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Effects of ultraviolet radiation on 25-hydroxyvitamin D3 synthesis in red-eared slider turtles (Trachemys scripta elegans)

Authors:

Abstract

To determine whether there are increased concentrations of 25-hydroxyvitaminn D(3) in red-eared slider turtles (Trachemys scripta elegans) after exposure to UV radiation. 12 yearling turtles recently removed from aestivation. Turtles were randomly allocated to 2 groups (6 turtles/group). An initial blood sample was collected from all turtles for measurement of 25-hydroxyvitamin D(3) concentrations. Turtles of 1 group were then provided no supplemental lighting, whereas turtles of the other group were exposed to full-spectrum coil bulbs at a distance of 22.86 cm. The UV-A and UV-B radiation generated by the supplemental lighting was measured by use of a radiometer-photometer at weekly intervals. Measurements were collected 2.54 and 22.86 cm from the bulb surface. The study was continued for a 4-week period. At the end of the study, a second blood sample was collected from all turtles for measurement of 25-hydroxyvitamin D(3). Mean +/- SD 25-hydroxyvitamin D(3) concentrations differed significantly between turtles provided supplemental UV radiation (71.7 +/- 46.9 nmol/L) and those not provided UV radiation (31.4 +/- 13.2 nmol/L). Appropriate husbandry recommendations for raising and maintaining red-eared slider turtles should include use of sunlight that is unobstructed by UV-B filtering material or provision of an artificial source of UV-B radiation.
2046 AJVR, Vol 67, No. 12, December 2006
V
itamin D
3
is an important hormone that is involved
in numerous physiologic processes.
1,2
Although the
most widely recognized function of this hormone is the
regulation of calcium metabolism, which is needed for
the development and maintenance of healthy bones,
the reproductive success of some reptilian species has
also been associated with optimized amounts of vita-
min D
3
.
3,4
Vitamin D
3
can be obtained through the diet
or synthesized through exposure of the skin to UV-B
(290 to 320 nm) radiation.
5,6
There is wide variation
among vertebrate species between the need for dietary
vitamin D
3
and the ability to synthesize the hormone.
7,8
The source and function of vitamin D
3
have been
examined in mammals and birds.
9
Studies
4,8,10,11
that
have been performed in reptiles have focused on
dietary requirements and synthesis during basking in
various lizards. The authors are not aware of any stud-
ies to determine whether chelonians synthesize vita-
min D
3
during basking or obtain vitamin D
3
from the
diet. This is unfortunate because many of these species
are raised in captivity as pets. Because these animals
potentially have long lives, it is important that specific
husbandry requirements be elucidated for them.
In addition to maintaining these animals as pets,
there is an increased interest in the conservation of
these reptiles. Currently, programs are underway to
prepare juvenile chelonians in captivity for release
to the wild. These programs typically release larger
animals that are less likely to be preyed on. If expo-
sure to UV-B radiation is required for chelonians to
maximize serum 25-hydroxyvitamin D
3
concentra-
tions, then full-spectrum lights capable of inducing
production of this hormone should be used.
The purpose of the study reported here was to
determine whether red-eared slider turtles (Trachemys
scripta elegans) exposed to UV-B radiation under
controlled conditions would have increased concentra-
tions of 25-hydroxyvitamin D
3
concentrations, com-
pared with concentrations for control turtles. We pro-
posed to test 3 specific hypotheses. First, turtles
exposed to UV-B radiation would have higher concen-
trations of 25-hydroxyvitamin D
3
, compared with con-
centrations for control turtles. Second, turtles fed a diet
that included amounts of vitamin D
3
would have an
increase in 25-hydroxyvitamin D
3
concentrations over
time. Finally, the amount of UV-B radiation generated
by commercially available coil fluorescent bulbs would
decrease over time.
Materials and Methods
Animals—Twelve yearling red-eared slider turtles were
used in the study. The turtles were being housed outdoors for
the winter of 2005 to 2006 and were removed from aestiva-
tion for the study. Turtles were allowed to acclimate in the
laboratory environment for 7 days prior to the start of the
study. This project was performed in accordance with the
regulations established by the Institutional Animal Care and
Use Committee at Louisiana State University (protocol No.
05-112).
