CHAPTER
Vitamin Metabolism
The word
vitam in
is used to describe any of a heteroge-
neous group of organic molecules that are needed in small
quantities for normal growth, reproduction, and home-
ostasis but that the human body is unable to synthesize
in adequate amounts. The group includes the fat-soluble
vitamins (A, D, E, and K) and the water-soluble vita-
mins (B-complex and C). Vitamins are generally needed
in catalytic quantities and do not function as structural el-
ements in the cell. Other organic compounds are not syn-
thesized in the body but are required for maintenance of
normal metabolism, such as essential fatty acids (some-
times inaccurately called “vitamin F”; Chapter 18) and
essential amino acids (Chapter 2). These substances are
needed in relatively large quantities, serve as nonregen-
erated substrates in metabolic reactions, and are used
primarily as structural components in lipids and pro-
teins, respectively. A number of other substances are es-
sential food factors in various nonhuman species, e.g.,
biopterin, inositol, ubiquinone, lipoic acid, phosphatidyl-
choline, and paraaminobenzoic acid. They are often classi-
fied as “vitamin-like.”
Vitamins discussed in other chapters include vitamin D
(Chapter 37) and vitamin K (Chapter 36). All the B vita-
mins function as cofactors or precursors for cofactors in
enzyme-catalyzed reactions and are discussed in appropri-
ate chapters. Less well-defined actions are reviewed here.
General Considerations
Classification of vitamins into fat-soluble and water-
soluble groups reflects the history of their discovery. This
grouping is still useful, despite the lack of chemical relat-
edness within each class, because it mirrors other under-
lying similarities.
Vitamers
are chemically similar substances that have a
qualitatively similar vitamin activity. Thus, “vitamin D”
refers to ergocalciferol (D
2
) and cholecalciferol (D
3
)
and sometimes to their 25-hydroxy- and 1,25-dihydroxy
derivatives (Chapter 37). Similarly, pyridoxine (pyri-
doxol), pyridoxal, and pyridoxamine are vitamin B
6
vitamers, riboflavin is the active form of vitamin B
2
and
cobalamin is vitamin B
12
. The members of a particular
vitamin “family” are functionally interchangeable and
protect against deficiency symptoms for that vitamin. A
vitamin and its corresponding deficiency disease are re-
lated as follows:
1. The putative vitamin is a normal dietary constituent of
healthy individuals.
2. The diet consumed by persons exhibiting symptoms
of the putative deficiency disease is lacking in the
vitamin.
3. Symptoms of such persons can be alleviated by
addition of the vitamin to their diet.
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