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REVIEW
The risks and benefits of sun exposure 2016
David G. Hoel
a
, Marianne Berwick
b
, Frank R. de Gruijl
c
, and Michael F. Holick
d
a
Department of Public Health Sciences, Medical University of South Carolina, Charleston, SC, USA;
b
Department of Internal Medicine and
University of New Mexico Cancer Center, Division of Epidemiology and Biostatistics, University of New Mexico, Albuquerque, NM, USA;
c
Department of Dermatology, Leiden University Medical Center, Leiden, The Netherlands;
d
Section of Endocrinology, Diabetes and Nutrition,
Department of Medicine, Boston University Medical Center, Boston, MA, USA
ARTICLE HISTORY
Received 30 September 2016
Accepted 11 October 2016
ABSTRACT
Public health authorities in the United States are recommending that men, women and children
reduce their exposure to sunlight, based on concerns that this exposure will promote skin cancer.
On the other hand, data show that increasing numbers of Americans suffer from vitamin D
deficiencies and serious health problems caused by insufficient sun exposure. The body of science
concerning the benefits of moderate sun exposure is growing rapidly, and is causing a different
perception of sun/UV as it relates to human health. Melanoma and its relationship to sun exposure
and sunburn is not adequately addressed in most of the scientific literature. Reports of favorable
health outcomes related to adequate serum 25(OH)D concentration or vitamin D supplementation
have been inappropriately merged, so that benefits of sun exposure other than production of
vitamin D are not adequately described. This review of recent studies and their analyses consider
the risks and benefits of sun exposure which indicate that insufficient sun exposure is an emerging
public health problem. This review considers the studies that have shown a wide range health
benefits from sun/UV exposure. These benefits include among others various types of cancer,
cardiovascular disease, Alzheimer disease/dementia, myopia and macular degeneration, diabetes
and multiple sclerosis. The message of sun avoidance must be changed to acceptance of non-
burning sun exposure sufficient to achieve serum 25(OH)D concentration of 30 ng/mL or higher in
the sunny season and the general benefits of UV exposure beyond those of vitamin D.
KEYWORDS
cancer; cardiovascular
disease; melanoma;
ultraviolet radiation; vitamin
D; 25-hydroxyvitamin D
Introduction
Public health authorities in the United States are cur-
rently advising that human sun exposure be reduced.
1
At the same time, NHANES data show that 32% of
Americans suffer from vitamin D insufficiency.
a
In this paper we review the current state of the science of
the risks and benefits of sun exposure and suggest that pub-
lic health advice be changed to recommend that all men,
women and children accumulate sufficient non-burning
sun exposure to maintain their serum 25hydroxyvitaminD
[25(OH)D]levelsat30ng/mLormoreyear-round.
History
The first scientifically-established health benefitofsun
exposure was the discovery in 1919 that sunlight cured
rickets.
4-5
This was followed in 1924 up by the discovery
that an inactive lipid in the diet and skin could be con-
verted by UV light into an antirachitic substance.
6
The
identification of vitamin D occurred in 1931.
7
The asso-
ciation between sun exposure and reduced cancer mor-
tality in North America was identified in the 1960s. In
the 1980s, it was hypothesized that vitamin D was the
protective factor. For most of the intervening years,
instead of pursuing further benefits of sun exposure,
scientific inquiry focused on the health risks of sun expo-
sure, especially melanoma and other types of skin
cancer.
8
Chemical sunscreens were developed in 1928.
9
Avoidance of intentional sun exposure and use of chemi-
cal sunscreens persisted as the standard advice of physi-
cians and public health authorities for reducing the risk
of melanoma and other forms of skin cancer.
1,8
The risks
of inadequate sun exposure have been largely ignored.
Recently, however, scientific inquiry has increasingly
CONTACT David G. Hoel dghoel@gmail.com 36 S. Battery, Charleston, SC 29401, USA.
a
The 2010 Institute of Medicine vitamin D report defined vitamin D deficiency as serum 25(OH)D levels of less than 12ng/mL and vitamin D insufficiency as serum
25(OH)D levels of less than 20 ng/mL.
2
NHANES data for 2001–2006 show that 8% of Americans had 25(OH)D below 12 ng/mL and 32% had 25(OH)D below
20 ng/mL.
3
© 2016 David G. Hoel, Marianne Berwick, Frank R. de Gruijl, and Michael F. Holick. Published with license by Taylor & Francis.
This is an Open Access article distributed under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/),
which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way.
DERMATO-ENDOCRINOLOGY
2016, VOL. 0, NO. 0, e1248325 (17 pages)
http://dx.doi.org/10.1080/19381980.2016.1248325
turned to the benefits of moderate sun exposure and the
public health risks of inadequate sun exposure.
10
Risks of sun exposure
Melanoma
The mechanism of melanoma is unknown, but is
believed to be linked to genetic factors.
11
The principal
identified non-genetic risk factor is ultraviolet radia-
tion (UVR) exposure, and the relationship between
melanoma and UVR is 2-sided: non-burning sun
exposure is associated with a reduced risk of mela-
noma, while sunburns are associated with a doubling
of the risk of melanoma.
12
It has long been observed
that outdoor workers have a lower incidence of mela-
noma than indoor workers.