Turtles were housed in 78 X 53 X 41.9-cm plastic con-
tainers.
a
Each container was filled with 35 L of chlorinated
tap water. Two concrete stones were placed in each contain-
er to provide a basking area. The laboratory environment was
maintained at a temperature of 30.0
o
to 31.1
o
C. Water tem-
perature in each container was maintained at 25.5
o
to 26.6
o
C.
Water in the containers was replaced every 48 hours with
fresh chlorinated tap water; a water filtration system was not
used. Each day, commercially available chow formulated for
turtles
b
was provided in each container.
Received June 30, 2006.
Accepted August 2, 2006.
From the Department of Veterinary Clinical Sciences, School of
Veterinary Medicine, Louisiana State University, Baton Rouge, LA
70810.
Supported by Fluker Farms.
Address correspondence to Dr. Acierno.
Effects of ultraviolet radiation
on 25-hydroxyvitamin D
3
synthesis in red-eared
slider turtles (
Trachemys scripta elegans
)
Mark J. Acierno, DVM; Mark A. Mitchell, DVM, PhD; Marlana K. Roundtree; Trevor T. Zachariah, DVM
Objective—To determine whether there are
increased concentrations of 25-hydroxyvitaminn D
3
in
red-eared slider turtles (
Trachemys scripta elegans
)
after exposure to UV radiation.
Animals—12 yearling turtles recently removed from
aestivation.
Procedures—Turtles were randomly allocated to 2
groups (6 turtles/group). An initial blood sample was
collected from all turtles for measurement of 25-
hydroxyvitamin D
3
concentrations. Turtles of 1 group
were then provided no supplemental lighting, whereas
turtles of the other group were exposed to full-spec-
trum coil bulbs at a distance of 22.86 cm. The UV-A and
UV-B radiation generated by the supplemental lighting
was measured by use of a radiometer-photometer at
weekly intervals. Measurements were collected 2.54
and 22.86 cm from the bulb surface. The study was
continued for a 4-week period. At the end of the study,
a second blood sample was collected from all turtles
for measurement of 25-hydroxyvitamin D
3
.
Results—Mean ± SD 25-hydroxyvitamin D
3
concen-
trations differed significantly between turtles provided
supplemental UV radiation (71.7 ± 46.9 nmol/L) and
those not provided UV radiation (31.4 ± 13.2 nmol/L).
Conclusions and Clinical Relevance—Appropriate
husbandry recommendations for raising and main-
taining red-eared slider turtles should include use of
sunlight that is unobstructed by UV-B filtering materi-
al or provision of an artificial source of UV-B radiation.
(
Am J Vet Res
2006;67:2046–2049)
06-06-0192r.qxp 11/15/2006 10:18 AM Page 2046
AJVR, Vol 67, No. 12, December 2006 2047
Experimental procedures—After the initial 7-day accli-
mation period, a blood sample (0.5 mL) was collected from
the subcarapacial sinus of each turtle (day 0). Blood samples
were stored in lithium heparin microtainers.
c
Blood samples
were centrifuged within 60 minutes after collection. Plasma
was harvested and frozen. Plasma samples were submitted on
frozen gel packs to a university laboratory
d
for measurement
of 25-hydroxyvitamin D
3
concentrations.
After collection of the initial blood sample, turtles were
allocated into 2 groups (6 turtles/group) by use of a random
number generator. Turtles of 1 group were not provided
supplemental lighting, whereas turtles of the other group
were provided with supplemental lighting. Lights
e
used to
provide supplemental lighting were positioned at a height
of 22.86 cm directly over the basking stones. Lighting was
provided for 12 continuous hours each day.
Radiation (UV-A and UV-B) generated by the coil bulbs
was measured by use of a radiometer-photometer.
f
Measurements were collected at a distance of 2.54 and
22.86 cm from the bulb surface. Amounts of UV-A and UV-
B radiation were measured in triplicate at each distance, and
the arithmetic mean value was calculated and used for sta-
tistical analysis. Measurements of UV-A and UV-B were also
collected at the surface of the basking stone for the turtles
that did not receive supplemental lighting. Amounts of UV-
A and UV-B were measured on a weekly basis at the same
time of day during each successive week, with the excep-
tion of the first measurement, which was recorded immedi-
ately after the lights were turned on.
The turtles were weighed weekly. Weight measurements
were rounded to the nearest 0.1 g. Weight measurements
were collected after the bulbs had been on for 4 hours.
