Chemistry Reference
In-Depth Information
chronic diseases and the influence of these nutrients at the cellular and
molecular levels. For example, the discovery of vitamin D receptors in over
30 different body tissues suggests that this vitamin has important regulatory
effects in tissues outside its classical role in the skeleton. In relation to vitamin
intake and vitamin status, emphasis is now increasingly placed on determin-
ing dietary recommendations to take account of the amounts of vitamin
needed to optimize physiological and mental functions and to minimize the
development of degenerative diseases. Milk makes a significant contribution
to the intake of vitamins A and D in American children and adults, with a
minor contribution to the intake of vitamins C, E and K. For example, milk
contributes between 10-30% to vitamin A intake in children and adults.
Fortified milk contributes 40-60% of vitamin D intake by toddlers and adults
and is thus the most important source of this vitamin in the American diet.
12.2.
Vitamin A; Retinol and
b
-Carotene
12.2.1.
Chemistry and Analysis
Vitamin A is a generic term used to designate any compound possessing
the biological activity of retinol (Figure 12.1). The term 'retinoids' was
designated to include compounds consisting of four isoprenoid units joined
in a head-to-tail manner. All retinoids may be derived from a monocyclic
parent compound containing five carbon-carbon double bonds and a func-
tional terminal group at the terminus of the acyclic portion. All three basic
forms (retinol, retinal and retinoic acid) are found in two variants: with the
-ionone nucleus (vitamin A
1
) or the dehydrogenated
-ionone nucleus
(vitamin A
2
) with about half the vitamin A activity. The parent retinoid
compound, all-trans-retinol, is a primary alcohol with a molecular mass of
286 Da. In most animal tissues, the predominant retinoid is retinyl palmitate
but other fatty acid esters, such as retinyl oleate and retinyl stearate, are also
found. Most of these metabolites occur in the all-trans configuration. The 11-
cis-aldehyde form, 11-cis-retinal, is present in the retina of the eye, and several
acid forms, such as all-trans and 13-cis-retinoic acid, are metabolites of retinol
found in many tissues. Carotenoids may contribute significant vitamin A
activity to foods of both animal and plant origins. Of the estimated 500
known carotenoids,
50 exhibit some pro-vitamin activity (i.e. are partially
converted to vitamin A in vivo).
For a compound to have vitamin A or pro-vitamin A activity, it must
exhibit certain structural similarities to retinol, including (a) at least one
intact monooxygenated
-ionone ring and (b) an isoprenoid side chain ter-
minating in an alcohol, aldehyde or carboxyl functional group (Figure 12.1).
