Conversion of provitamin D (ergosterol or 7-dehydrocholesterol) to vitamin D in the skin and its transport in the blood
compiexed with vitamin D-binding protein (also known as transcalciferin, Gc-protein, and group-specific component).
Increased oxalate absorption can be reduced by calcium
adm inistered w ith m eals as a w ater-soluble salt.
Vitamin D Metabolism and Function
A lthough rickets w as first described in the m id -1600s,
it w as not until the 1920s that deficiency o f vitam in D
w as recognized as its cause. D espite its designation as
a vitam in, dietary vitam in D is needed only if a per-
son receives inadequate exposure to sunlight. Norm ally,
vitam in D
is synthesized in the skin by irradiation o f
7-dehydrocholesterol (Figure 37-1). Sufficient exposure
to ultraviolet radiation can cure rickets.
The principal com pound form ed in the skin is
(vitam in D 3). The critical step requiring irra-
diation is the breaking o f the 9 ,1 0 bond in the sterol B
ring to form a secosterol. A secosterol occurs w hen one
o f the rings o f the steroid skeleton cyclopentanoperhy-
drophenanthrene has undergone carbon-carbon breakage.
R ing opening is accom plished by light o f w avelength from
290 to 320 nm (ultraviolet-B radiation) with a m axim al
effect at 297 nm. 7-D ehydrocholesterol is present at high
concentration in the stratum spinosum and stratum basale
o f the epiderm is. Thermal isom erization o f previtamin D
to vitam in D
occurs slow ly, over 2 -3 days. The vitam in
form ed diffuses gradually through the basal layers o f
the skin into the circulation. E xcessive exposure to solar ra-
diation causes conversion o f previtam in D
and Iumisterol3, w hich are b iologically inactive. M elanin
can reduce form ation o f previtam in D
by absorbing part
o f the solar radiation. This effect m ay partly explain the
greater susceptibility o f dark-skinned children to rickets.
T he concentration o f 7-dehydrocholesterol in skin is re-
ported to decrease with increasing age; if so, inadequate
synthesis o f vitam in D
m ay contribute to senile osteo-
porosis. The R D A for vitam in D is 4 0 0 IU.
(vitam in D 2) is form ed by irradiation
o f ergosterol, a plant sterol com m on in the diet. It dif-
fers from ch olecalciferol in the sid e chain attached to the
D ring (Figure 37-1). Irradiation o f ergosterol is an im -
portant com m ercial m ethod for the synthesis o f vitam in
D 2, w hich is used for enriching c o w ’s m ilk. The practice