13-19
A 1997 meta-analysis
found an OR of 0.86 (95% CI: 0.77–0.96) for occupa-
tional sun exposure.
18
Biologically, UVB is known to induce DNA damage
through the creation of pyrimidine dimers while UVA
does so at orders of magnitude less efficiently.
20
Oxidative damage through the creation of free radicals
(singlet oxygen and hydrogen peroxide) occurs at all
UVR frequencies.
20
However, the human body has
many defenses against such damage including DNA
repair mechanisms, cell cycle and growth inhibitions,
reduced proliferation, enhanced sensitivity to apopto-
sis, enhancement of cellular differentiation and anti-
inflammatory effects; many of which are related to
vitamin D produced by exposure to UVB.
21-25
With respect to sunburns, melanocytes are not rep-
licating cells, so once DNA damage has occurred, it is
necessary for cellular replication to take place for the
possibility of unrepaired or mis-repaired melanocytes
to develop into malignant melanoma.
20
Sunburns cor-
respond with rare occasions of cell divisions and ensu-
ing vulnerability to mutations in otherwise indolent
melanocytes.
20
With respect to chronic non-burning
sun exposure, it is thought that protection against
sunburn and development of melanoma derives from
photo-adaptation (increased melanisation and epider-
mal thickening) or from the induction of higher levels
of vitamin D, or possibly both.
12,25-28
Vitamin D
produced by UVB exposure is converted to the active
form of vitamin D by its sequential metabolism in the
liver to form the major circulating form of vitamin D,
25-hydroxyvitamin D [25(OH)D] which is then
converted in the kidneys to 1,25-dihydroxyvitamin D
[1,25(OH)2D]. Evidence suggests that vitamin D that
is produced in the skin can also be converted in the
skin to its active form 1,25(OH)2D
25
, thereby enhanc-
ing DNA repair
29
and lowering cancer risk.
The incidence of melanoma in the United States has
increased dramatically from 1 per 100,000 people per
year in 1935 to 23 per 100,000 per year in 2012. Various
explanations for this phenomenon have been suggested,
including diagnostic drift,
30
depletion of the ozone
layer,
31
the widespread use of artificial UVR devices,
32
and the proliferation of large windows in office build-
ings.
15
None of these explanations is particularly satis-
factory for the reason that none explains the steady
increase in melanoma incidence since 1935. While sun-
burns have been associated with a doubling of mela-
noma risk,
12
chronic non-burning sun exposure and
outdoor occupations have been associated with reduced
risk of melanoma.
12-19
Indoor occupations such as pro-
fessional, managerial, clerical, sales and service workers
grew from 25% to 75% of total employment between
1910 and 2000.
33
25% of Americans lived on farms in
1930 whereas only 2% do so today.
34
Indoor attractions
such as air conditioning, television, computers and the
internet probably have led to Americans spending more
of their leisure time indoors, the prevalence of sunburns
is high and has been increasing
b
,andserum25(OH)D
levels of the American public, a likely marker for sun
exposure, are low and have been declining.
c
A more
plausible explanation for the rise in melanoma incidence
since 1935 may be the continually-increasing insuffi-
cient non-burning sun exposure and related increasing
vitamin D deficiency/insufficiency, and the increasing
sunburn prevalence experienced by the American
public over the same time period
d
. Furthermore,
b
According to the Centers for Disease Control and Prevention, the prevalence
of sunburns increased from 32% of all adults in 1999 to 34% in 2004
35
and
up to 50% in 2012.
36
Among adolescents aged 12–18 in 1999, 83% reported
at least one sunburn in the previous summer and 36% reported three or
more sunburns in the previous summer.
37
c
Data on temporal trends in vitamin D levels are contained in study by Ginde
et al. 2009
38
who reported that NHANES data on serum 25(OH)D levels
show that the prevalence of 25(OH)D of less than 10 ng/mL increased from
2% in 1988–1994 to 6% in 2001–2004 while over the same time period the
prevalence of 25(OH)D of less than 20 ng/mL increased from 22% to 36%,
and for 25(OH)D of less than 30 ng/mL increased from 55% to 77%.
d
Such an explanation is not new. White et al. 1988
39
(published as Garland
et al. 1990
40
) proposed that low levels of vitamin D (either locally available
in skin or circulating in plasma) allow melanomas which were previously ini-
tiated by sunlight exposure to develop into clinically apparent disease in
continually sunlight deprived individuals. This proposal was apparently
ignored as precautions against melanoma focused on sun avoidance and
liberal use of chemical sunscreens, with inadequate attention paid to the
role of sunburns in melanomagenesis and to the role of vitamin D in inhibit-
ing cancer. The first cancer cell line shown in 1980 to be inhibited in growth
by 1,25(OH)
2
D was in fact a melanoma cell line. In 1989 Gallagher et al.
41
suggested that part of the increased incidence in melanoma could be attrib-
uted to the decline in outdoor workers.
e1248325-2 D. G. HOEL ET AL.
epidemiological studies do not indicate any difference in
melanoma risk based on the age at which UVR exposure
occurs.
12,17,18
Sunburns appear to be equally risky at any
age.
17
The public health messages of the past 50 y to
avoid sun exposure and to use chemical sunscreens may
have contributed to the rise in melanoma incidence.