The study was continued for 4 weeks. At the end of that
period, a second blood sample was collected (day 30) from
each turtle for use in measuring 25-hydroxyvitamin D
3
con-
centrations. Collection of blood samples, processing, and
shipment to a university laboratory for testing were similar to
the techniques described for the blood samples obtained on
day 0.
Statistical analysis—Distribution of the data was evalu-
ated by use of the Shapiro-Wilk test. Mean, SD, minimum,
and maximum values were reported for data that had a nor-
mal distribution, whereas the median, 10th to 90th per-
centiles, minimum, and maximum values were reported for
data that did not have a normal distribution. Data that were
not normally distributed were logarithmically transformed
for parametric analysis.
A paired-sample t test was used to determine within-
subject differences throughout the study for 25-hydroxyvita-
min D
3
concentrations and body weight. An unpaired t test
was used to assess differences in 25-hydroxyvitamin D
3
con-
centrations and body weight between turtles provided UV
radiation and those that did not receive UV radiation. A
repeated-measures ANOVA was used to assess the quantity of
UV-A and UV-B radiation generated at the bulb surface and
the surface of the basking stone during the 4-week study.
When differences were found, post hoc comparisons were
made by estimating the marginal means. A value of P 0.05
was used to determine significance. A commercially available
statistical program
g
was used to analyze the data.
Results
Concentrations of 25-hydroxyvitamin D
3
differed
significantly (P = 0.001) between days 0 and 30 for
both groups (Table 1). Mean ± SD concentrations of
25-hydroxyvitamin D
3
differed significantly between
turtles provided supplemental UV radiation (71.7 ±
46.9 nmol/L) and turtles that were not provided sup-
plemental UV radiation (31.4 ± 13.2 nmol/L).
Body weight did not differ significantly (P = 0.50)
between the 2 groups of turtles. Therefore, body
weights for both groups were pooled and evaluated
over time. Body weight increased significantly (P =
0.001) during the course of the study. At the beginning
of the study, mean ± SD body weight for the turtles was
115.9 ± 23.4 g (minimum, 88 g; maximum, 170 g). At
the end of the study, body weight of the turtles had
increased by 10% (mean, 128.6 ± 21.8 g; minimum,
102 g; and maximum, 174 g).
We detected significant differences in the amount
of UV-B radiation at the bulb surface (F = 20.9; P =
0.006) and surface of the basking stone (F = 11.9; P =
0.002) during the course of the study (Table 2). There
was also a significant (F = 89.8; P < 0.001) difference
in the amount of UV-A radiation at the bulb surface
during the course of the study (Table 3). However,
Table 1—Plasma concentrations of 25-hydroxyvitamin D
3
for 2 groups of red-eared slider turtles (6 tur-
tles/group) at the beginning (day 0) and end (day 30) of the study in which 1 group of turtles received
supplemental UV radiation and the other group was exposed to only ambient light.
Mean
SD Minimum Maximum
Sample Group (nmol/L) (nmol/L) (nmol/L)
Day 0 No UV radiation 11.2 4.3 6.0 16.0
Supplemental UV radiation 10.7 3.4 5.0 14.0
Day 30 No UV radiation 31.4 13.2* 15.0 44.0
Supplemental UV radiation 71.7 46.9*† 34.0 155.0
*Value differs significantly (
P
= 0.001) from value for the same group on day 0. †Value differs significantly
(
P
0.05) from the value for the other group on day 30.
Table 2—Amount of UV-B radiation measured at the bulb sur-
face and surface of the basking stone during the course of the
study.
Mean
SD Minimum Maximum
Location Day (
μμ
W/cm
2
)(
μμ
W/cm
2
)(
μμ
W/cm
2
)
Bulb* 0 686 109.4
a,b,c
495 803
7 488 67.5
a
400 564
14 414 67.3
b
309 489
21 426 58.4
c
348 489
Basking stone† 0 14.1 1.7
e,f,g
12.1 16.1
7 18.6 3.3
e
14.0 24.3
14 17.3 2.3
f
13.4 19.9
21 18.2 3.6
g
15.0 24.9
*Radiation was measured 2.54 cm from the bulb surface.
†Radiation was measured 22.86 cm from the bulb surface.
Day 0 = First day of the study.
a-g
Values with different superscript letters differ significantly
(
P
0.05).
