Introduction to Human Nutrition

The Vitamins 133

known as vitamin B 2. Biotin is still sometimes called
vitamin H. Vitamin K was discovered by Henrik Dam,
in Denmark, as a result of studies of disorders of
blood coagulation, and he named it for its function:
koagulation in Danish.
As the chemistry of the vitamins was elucidated, so
they were given names as well, as shown in Table 8.1.
When only one chemical compound has the biologi-
cal activity of the vitamin, this is quite easy. Thus,
vitamin B 1 is thiamin, vitamin B 2 is ribofl avin, etc.
With several of the vitamins, a number of chemically
related compounds found in foods can be intercon-
verted in the body, and all show the same biological
activity. Such chemically related compounds are
called vitamers, and a general name (a generic descrip-
tor) is used to include all compounds that display the
same biological activity.
Some compounds have important metabolic func-
tions, but are not considered to be vitamins, since, as
far as is known, they can be synthesized in the body

in adequate amounts to meet requirements. These

include carnitine, choline, inositol, taurine, and

ubiquinone.

Two compounds that are generally considered to

be vitamins can be synthesized in the body, normally

in adequate amounts to meet requirements: vitamin

D, which is synthesized from 7-dehydrocholesterol in

the skin on exposure to sunlight, and niacin, which is

synthesized from the essential amino acid tryptophan.

However, both were discovered as a result of studies

of defi ciency diseases that were, during the early

twentieth century, signifi cant public health problems:

rickets (due to vitamin D defi ciency and inadequate

sunlight exposure) and pellagra (due to defi ciency of

both tryptophan and preformed niacin).

8.2 Vitamin A

Vitamin A was the fi rst vitamin to be discovered, ini-

tially as an essential dietary factor for growth. It has

Table 8.1 The vitamins, their principal functions and defi ciency diseases

Vitamin Functions Defi ciency disease

A Retinol
β-Carotene

Visual pigments in the retina; cell differentiation;

β-carotene is an antioxidant

Night blindness, xerophthalmia; keratinization of skin

D Calciferol Maintenance of calcium balance; enhances
intestinal absorption of Ca^2 + and mobilizes
bone mineral

Rickets (poor mineralization of bone); osteomalacia

(demineralization of bone)

E Tocopherols
Tocotrienols

Antioxidant, especially in cell membranes Extremely rare: serious neurological dysfunction

K Phylloquinone
Menaquinones

Coenzyme in formation of γ-carboxyglutamate in

enzymes of blood clotting and bone matrix

Impaired blood clotting, hemorrhagic disease

B 1 Thiamin Coenzyme in pyruvate and 2-keto-glutarate
dehydrogenases, and transketolase; poorly
defi ned function in nerve conduction

Peripheral nerve damage (beriberi) or central nervous

system lesions (Wernicke–Korsakoff syndrome)

B 2 Ribofl avin Coenzyme in oxidation and reduction reactions;
prosthetic group of fl avoproteins

Lesions of corner of mouth, lips, and tongue;
seborrheic dermatitis
Niacin Nicotinic acid
Nicotinamide

Coenzyme in oxidation and reduction reactions,

functional part of NAD and NADP

Pellagra: photosensitive dermatitis, depressive
psychosis
B 6 Pyridoxine
Pyridoxal
Pyridoxamine

Coenzyme in transamination and decarboxylation

of amino acids and glycogen phosphorylase;

role in steroid hormone action

Disorders of amino acid metabolism, convulsions

Folic acid Coenzyme in transfer of one-carbon fragments Megaloblastic anemia
B 12 Cobalamin Coenzyme in transfer of one-carbon fragments
and metabolism of folic acid

Pernicious anemia (megaloblastic anemia with

degeneration of the spinal cord)

Pantothenic acid Functional part of coenzyme A and acyl carrier

protein

Peripheral nerve damage (burning foot syndrome)

H Biotin Coenzyme in carboxylation reactions in
gluconeogenesis and fatty acid synthesis

Impaired fat and carbohydrate metabolism, dermatitis

C Ascorbic acid Coenzyme in hydroxylation of proline and lysine
in collagen synthesis; antioxidant; enhances
absorption of iron

Scurvy: impaired wound healing, loss of dental

cement, subcutaneous hemorrhage