We can find no consistent evidence that use of
chemical sunscreens reduces the risk of melanoma.
Green et al. 2011,
42
found in a prospective study that
there may be an association between sunscreen use
and reduced risk of melanoma. However, since the
participants were told they were participants in a skin
cancer prevention trial and were questioned periodi-
cally during the trial on their use of sunscreen, the
likelihood that they were significantly more diligent in
applying sunscreen in accordance with manufacturers’
instructions than ordinary users of sunscreen cannot
be discounted.
e
In addition, this study took place in a
tropical environment, differing significantly from the
environments of North America and Europe. Use of a
placebo sunscreen was barred by ethical concerns.
Sunscreens do, however, reduce acclimatization to
UVR and vitamin D production in the skin.
46
Since
public health authorities recommend liberal use of
sunscreens for good health, the labeling of sunscreens
should contain a statement about the possibility of
vitamin D deficiency that may result from excessive
use of sunscreens. Labeling should also state that
sunscreens have not been shown to be effective in
reducing the risk of melanoma. Sunscreens have been
shown in one study to be effective in reducing the risk
of squamous cell, but not basal cell, skin cancer.
47
Nonmelanoma skin cancer (NMSC)
There are no official registries for basal cell carcinoma
(BCC) or squamous cell carcinoma (SCC), and esti-
mates of the prevalence of these carcinomas vary
widely. One group of investigators examined Medicare
fee-for-service data, extrapolated to the entire United
States population, and estimated that 2,152,500
persons were treated for 3,507,693 NMSCs in 2006.
48
Several of the same investigators estimated that
3,315,554 persons were treated for 5,434,193 NMSCs
in 2012 and revised the 2006 estimates to 2,463,567
persons and 4,013,890 NMSCs.
49
These latter
estimates indicated a 14% increase in Medicare
NMSCs over the 6-year period 2006–2012 and a 54%
increase in non-Medicare NMSCs over the 6-year
period. It is not clear in this analysis that all treat-
ments for NMSCs were in fact treatments for malig-
nancies rather than for non-cancerous lesions, and
these investigators found the ratio of BCC to SCC to
be 1 to 1 instead of the expected 4 to 1. Another recent
study
50
which histologically confirmed all cases but
studied only BCCs, calculated based on an analysis of
a Kaiser Permanente BCC registry that approximately
2 million BCCs are treated annually in the United
States in an undisclosed number of persons. Assuming
a 4 to 1 ratio of BCC to SCC, this would indicate that
2.5 million NMSCs are treated annually. This study
found that the incidence of BCC increased 17% during
the 15-year period from 1998 to 2012.
As with melanoma, sunburns are associated with
increased risk of SCC and BCC.
16,17,51
Cumulative sun
exposure, which is associated with decreased risk of
melanoma, is apparently associated with increased
risk of SCC and BCC, although the relationship
between cumulative sun exposure and NMSC is not
entirely clear. Armstrong and Kricker 2001
17
found
that only SCC, not BCC, is related to total sun expo-
sure, and Rosso et al. 1998
52
found no association
between cumulative lifetime sun exposure and BCC.
Kennedy et al. 2003
16
found a positive association
between increasing lifetime sun exposure and the
development of SCC and BCC but statistical signifi-
cance was not always reached after age adjustment.
English et al. 1998
53
found that total time spent out-
doors was only weakly associated with SCC. Gallagher
et al. 1995a,b
54,55
found no association between cumu-
lative lifetime sun exposure and risk of SCC or BCC,
but Gallagher et al. 1995b
55
found that occupational
sun exposure in the 10 y prior to diagnosis was associ-
ated with increased risk of SCC. Many studies have
found increased risk of SCC and to a lesser extent
BCC from occupational sun exposure.
17,51,56,57
Alam
et al. 2001
58
found that the risk of SCC, but not BCC,
is directly related to cumulative total dose of ionizing
radiation from x-rays, that SCC may develop on sun-
exposed areas in people with certain genodermatoses,
such as oculocutaneous albinism, that chemical agents
such as soot, arsenic and polycyclic hydrocarbons
have historically been a major cause of SCC, and that
e
Sunscreens are intended to prevent sunburn when used in thickness and
frequency recommended by manufacturers or used in setting SPFs. How-
ever, studies have shown that the incidence of sunburn is higher or the
same in people who almost always use sunscreens compared with those
who rarely use sunscreens.
43-45
DERMATO-ENDOCRINOLOGY e1248325-3
human papillomavirus infection has been associated
with SCC. The US. Preventive Services Task Force, in
its May 2012 Final Recommendation Statement on
skin cancer counseling,
59,60
stated that studies that
measured long-term or total sun exposure had found
no association between cumulative sun exposure and
either SCC or BCC.
Benefits of sun exposure; Risks of inadequate
sun exposure
General
Scientific inquiry into the benefits of sun exposure lan-
guished for many decades following the observation in
the 1920s that farmers in Europe developed non-
melanoma skin cancer on their most sun-exposed
areas - their ears, face, nose and backs of their hands.
61
Research on the benefits of sun exposure has acceler-
ated in the past 15 y and particularly in the past 5 y.