06-06-0192r.qxp 11/15/2006 10:18 AM Page 2047
there was not a significant (F = 0.4; P = 0.70) differ-
ence in UV-A radiation at the surface of the basking
stone during the study. The amount of UV-B (< 0.01
W/cm
2
) and UV-A (< 10 W/cm
2
) radiation measured
at the surface of the basking stone for turtles provided
only ambient light (ie, no supplemental radiation) was
negligible.
Discussion
All vitamin D is ultimately the result of the photo-
synthetic conversion of 7-dehydrocholesterol to previt-
amin D
3
in the skin of vertebrates exposed to UV-B.
1,12
Previtamin D
3
is an unstable molecule that undergoes
temperature-dependent isomerization to become vita-
min D
3
.
12
Newly formed vitamin is transported to the
liver, where it is hydroxylated to form 25-hydroxyvita-
min D
3
.
3,11
This represents the storage form of the
hormone, which is bound to protein for systemic circu-
lation.
11
The kidneys are responsible for the final con-
version of 25-hydroxyvitamin D
3
to 1,25-dihydroxyvit-
amin D
3
, which is the active form of the hormone.
1,3,11
Vitamin D
3
can be obtained directly through expo-
sure of the skin to UV-B radiation (290 to 320 nm) or
through consumption of prey that has already per-
formed the biosynthesis. The need for appropriate
plasma concentrations of vitamin D
3
is so important
that the skin of some nocturnal species of lizards, such
as the Mediterranean house gecko (Hemidactylus turci-
cus), has developed the ability to synthesize the hor-
mone in minimal light conditions, whereas other
species can modify their basking to compensate for
variations in dietary amounts of vitamin D
3
.
13-15
Interestingly, animals vary in their capacity to photo-
synthesize or extract this important hormone from
their diet. Some carnivorous mammals, such as cats,
are unable to photosynthesize vitamin D and must rely
totally on dietary sources, whereas some lizards rely
primarily on photosynthesis to produce vitamin D.
7,16
In the study reported here, 25-hydroxyvitamin D
3
concentrations increased significantly in all turtles
from the time they were removed from aestivation until
the end of the study. Although we cannot eliminate
ambient UV-B radiation as a possible cause for this
increase, half of the turtles had exposure to only insub-
stantial amounts of UV-B radiation (< 0.01 W/cm
2
).
Therefore, absorption of vitamin D
3
from the diet is the
most likely explanation for the observed increase.
Turtles that were exposed to supplemental UV-B radia-
tion had significantly higher 25-hydroxyvitamin D
3
concentrations than the turtles that did not receive
supplemental lighting.
The information reported here reinforces results of
a study
17
of another species of freshwater turtle
(Emydura signata) in which it was reported that those
turtles were commonly observed basking, but their
body temperature was in thermoconformity with the
water. Considering the thermal conductivity of water,
compared with that of air, it seems logical that an
aquatic species would gain little thermal benefit from
basking for relatively short periods. Thus, it would
appear that in at least some freshwater turtles, sun-
seeking behavior is a strategy for vitamin D synthesis
rather than a thermoregulatory adaptation.
Body weight of all turtles increased significantly
from the time they were removed from aestivation until
the end of the study. There was no difference in body
weight between turtles exposed to supplemental UV-B
and turtles that did not receive supplemental lighting.
Vitamin D
3
is important in the development and main-
tenance of healthy bones. Because these turtles were all
from the same colony and had been housed outdoors,
it seems unlikely that measurable differences in body
weight from bone demineralization or changes in gen-
eral health would be detectable in such a short study
(ie, 4 weeks). Even had the study period been longer,
we would have needed to conduct a more sensitive
measure of bone density to detect weight attributable
to demineralization of bone.
The coil fluorescent bulbs used in the study were
the most current addition to the selection of full-spec-
trum lights available for reptiles. Historically, fluores-
cent tubes have been the most commonly used
lights.
18,19
Those bulbs provide adequate UV-B radiation
in the range of 290 to 320 nm.
19
In the experience of
one of the authors, the coil bulbs used in the study
reported here generated greater quantities of UV-B
radiation at the bulb surface and distances of 15 and
30 cm, compared with the amount generated by the
more common full-spectrum fluorescent tubes. It was
for this reason that the coil fluorescent bulbs were
selected for use in the study.
