids, essential fatty acids, and minerals) and indicates the needs in various normal physiological functions. It also denotes the specificity of deficiency syndromes by which the vitamins were discovered. Further, it places the vitamins in that portion of the external chemical environment on which animals (including humans) must depend for survival, thus distinguishing vitamins from hormones.

Some Caveats It will quickly become clear, however, that, despite its utility, this operating definition has limitations, notably with respect to the last clause. Many species can, indeed, synthesize at least some of the vitamins, although not always at the levels required to prevent deficiency disorders. Four examples illustrate this point: Vitamin C: Most animal species have the ability to synthesize ascorbic acid. Only those few that lack the enzyme l-gulonolactone oxidase (e.g., the guinea pig, humans) cannot. For those species, ascorbic acid is properly be called vitamin C. Vitamin D: Individuals exposed to modest amounts of sunlight can produce cholecalciferol, which functions as a hormone. Only individuals without sufficient exposure to ultraviolet light (e.g., livestock raised in indoor confinement, people spending most of their days indoors) require dietary sources of vitamin D. Choline: Most animal species have the metabolic capacity to synthesize choline; however, some (e.g., the chick, the rat) may not be able to employ that capacity if they are fed insufficient amounts of methyl donor compounds. In

What Is a Vitamin? Chapter | 1  5

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addition, some (e.g., the chick) do not develop that capacity completely until they are several weeks of age. Thus, for the young chick and for individuals of other species fed diets providing limited methyl groups, choline is a vitamin. Niacin: All animal species can synthesize nicotinic acid mononucleotide from the amino acid tryptophan. Only those for which this metabolic conversion is particularly inefficient (e.g., the cat, fishes) and others fed low dietary levels of tryptophan require a dietary source of niacin. With these counterexamples in mind, the definition of a vitamin has specific connotations for animal species, stage of development, diet or nutritional status, and physical environmental conditions.5

 

The “Vitamin Caveat” Some compounds are vitamins for one species and not another. l  Some compounds are vitamins only under specific dietary or environmental conditions. l

4. The concept map can be a useful device for organizing thought, as its discipline can serve to assist in identifying the nature and extent of concepts related to the one in question. A concept map should be laid out as a hierarchy of related concepts with the superordinate concept at the top and all relationships between concepts identified with a verb phrase. Thus, it can be “read” from top to bottom. One of the authors (GFC) has used concept mapping in graduate-level teaching, both as a group exercise and testing device. For a useful discussion of the educational value of the concept map, the reader is referred to Learning How to Learn, 1984, J.D. Novak and D.B. Gowin, Cornell University Press, Ithaca, NY, pp. 199. 5. For this reason, it is correct to refer to vitamin C for the nutrition of humans but ascorbic acid for the nutrition of livestock.

4. THE RECOGNIZED VITAMINS Thirteen substances or groups of substances are now generally recognized as vitamins (Table 1.1); others have been proposed.6 In some cases, the familiar name is actually the generic descriptor for a family of chemically related compounds having qualitatively comparable metabolic activities. For example, the term vitamin E refers to those analogs of tocol or tocotrienol7 that are active in preventing such syndromes as fetal resorption in the rat and myopathies in the chick. In these cases, the members of the same vitamin family are called vitamers. Some carotenoids can be metabolized to yield the metabolically active form of vitamin A; such a precursor of an actual vitamin is called a provitamin.

5. STUDY QUESTIONS AND EXERCISES 1. What are the key features that define a vitamin? 2. What are the fundamental differences between vitamins and other classes of nutrients… between vitamins and hormones? 3. Detail, citing a specific example, a situation in which a vitamin may be nutritionally essential for one species but not another. 4. Using key words and phrases, list briefly what you know about each of the recognized vitamins.

6. These include such factors as inositol, carnitine, bioflavonoids, pangamic acid, and laetrile, for some of which there is evidence of vitamin-like activity (Chapter 19). 7. Tocol is 3,4-dihydro-2-methyl-2-(4,8,12-trimethyltridecyl)-6-chromanol; tocotrienol is the analog with double bonds at the 3, 7, and 11′ positions on the phytol side chain (Chapter 7).

6  PART | I  Perspectives on the Vitamins in Nutrition

TABLE 1.1  The Vitamins: Their Vitamers, Provitamins, and Functions Group

Vitamers

Provitamins

Physiological functions

Vitamin A

Retinol Retinal Retinoic acid

β-Carotene Cryptoxanthin

Visual pigments; epithelial cell differentiation

Vitamin D

Cholecalciferol (D3) Ergocalciferol (D2)

Calcium homeostasis; bone metabolism; transcription factor

Vitamin E

α-Tocopherol γ-Tocopherol

Membrane antioxidant

Vitamin K

Phylloquinones (K1) Menaquinones (K2) Menadione (K3)

Blood clotting; ­ calcium metabolism

Vitamin C

Ascorbic acid Dehydroascorbic acid

Reductant in hydroxylations in the formation of collagen and carnitine, and in the metabolism of drugs and steroids

Vitamin B1

Thiamin

Coenzyme for decarboxylations of 2-keto acids (e.g., pyruvate) and transketolations

Vitamin B2

Riboflavin

Coenzyme in redox reactions of fatty acids and the tricarboxylic acid (TCA) cycle

Niacin

Nicotinic acid Nicotinamide

Coenzyme for several dehydrogenases

Vitamin B6

Pyridoxol Pyridoxal Pyridoxamine

Coenzyme in amino acid metabolism

Folic acid

Folic acid Polyglutamyl folacins

Coenzyme in single-carbon metabolism

Biotin

Biotin

Coenzyme for carboxylations

Pantothenic acid

Pantothenic acid

Coenzyme in fatty acid metabolism

Vitamin B12

Cobalamin

Coenzyme in the metabolism of propionate, amino acids, and single-carbon units