62
Vitamin D
Biological plausibility
Vitamin D is a hormone and most cells and organs in
the human body have a vitamin D receptor, which
explains the wide variety of diseases and disorders
that have been linked to vitamin D insufficiency in
epidemiological studies.
63
The production of vitamin
D by UV B radiation, the availability of vitamin D in
food and supplements, and the biological plausibility
of vitamin D as a mediator for a large variety of favor-
able health outcomes are well described in the litera-
ture.
22-25,63-65
Recommended vitamin D status
There is considerable controversy within the scientific
community regarding optimum 25(OH)D levels for
human health. In 2010, the Institute of Medicine
defined vitamin D deficiency as 25(OH)D of less than
12 ng/mL and vitamin D insufficiency as 25(OH)D of
less than 20 ng/mL.
2
In 2011, The Endocrine Society
defined vitamin D deficiency as 25(OH)D below
20 ng/mL and vitamin D insufficiency as 25(OH)D of
21–29 ng/mL.
66
Others have suggested even higher
levels.
22,67-69
A letter signed by many respected
vitamin D scientists and physicians recommends
40–60 ng/mL
70
which is in line with what the Endo-
crine Society recommended as the preferred range for
health –i.e, a 25(OH)D of 40–60 ng/mL.
66
Most
reference laboratories have raised the lower boundary
of the normal range to 30 ng/mL.
68
Prevalence of vitamin D deficiency/insufficiency
Ginde et al. 2009
38
reported that NHANES data on
serum 25(OH)D levels show that the prevalence of 25
(OH)D of less than 10 ng/mL increased from 2% of
the US population in NHANES III (1988–1994) to 6%
in NHANES 2001–2004, and that over the same
period the prevalence of 25(OH)D of less than 20 ng/
mL increased from 22% of the US population to 36%.
f
The IOM report did not offer a solution to this prob-
lem since that was not its purpose; the IOM was
charged with determining the DRI of vitamin D sup-
plements and found that there was insufficient scien-
tific evidence on the benefits of vitamin D
supplementation to support raising the DRI of vitamin
D supplements to more than 600 International Units
(IUs) per day.
g
Using the Endocrine Society’sdefini-
tion of vitamin D sufficiency of 30 ng/mL, the level of
vitamin D insufficiency increased from 55% of the US
population in NHANES III to 77% in NHANES
2001–2004,
38
which indicates that the vast majority of
Americans have an insufficient vitamin D status.
Mediators other than vitamin D
Several studies, discussed below, have found that
mediators other than vitamin D are or may be
involved in the beneficial effects of adequate sun
exposure.
Benefits of vitamin D/sun exposure; Risks of vitamin
D insufficiency/inadequate sun exposure
We next examined the health benefits associated with
increasing levels of sun exposure and/or circulating
serum 25(OH)D and the health risks associated with
inadequate sun exposure and/or inadequate serum 25
(OH)D, with particular emphasis on studies published
since the 2010 IOM report.
f
The differences between NHANES III and NHANES 2001- 2004 may be atten-
uated by approximately 4 ng/mL after adjustment for improvements in the
serum 25(OH)D assay performance from NHANES III to NHANES 2001-
2004.
71
g
The Endocrine Society’s 2012 review of the nonskeletal effects of vitamin D
also found there was insufficient evidence to support a role of vitamin D
supplementation in correcting vitamin D insufficiency.
66
e1248325-4 D. G. HOEL ET AL.
All-cause mortality
Chowdhury et al. 2014
72
performed a meta-analysis of
data from 73 cohort studies with 849,000 participants
and 22 randomized controlled trials with 31,000 par-
ticipants. This study found an inverse association of
circulating 25(OH)D with risks of death due to cardio-
vascular diseases, cancer and other causes (RR 1.35,
95% CI 1.22–1.49 for all cause mortality, comparing
the bottom third versus top 2-thirds of baseline circu-
lating 25(OH)D distribution), but found that, with
respect to possible benefits of vitamin D supplementa-
tion, further investigation is required before any wide-
spread supplementation occurs. The prevalence of
vitamin D insufficiency (defined as 25(OH)D less
than 30 ng/mL) was found to be 69.5% for the United
States and 86.4% for Europe. The authors further esti-
mate that 9.4% of all deaths in Europe and 12.8% in
the United States could be attributable to vitamin D
insufficiency. Other meta analyses include Garland
et al. 2014
73
who pooled the data from 32 studies
(30 cohort studies and 2 nested case-control studies)
that examined age-adjusted all-cause mortality and
serum 25(OH)D levels and found that the overall age-
adjusted hazard ratio for all-cause mortality compar-
ing the lowest (0–9 ng/mL) group to the highest
(greater than 50 ng/mL) was 1.9 (95% CI 1.6–2.2),
indicating that individuals in the lowest group had
nearly twice the age-adjusted death rate as those in the
highest quantile. Schottker et al. 2014
74
conducted a
meta-analysis of 8 cohort studies with 26,000 partici-
pants and found a 1.6-fold higher all-cause mortality
in the bottom quintile (25(OH )D approximately
<12 ng/mL) compared with the top quintile (25(OH)
D approximately >24 ng/mL) (RR 1.57, 95% CI
1.36–1.81).