The bulbs produced 40% and 27% less UV-B and
UV-A radiation, respectively, at the bulb surface during
the course of the study. Investigators in another study
h
also found that the quantities of UV-B radiation pro-
duced by fluorescent tubes significantly decreased over
time. Interestingly, the amount of UV-A radiation at the
surface of the basking stone did not decrease during the
course of the study. One possibility is that the initial
measurements for the bulbs were obtained as soon as
they were energized, and they may not have had suffi-
cient time to generate maximum output. In our experi-
ence, these bulbs do not necessarily produce a maximal
quantity of radiation immediately after being turned on.
It is also possible that initial measurements at the sur-
face of the basking stone were conducted incorrectly.
However, the same 2 investigators (MJA and MKR)
measured the UV-A and UV-B radiation throughout the
study in an effort to minimize the chance of error. If
either scenario was true, then the difference at the bulb
surface would have been even greater.
2048 AJVR, Vol 67, No. 12, December 2006
Table 3—Amount of UV-A radiation measured at the bulb sur-
face and surface of the basking stone during the course of the
study.
Mean
SD Minimum Maximum
Location Day (
μμ
W/cm
2
)(
μμ
W/cm
2
)(
μμ
W/cm
2
)
Bulb* 0 3,463 248.3
a
3,160 3,800
7 2,595 160.6
b
2,340 2,750
14 2,438 258.0
b
2,160 2,910
21 2,318 104.2
b
2,180 2,470
Basking stone† 0 73.7 9.5 61.9 82.9
7 73.5 8.6 65.6 84.5
14 76.4 6.1 69.7 85.5
21 78.1 9.7 65.0 91.0
See
Table 2 for key.
06-06-0192r.qxp 11/15/2006 10:18 AM Page 2048
AJVR, Vol 67, No. 12, December 2006 2049
Currently, we are evaluating the long-term pro-
duction of UV-B and UV-A radiation by these bulbs to
determine their spectral characteristics and longevity.
Interestingly, UV-B radiation at the surface of the bask-
ing rock generated by the coil florescent tube was con-
sistent with UV-B radiation measured during overcast
conditions in early May in Louisiana. Red-eared slider
turtles have the potential for living long periods in cap-
tivity and are popular as pets. Therefore, it is important
to consider the effect of UV radiation on the human
caregivers. As newer, more powerful bulbs are devel-
oped, they may begin to pose health risks to humans.
To the authors’ knowledge, this risk has not yet been
evaluated.
The study reported here provides important new
information regarding the husbandry of red-eared slid-
er turtles. It would appear that the appropriate hus-
bandry recommendations for raising and maintaining
these aquatic turtles should include sunlight that is
unobstructed by UV-B filtering material or an artificial
source of UV-B radiation (290 to 320 nm) that is locat-
ed no more than 23 cm from the basking area.
a. Rubbermaid Home Products, Fairlawn, Ohio.
b. Aquatic turtle diet, Fluker Farms, Port Allen, La.
c. Microtainer tube with lihium heparin, Becton-Dickinson,
Franklin Lakes, NJ.
d. Diagnostic Center for Population and Animal Health, College
of Veterinary Medicine, Michigan State University, East
Lansing, Mich.
e. Sun-glow coil bulbs, Fluker Farms, Port Allen, La.
f. Model No. 1400, International Light Inc, Newburyport, Mass.
g. SPSS, version 11.0, SPSS Inc, Chicago, Ill.
h. Bernard JB. Spectral irradance of fluorescent lamps and their effi-
cacy for promoting vitamin D synthesis in herbivorous reptiles.
PhD thesis, Department of Science, Michigan State University,
East Lansing, Mich, 1995.
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... Vit D is generally obtained through food that is rich in vit D or through UV radiation, such as sun exposure [25]. Many vertebrate species rely on sun exposure for health, as not all diets are rich in vit D, and studies have shown that supplemental UV radiation can help boost vit D levels in reptiles [35], including corn snakes (Elaphe guttata) [35] and red-eared sliders (Trachemys scripta elegans) [36]. Agamid and iguanid lizards exposed to natural sunlight also had significantly higher levels of vit D than those exposed only to artificial UV light and/or dietary vit D [37]. ...
... PCV values at intake/capture were highest in IRG turtles (range 24.5-42.5%) and lowest in VT+ turtles (range 5-27%), with VT− turtles intermediate (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39).5%) ( Figure 1). Even though VT− turtles had intermediate values, there was a significant difference between VT+ and both IRG and VT− turtles (p < 0.005). ...
