Historical overview of Vitamin D

The discovery of vitamin D arose from research into
rickets – a bone disease of infancy and early childhood
that reached epidemic proportions in the industrial
cities of Europe and the United States of America during
the industrial revolution. The benefi cial effect of
sunlight in curing rickets had been recognized in
the early nineteenth century. It was the work of Sir
Edward Mellanby published in 1919 that fi nally led
to the acceptance of rickets as a nutritional disease.
In the same year, Huldschinsky cured four children
with severe rickets by exposing them to the rays of a
mercury quartz lamp, thus demonstrating that UV radiation
from an artifi cial source was equally effective
as solar radiation. Huldschinsky further showed that
the effect was not localized, since exposing one of each
child’s arms to the radiation resulted in the healing of
both arms. Two years later, Powers showed that codliver
oil and UV radiation had similar curative effects
on rachitic rats, thus establishing the dual source for
antirachitic activity – diet and UV radiation.
In 1922 McCollum and associates published the results
of experiments designed to determine whether
the antirachitic factor in cod-liver oil was identical to
or distinct from the previously discovered ‘fat-soluble
vitamin A’. They found that cod-liver oil retained its
antirachitic properties after destruction of the vitamin
A by heating and aeration. Thus, in addition to
vitamin A, cod-liver oil contained a new fat-soluble
vitamin, which McCollum later (1925) called ‘fatsoluble
vitamin D’. Zucker and co-workers in 1922
found that vitamin D was present in the unsaponifi -
able fraction of cod-liver oil, and suggested that it was
closely related to cholesterol.
In 1923, Goldblatt and Soames irradiated rachitic
rats and fed their livers to other rachitic rats which
were not irradiated. The latter rats were cured of their
rickets, thereby demonstrating that exposure to UV
radiation induces the production of an antirachitic
substance in the liver. Later, in a similar experiment,
Hess and Weinstock fed small portions of irradiated
skin to rachitic rats and noted a curative effect. In
1924, Steenbock and Black discovered that rat rations
exposed to UV radiation had the same benefi cial effects
as when rachitic rats were irradiated. A year later,
Hess and Weinstock induced antirachitic activity
by irradiating such foods as milk, butter, bread and
meats. It was further demonstrated that it is the sterols
in foods that are activated and converted to vitamin D.
It was fi nally realized that skin and certain foods contain
a precursor of vitamin D that can be converted to
the active vitamin by exposure to UV irradiation.
Irradiation of ergosterol, a sterol obtained from
yeast, led to the isolation of a photo-product that was
originally designated as vitamin D1. It was later realized
that vitamin D1 was a mixture of substances, which
explains its non-existence as a D vitamer in present
nomenclature. Further purifi cation of the irradiation
mixture by Askew in 1931 yielded a single compound
which was called ergocalciferol or vitamin D2. It was
assumed at the time that the vitamin D produced in
human skin during exposure to UV radiation was vitamin
D2. However, in the following year, Steenbock
noted that rachitic chickens did not respond to irradiated
ergosterol, but did so to irradiated cholesterol
preparations and cod-liver oil. This observation led to
the discovery of 7-dehydrocholesterol as the vitamin
D precursor in the cholesterol preparations and the
isolation of cholecalciferol (vitamin D3) by Windaus
in 1936. Vitamin D3, unlike vitamin D2, had antirachitic
activity in both chicks and rats. It was concluded
that 7-dehydrocholesterol, rather than ergosterol, was
the precursor for vitamin D3 in the skin.
A major breakthrough in our understanding of vitamin
D function arose from the discovery of the biologically
active metabolite, 1α,25-dihydroxyvitamin
D3. This event was initiated by Carlsson who, in 1952,
noted a lag between the time of vitamin D administration
and the appearance of its physiological response,
namely intestinal calcium transport. The discovery of
a metabolite of vitamin D3 in intestinal mucosa chromatin
was published by Norman’s group in 1968, and
its biological signifi cance was reported in 1970 by the
same group. In 1970, Fraser and Kodicek showed that
the kidney was the source of synthesis of the newly
discovered metabolite. The chemical characterization
of 1α,25-dihydroxyvitamin D3 was reported in 1971
simultaneously by three independent laboratories,
those of Norman, Kodicek, and Deluca, all within
a six-week period in February/March of that year.