Lindqvist et al. 2014
75
assessed the avoidance of sun
exposure as a risk factor for all-cause mortality for
29,518 Swedish women in a prospective 20-year fol-
low-up of the Melanoma In Southern Sweden cohort
and found that the population attributable risk for all-
cause mortality for those habitually avoiding sun
exposure was 3%. As compared to the highest sun
exposure group, the all-cause mortality rate was dou-
bled (RR 2.0, 95% CI 1.6–2.5) among avoiders of sun
exposure and increased by 40% (RR 1.4, 95% CI
1.1–1.7) in those with moderate exposure. The authors
noted that Sweden has national guidelines providing
restrictive advice on sun exposure habits in order to
lower the risk of skin cancer, and stated that these
guidelines may be harmful in terms of overall health
of the population. Lindqvist et al. 2016
76
found that
women with active sun exposure habits were mainly
at lower risk of cardiovascular disease mortality and
other non-cancer mortality, and noted that avoidance
of sun exposure is a risk factor for death of a similar
magnitude as smoking. “Our finding that avoidance of
sun exposure was a risk factor for all-cause death of
the same magnitude as smoking is novel.”
Afzal et al. 2014
77
conducted a Mendelian randomi-
zation analysis showing that genetically low 25(OH)D
levels were associated with increased all-cause mortal-
ity, but not with cardiovascular mortality. These
results confirm that the measured low 25(OH)D levels
in the general population associated with increased
mortality as indicated in the above meta-analyses are
related to vitamin D rather than simply a consequence
of poor health or sequestration of vitamin D in adi-
pose tissue, but indicate that some mediator other
than vitamin D may be involved in cardiovascular
mortality. Afzal et al. 2014
77
was the first study with
sufficient sample size to investigate the association of
genetically low 25(OH)D levels with increased
mortality.
Colorectal cancer
Rebel et al. 2014
78
showed for the first time the causal-
ity of the relationship between moderate UVR expo-
sure and primary intestinal tumors in mice. The
UVR-induced reduction in intestinal cancer in mice
could at least in part be attributed to vitamin D. How-
ever, the investigators also found a reduced progres-
sion to malignancy as a result of UVR exposure which
appeared not to be attributable to vitamin D. Three
groups of hairless mice were compared: one on a low-
vitamin D diet without vitamin D supplementation or
UVR exposure, one on a low-vitamin D diet with vita-
min D supplementation but without UVR exposure,
and one on a low-vitamin D diet without vitamin D
supplementation but with moderate UVR exposure.
This permitted the comparison of effects of dietary
vitamin D supplementation and UVR exposure. The
tumor load (area) was similarly and significantly
reduced in both the vitamin D supplementation group
and the UVR exposure group, but only the UVR expo-
sure group had a lower percentage of malignant
adenocarcinomas. Thus the study provided the first
DERMATO-ENDOCRINOLOGY e1248325-5
experimental evidence that physiologically relevant,
moderate UVR exposure can reduce the load of pri-
mary intestinal tumors, which reduction can at least
in part be explained by an increase in vitamin D status
as a comparable reduction in tumor load was observed
in the vitamin D supplementation group that had a
similar increase in vitamin D status. However, a
reduction in malignant progression and growth of
adenocarcinomas could not be attributed to vitamin D
as these effects were only observed with moderate
UVR exposure and not with dietary vitamin D supple-
mentation. Rebel et al. 2014
78
noted that prior studies
had long shown that low exposure to solar UVR is sig-
nificantly associated with increased risk of colon can-
cer, and that several recent studies showed that
increased risk of colon cancer was significantly associ-
ated with prediagnostic low vitamin D status. The
2010 IOM report
64
acknowledged that epidemiological
studies examining associations between vitamin D
status and colorectal cancer incidence generally sup-
ported an inverse association, but declined to base
vitamin D DRI’s on colon cancer outcomes because of
the paucity and conflicting findings of prospective
randomized controlled trials involving vitamin D sup-
plementation. Notably, the most recent, and only
observational, study reviewed in the IOM report
found no association of vitamin D supplementation
with colon cancer risk, but found that patients in the
highest quintile of prediagnostic circulating 25(OH)D
concentration (more than 40 ng/mL) had a 42%
reduced risk of colon cancer as compared to patients
with the lowest quintile (less than 10 ng/mL).
79
Breast cancer incidence and mortality
Mohr et al. 2014
80
conducted a meta-analysis of data
from 5 studies on the relationship between serum 25
(OH)D levels at time of breast cancer diagnosis and
breast cancer mortality which found that patients in
the highest quintile of 25(OH)D (more than 32 ng/
mL) had approximately half the death rate from breast
cancer as those in the lowest quintile (less than 14 ng/
mL) (HR 0.56; 95% CI: 0.4–0.7). The authors recom-
mended that serum 25(OH)D levels in all breast can-
cer patients should be restored to the normal range,
which the authors defined as 30–80 ng/mL.
Engel et al. 2010
81
found a 27% reduced risk of
breast cancer incidence in women in the highest tertile
of 25(OH)D (greater than 27ng/mL) as compared to
the lowest tertile (less than 19.8 ng/mL) in a nested
case-control study (OR 0.73; 95% CI: 0.55–0.96). The
authors noted that all 6 previous case-control studies
on the subject have reported a significant inverse asso-
ciation between serum 25(OH)D levels and breast
cancer and that an inverse effect between sun expo-
sure and breast cancer has previously been observed.