... Studies of both mammals and reptiles show greater vitamin D levels when animals are exposed to sunlight. Dairy cows exposed to natural sunlight or artificial UVB light for 73 days had increased plasma vitamin D levels [54], for example, while red eared slider turtles provided UV supplementation had significantly higher plasma vitamin D levels after 30 days compared to individuals kept only in ambient light [36]. Corn snakes also showed higher plasma vitamin D levels when exposed to supplemental UV light [35], as did agamid and iguanid lizards exposed to natural sunlight vs. those exposed only to artificial lights indoors [37]. ...
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To assess the importance of diet and light for indoor maintenance, hatchling panther chameleons were reared for 1 year on crickets fed diets that differed in vitamin concentrations and in different light environments. Dietary transfer of vitamins from the cricket diet to the lizards via the crickets was quantified, as was UV irradiance. There was a statistically significant dietary enhancement of growth by both vitamins on males. UV-A irradiation significantly suppressed growth of females. Low vitamin A shortened life span and resulted in a number of gross and histological pathologies. Hepatocellular lipidosis, indicating a possible toxicosis, occurred with all diets and light treatments. Higher vitamin A resulted in mild soft-tissue mineralization, and high vitamin D shortened the life span of females. Low vitamin A drastically reduced reproduction in both sexes. The intermediate levels of dietary vitamins resulted in the best production of viable eggs by females. However, without high UV-B irradiation, all viable eggs died at term and contained different vitamin levels than hatching eggs from wild-caught females. Baseline levels of egg calcium are given for hatching eggs from wild-caught females. Modifications in current husbandry procedures are recommended. © 1996 Wiley-Liss, Inc.
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Having previously documented experimentally the need for ultraviolet B (UVB) irradiation (290–315 nm) in the light environment of captive female panther chameleons (Furcifer pardalis) to ensure hatching success of their eggs, we investigated optimal UVB irradiation levels. From 1996–1998 28 hatchling female panther chameleons were raised to maturity and bred (using vitamin and mineral-fortified insect diets low in vitamin D) in nine different artificial UVB light environments. Seven of the environments included long (12 hr/day) low irradia-tion exposures ranging from 1.7 to 22 μW/cm2 UVB, with a corresponding conversion of provitamin D3 to photoproducts in in vitro models of 0.18 to 1.52% in 2 hr. Two environments included short (0.5 and 1.0 hr/day), high irradiation exposures of 55 and 49 μW/cm2 UVB, respectively, with a corres-ponding conversion of provitamin D3 to photoproducts in in vitro models of 8.3% to 14.6% in the respective 0.5- and 1.0-hr time periods. Females raised with the mid-level long/low exposures (5–15 μW/cm2 UVB; 0.52–1.32% conversion to photoproducts in in vitro models) produced viable eggs with a significantly higher percentage of hatching compared to those with the extreme (highest or lowest) long/low exposures. Those raised with the short-/high-exposure environments produced viable eggs with a generally high percentage of hatching, but success was variable. The results and techniques for light quality assessment are interpreted, with recommendations for practical application by the herpetoculturist desiring to successfully breed panther chameleons in captivity. Zoo Biol 21:525–537, 2002. © 2002 Wiley-Liss, Inc.
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Sunlight has long been recognized as a major provider of vitamin D for humans; radiation in the UVB (290-315 nm) portion of the solar spectrum photolyzes 7-dehydrocholesterol in the skin to previtamin D3, which, in turn, is converted by a thermal process to vitamin D3. Latitude and season affect both the quantity and quality of solar radiation reaching the earth's surface, especially in the UVB region of the spectrum, but little is known about how these influence the ability of sunlight to synthesize vitamin D3 in skin. A model has been developed to evaluate the effect of seasonal and latitudinal changes on the potential of sunlight to initiate cutaneous production of vitamin D3. Human skin or [3 alpha-3H]7-dehydrocholesterol exposed to sunlight on cloudless days in Boston (42.2 degrees N) from November through February produced no previtamin D3. In Edmonton (52 degrees N) this ineffective winter period extended from October through March. Further south (34 degrees N and 18 degrees N), sunlight effectively photoconverted 7-dehydrocholesterol to previtamin D3 in the middle of winter. These results quantify the dramatic influence of changes in solar UVB radiation on cutaneous vitamin D3 synthesis and indicate the latitudinal increase in the length of the "vitamin D winter" during which dietary supplementation of the vitamin may be advisable.