The major circulating metabolite of vitamin D3, 25-
hydroxyvitamin D3, was identifi ed by DeLuca’s group
in 1968 and subsequently was shown to be produced
primarily in the liver.
In 1969, Norman’s group reported the existence of
the vitamin D receptor in the chromatin fraction of
the intestinal mucosa. The interaction of the receptor
190 Vitamins: their role in the human body
with the transcriptional machinery inside vitamin D
target cells demonstrated that 1α,25-dihydroxyvitamin
D3 has a similar mechanism of action to that of
steroid hormones.

Dietary sources
The vitamin D activity in the human diet is contributed
mainly by vitamin D itself and its immediate
metabolite, 25(OH)D. The proportion of vitamin D
obtained from the diet is normally very small compared
with that synthesized in skin in response to
sunlight. The richest natural sources of vitamin D3
are fi sh-liver oils, especially halibut-liver oil. Fatty fi sh,
such as herring, sardines, pilchards and tuna, are rich
natural food sources; smaller amounts of the vitamin
are found in mammalian liver, eggs and dairy products.
Cereals, vegetables and fruit contain no vitamin
D, whilst meat, poultry and white fi sh contribute insignifi
cant amounts.
Foodstuffs commonly enriched with vitamin D include
margarine, skimmed milk powder, evaporated
milk, milk-based beverages, breakfast cereals, dietetic
products of all kinds, baby foods and soup powders.
8.4 Cutaneous synthesis, intestinal
absorption, transport and metabolism
8.4.1 Overview
Solar radiation converts 7-dehydrocholesterol in the
skin to previtamin D3, which in turn is converted by
CH2
H
HO
H
H3C OH
H3C
OH
CH2
H
HO
H
H3C
H3C
CH2
H
HO
H
H3C OH
H3C
CH2
H
HO
H
H3C OH
H3C
OH
Liver Kidney
Vitamin D3 25-Hydroxyvitamin D3 1α,25-Dihydroxyvitamin D3
Kidney
24R,25-Dihydroxyvitamin D3
Fig. 8.2 Conversion of vitamin D3 to hormonal metabolites.
192 Vitamins: their role in the human body
body heat to vitamin D3. Vitamins D2 and D3 can be
obtained orally from natural and fortifi ed foods, commercial
fi sh-liver oil preparations and vitamin tablets.
Unlike excessive ingestion of vitamin D supplements,
the cutaneous source of vitamin D3 does not result
in toxicity when the skin is overexposed to sunlight.
Vitamin D of both cutaneous and dietary origin is
converted in the liver to 25(OH)D, the major circulating
form of the vitamin. The 25(OH)D is converted
in the kidney to 1α,25(OH)2D, which circulates at low
concentrations and acts in the manner of a steroid
hormone.
8.4.2 The vitamin D-binding protein
In the plasma, 25(OH)D, and indeed all vitamin D
metabolites, are mainly bound to a specifi c glycoprotein,
known as the vitamin D-binding protein (DBP).
Much smaller amounts of circulating vitamin D metabolites
are bound with low affi nity to albumin. DBP
is synthesized principally in the liver and belongs to
the same gene family as albumin. It has a molecular
weight of 58 kDa and contains a single binding site for
vitamin D metabolites. At normal circulating concentrations
of vitamin D metabolites, less than 5% of the
available binding sites on DBP are occupied. The features
of the secosteroid molecule necessary for binding
activity are the three conjugated double bonds and
a hydroxyl group at C-25. The binding affi nities of the
DBP for vitamin D and its metabolites are 25(OH)D3
= 24R,25(OH)2D3 = 25,26(OH)2D3 > 1α,25(OH)2D3
> vitamin D3. The difference in the dissociation constants
for 25(OH)D3 and 1α,25(OH)2D3 is about 10-
fold. In mammals, the metabolites of vitamins D2 and
D3 exhibit the same relative affi nitites for DBP (Brown
et al. 2000).