John et al. 1999
82
found that women with higher solar
UVB exposure in NHANES III had only about half
the incidence of breast cancer as those with lower solar
exposure (RR 0.50; 95% CI: 0.33–0.80) and Knight
et al 2007
83
found that increasing sun exposure from
ages 10 to 19 reduced breast cancer risk by 35% (OR
0.65, 95% CI 0.50–0.85 for the highest quartile of out-
door activities vs. the lowest).
Non-hodgkins lymphoma, colorectal, prostate
and breast cancer, and multiple sclerosis
Van der Rhee et al. 2013
84
noted that the association
between solar radiation and reduced cancer mortality
in North America was identified more than 60 y ago
85
and that in 1980 it was hypothesized that vitamin D
was the protective factor.
40
The authors conducted a
systematic review to verify if epidemiological evidence
is in line with the hypothesis that the possible preven-
tive effect of sunlight on cancer is more than just the
effect of vitamin D. Vitamin D intake studies were
excluded from the review and the authors stated that
their review presented the sum of epidemiological
knowledge on the influence of sun exposure and circu-
lating 25(OH)D levels on the risk of colorectal cancer,
prostate cancer, breast cancer and non-Hodgkin’s
lymphoma (NHL). They concluded that: 1) there is an
inverse association between sun exposure and both
colorectal cancer risk and colorectal cancer mortality;
2) there is an inverse association between vitamin D
status and both colorectal cancer risk and colorectal
cancer mortality; 3) there is a negative association
between sun exposure and prostate cancer risk and
prostate cancer mortality but not between vitamin D
status and prostate cancer risk or mortality; 4) there is
an inverse correlation between sun exposure and
breast cancer risk and breast cancer mortality, and
possibly between 25(OH)D and breast cancer mortal-
ity, but studies on the association between 25(OH)D
and breast cancer risk are inconclusive; 5) there is a
negative association between sun exposure and NHL
risk and NHL mortality but not between vitamin D
e1248325-6 D. G. HOEL ET AL.
status and NHL risk or mortality; 6) there is a negative
association between sun exposure and lymphoma risk,
but no association between lymphoma risk and vita-
min D intake or 25(OH)D levels; and, 7) for multiple
sclerosis, both experimental and epidemiological stud-
ies show that the preventative role of sun exposure is
independent of vitamin D production. The authors
concluded that for colorectal cancer and breast cancer
the benefit of sun exposure is mediated by high vita-
min D levels produced by sun exposure, whereas for
prostate cancer, NHL and multiple sclerosis the bene-
fit of sun exposure is independent of vitamin D.
84
Bladder cancer
Zhao et al. 2016
86
found a 30% reduced risk of bladder
cancer associated with 25(OH)D concentrations above
30 ng/mL compared to less than 15 ng/mL.
Cardiovascular disease (CVD)
Liu et al. 2014
87
found that hypertension is reduced by
UVR-induced nitric oxide independent of vitamin D.
They showed that stores of nitrogen oxides in the
human skin are mobilized to the systemic circulation
by exposure of the body to UVA radiation, causing
arterial vasodilation and a resultant decrease in blood
pressure independent of vitamin D, confirming the
hypothesis of Feelisch et al. 2010.
88
These results cor-
relate with the findings of Afzal et al. 2014
77
that
genetically low 25(OH)D levels were associated with
increased all-cause mortality but not with cardiovas-
cular mortality, indicating that a mediator other than
vitamin D may be involved in cardiovascular mortal-
ity, and with the results of Tunstall-Pedoe et al. 2015
89
challenging vitamin D’s alleged role in cardiovascular
disease.
Metabolic syndrome (MetS) and type 2 diabetes
Vitezova et al. 2015
90
found that higher 25(OH)D lev-
els were associated with lower prevalence of metabolic
syndrome (OR 0.61, 95% CI 0.49–0.77 for more than
30 ng/mL versus less than 20 ng/mL) in the elderly in
an analysis of data from 3240 people (median age
71.2 years) imbedded in the Rotterdam Study, a pro-
spective population-based cohort study of middle-
aged and elderly adults. Importantly, after adjustment
for body mass index (BMI), higher 25(OH)D levels
were still significantly associated with lower odds of
MetS. Almost concurrent with Vitezova et al. 2015,
Clemente-Postigo et al. 2015
91
showed that low 25
(OH)D levels are associated with type 2 diabetes inde-
pendently of BMI. These findings are important in
light of the 2010 IOM report’s discounting of the asso-
ciation studies linking low 25(OH)D levels to
increased risk of type 2 diabetes on the ground that
they may be confounded by obesity, which not only
predispose individuals to type 2 diabetes but may also
cause lower 25(OH)D levels as a result of sequestra-
tion of vitamin D in adipose tissue and possibly other
mechanisms. Vitezova et al. 2015 noted that other
recent studies had found an inverse association
between vitamin D status and MetS in younger popu-
lations, but only one other study of older persons had
found the association while another study of older
persons had not. Neither Vitezova et al. 2015,
125
nor
Clemente-Postigo et al. 2015
91
cited Geldenhuys et al.
