Interesting points on vitamin D3 metabolism

Interesting points on vitamin D3 metabolism

Am J Clin Nutr. 2008 Jun;87(6):1738-42.

25-Hydroxylation of vitamin D3: relation to circulating
vitamin D3 under various input conditions.

Heaney RP, Armas LA, Shary JR, Bell NH,
Binkley N, Hollis BW.

Osteoporosis Research Center,
Creighton University,
Omaha, NE 68131, USA.
rheaney@creighton.edu

BACKGROUND:
Neither the efficiency of the 25-hydroxylation of
vitamin D nor the steady state relation between
vitamin D(3) and 25-hydroxyvitamin D [25(OH)D] has
been studied in humans.

OBJECTIVE:
We aimed to examine the relation between serum
vitamin D(3) and 25(OH)D in normal subjects after
either oral administration of vitamin D(3) or
ultraviolet-B radiation across a broad range of inputs.

DESIGN:
Values for serum vitamin D(3) and (OH)D(3) were
aggregated from 6 studies--1 acute and 5 near-steady
state--at various vitamin D(3) inputs. In 3 of
the steady state studies, vitamin D(3) had been
administered for 18-26 wk in doses of 0 to
11000 IU/d; in 2 studies, subjects had received
solar or ultraviolet-B irradiation.

RESULTS:
In the acute study, subjects receiving a single
100000-IU dose of vitamin D(3) had a rise in
serum cholecalciferol to a mean of 521 nmol/L
at 1 d and then a fall to near-baseline values
by 7-14 d. Serum 25(OH)D peaked at 103 nmol/L on
day 7 and fell slowly to baseline by day 112.
In the 5 steady state studies, the relation of
serum 25(OH)D to serum vitamin D(3) was biphasic
and was well described by a combined exponential
and linear function: Y = 0.433X +
87.81[1-exp (-0.468X)], with R(2) = 0.448.

CONCLUSIONS:
At physiologic inputs, there is rapid conversion
of precursor to product at low vitamin D(3)
concentrations and a much slower rate of conversion
at higher concentrations. These data suggest that,
at typical vitamin D(3) inputs and serum concentrations,
there is very little native cholecalciferol in
the body, and 25(OH)D constitutes the bulk of
vitamin D reserves. However, at supraphysiologic
inputs, large quantities of vitamin D(3) are
stored as the native compound, presumably in body
fat, and are slowly released to be converted
to 25(OH)D.


PMID: 18541563