2014,
92
which found that UVR exposure levels, not
vitamin D supplements or 25(OH)D levels, reduced
the risk of obesity and type 2 diabetes, indicating that
25(OH) levels may be to some extent a marker for
UVR exposure in this regard.
Afzal et al. 2013
93
measured 25(OH)D levels in
9841 persons of whom 810 developed type 2 diabetes
during 29 y of follow-up. The investigators observed
an association of low 25(OH)D with increased risk of
type 2 diabetes (HR 1.35, 95% CI 1.09–1.66 for lowest
(less than 5 ng.mL) vs. highest (more than 20 ng/mL)
quartile of 25(OH)D. This finding was substantiated
by the authors’meta-analysis of 14 studies represent-
ing 16 cohorts with a total of 72,204 participants and
4,877 type 2 diabetes events (HR 1.50, 95% CI 1.33–
1.70 for the bottom vs. top quartile of 25(OH)D). A
prior 2011 meta-analysis [134 Mitri 2011
94
] had
shown that individuals with 25(OH)D levels above
25 ng/mL had a 43% lower risk of developing type 2
diabetes (95% CI, 2457%–) compared with individuals
with 25(OH)D levels below 14 ng/mL, and that vita-
min D supplementation had no effect.
Alzheimer disease and cognitive decline
Littlejohns et al. 2014
95
[135] studied a group of 1,658
Americans age 65 and older who were able to walk
unaided and who were free of dementia. The partici-
pants were followed for 6 y to investigate who went on
to develop Alzheimer disease and other forms of
dementia. The investigators found that participants
DERMATO-ENDOCRINOLOGY e1248325-7
with serum 25(OH)D levels below 10 ng/mL were
more than twice as likely to develop Alzheimer disease
than participants with serum 25(OH)D levels greater
than 20 ng/mL (HR 2.22, 95% CI 1.02–4.83) and par-
ticipants with serum 25(OH)D levels of 10 ng/mL to
20 ng/mL were 69% more likely to develop Alzheimer
disease than participants with serum 25(OH)D levels
greater than 20 ng/mL (HR 1.69, 95% CI 1.06–2.69).
Similar results were obtained for all-cause dementia.
According to the authors, this was the first large, pro-
spective, population-based study incorporating a com-
prehensive adjudicated assessment of dementia and
Alzheimer to examine their relationship with vitamin
D concentrations. This study confirms other recent
studies linking low vitamin D levels with cognitive
decline.
96-102
Keeney et al. 2013
96
manipulated vitamin D status
in middle-age to old-age rats by dietary supplementa-
tion with low, moderate and high levels of vitamin D.
The results suggested that dietary vitamin D deficiency
contributes to significant nitrosative stress in the brain
and may promote cognitive decline in middle-age and
elderly humans.
Annweiler et al. 2013
97
was a systematic review
and meta-analysis finding that 25(OH)D levels
were lower in Alzheimer cases than in controls
(summary random effect size 1.40, 95% CI 0.26–
2.54), which means that the probability is about
140% that an individual without Alzheimer would
have a higher 25(OH)D level than an individual
with Alzheimer if both individuals were chosen at
random from a population.
Multiple sclerosis (MS), type 1 diabetes,
rheumatoid arthritis
Wang et al. 2014
103
found that UVR suppressed
experimental autoimmune encephalomyelitis (EAE
- an animal model of MS), independent of vitamin
D production, confirming the conclusions of van
der Rhee et al. 2013
84
and the findings of Becklund
et al. 2010.
104
The investigators showed that UVB
irradiation did not suppress immune response in
the periphery, but suppressed EAE by blocking
selectively the infiltration and binding of inflamma-
tory cells into the central nervous system. These
findings support the long-held view that the inci-
dence of MS is inversely related to UVR expo-
sure.
105-109
Baarnhielm et al. 2012
110
was an association study
finding that persons with low UVR exposure had a
significantly increased risk of MS compared with those
who reported the highest exposure (OR 2.2, 95% CI
1.5–3.3), and that this association persisted after
adjustment for vitamin D status. Wang et al. 2014
103
and Baarnhielm et al. 2012
110
confirmed the conclu-
sions of van der Rhee et al. 2013
83
that sun exposure
reduces the risk of MS through pathways independent
of vitamin D.
Ponsonby et al. 2005
108
stated that genetic factors
appear to be involved in MS, but the low concordance
among identical twins for MS
111
and trends of increas-
ing incidence of MS over time
112
suggest environmen-
tal factors are also important determinants, and that
UVR exposure may be one factor that can attenuate
MS through several mechanisms and that some the
pathways are independent of vitamin D. Similar con-
clusions were made about 2 other autoimmune dis-
eases, type 1 diabetes and rheumatoid arthritis. The
authors concluded that it was critical to consider the
benefits of sun exposure as well as the risks, and to
provide information to the public on the minimum
sun exposure required for beneficial health effects as
well as the maximal sun exposure to avoid the adverse
health effects associated with excessive sun exposure.
Mokry et al. 2015
113
was a Mendelian randomization
analysis showing that genetically low 25(OH)D levels
were associated with increased risk of MS. Jalkanen
et al. 2015
114
found a high level of vitamin D defi-
ciency during pregnancy in MS patients.
Jacobsen et al. 2015
115
found that more sun expo-
sure in the third gestational trimester was associated
with lower risk of type 1 diabetes in male children.
Sawah et al. 2016
116
found a high prevalence of vita-
min D deficiency (25(OH)D levels less than 20 ng/
mL) in children and adolescents with type 1 diabetes.
Kostoglou-Athanassiou et al. 2012
117
found a high
prevalence vitamin D deficiency in patients with rheu-
matoid arthritis.
Psoriasis
Gisondi et al. 2012
118
found that the prevalence of
25(OH)D of less than 20 ng/mL was 57.8% in
patients with psoriasis vs. 29.7% in healthy controls,
and that in a logistic regression analysis, vitamin D
deficiency was associated with psoriasis indepen-
dently of other factors (OR 2.50, 95% CI 1.18–4.89).
e1248325-8 D. G. HOEL ET AL.
The investigators noted that topical vitamin D
derivatives and UVB radiation are used in the treat-
ment of psoriasis. Vitamin D status was found to be
unrelated to levels of self-reported sun exposure, but
the measure used for sun exposure, which was
minutes per day of sun exposure from March to
September, may not have been appropriate for vita-
min D production since it apparently did not
include the time of day or the area of skin exposed.
Liver disease
Gorman et al. 2015
119
in a review stated that a large
number of studies in recent Years
92,120,121
have shown
that exposure to UVR has the potential to curtail the
development of non-alcoholic fatty liver disease
(NAFLD) through vitamin D dependent and vitamin
D independent mechanisms. The authors noted that
most observational studies support an inverse associa-
tion between serum 25(OH)D levels and NAFLD, but
that vitamin D supplementation did not produce the
same results. The authors further stated that circulat-
ing vitamin D levels may represent a proxy for bodily
exposure to sunlight
122
explaining the observation
that mediators induced by sun exposure other than
vitamin D may play important roles in curtailing
NAFLD.
Statin intolerance and muscle pain, weakness
Khayznikov et al. 2015
67
found that statin intoler-
ance because of myalgia, myositis, myopathy, or
myonecrosis associated with serum 25(OH)D less
than 23 ng/mL can be resolved with vitamin D
supplementation raising serum 25(OH)D to 53 ng/
mL. Aleksic et al. 2015
123
found that low vitamin
D levels are a potentially significant and correctible
risk factor for statin-related myopathy, especially in
African-Americans.
Macular degeneration
Millen et al. 2015
124
observed a 6.7-fold increased risk
of age-related macular degeneration (AMD) among
women with serum 25(OH)D levels less than 12 ng/
mL who also had genetic risk for AMD, and noted
that previous studies had found that decreased odds of
AMD are associated with high compared to low con-
centrations of 25(OH)D.
Dental caries in infants
Schroth et al. 2014
125
found that low prenatal 25(OH)
D concenratations were associated with increased risk
of dental caries among offspring in the first year of life.
Reverse causation
Autier et al. 2014
126
suggested that low serum 25(OH)
D levels may be the result rather than the cause of dis-
eases associated with low serum 25(OH)D levels in
observational studies (reverse causation). The authors
offer little evidence to support such a hypothesis, and
it is contraindicated by the prospective nature of
many of the studies linking serum 25(OH)D levels
with health outcomes, by Mendelian randomization
studies
77,113
and by the body of knowledge concerning
the bioactivity of vitamin D, particularly its cancer-
inhibiting properties.
Obesity
Geldenhuys et al. 2014
92
suggests that UVR exposure
may be an effective means of suppressing the develop-
ment of obesity and metabolic syndrome through
mechanisms that are independent of vitamin D but
dependent on other UVR-induced mediators such as
nitric oxide. This study investigated whether UVR
and/or vitamin D supplementation had an effect on
the development of obesity and type 2 diabetes in
mice fed a high-fat diet, and found that UVR signifi-
cantly suppressed weight gain but vitamin D supple-
mentation did not. These results indicate that low
vitamin D status in obese persons may only be a
marker for low UVR exposure or a result of sequestra-
tion of vitamin D in adipose tissue, and provide a new
view of previous studies showing a consistent associa-
tion between increasing body mass index and lower
serum 25(OH)D levels.
127
Myopia
French et al. 2015
128
was a review stating that
recent epidemiological evidence suggests that chil-
dren who spend more time outdoors are less likely
to be or to become myopic, irrespective of how
much near work they do or whether their parents
are myopic. The likely mechanism for this protec-
tive effect is visible light stimulating release of
dopamine from the retina, which inhibits increased
axial elongation, the structural basis of myopia.
DERMATO-ENDOCRINOLOGY e1248325-9
The authors describe the effect of time outdoors on
the risk of myopia as robust. The prevalence of
myopia in the US in persons 12 to 54 y old
increased 66% between 1971–1972 and 1999–2004,
from 25.0% to 41.6%, according to the National
Eye Institute of the National Institutes of
Health.
129,130
For African Americans, the increase
was 157.7%.
130
This high prevalence of myopia
presents a major public health problem since, in
addition to requiring corrective lenses, myopia
poses substantially increased risk of retinal detach-
ment, glaucoma, macular degeneration, amblyopia
and cataracts.
131,132
Other benefits of sun exposure
Lambertetal.2002
133
suggested that the prevailing
amount of sunlight affects brain serotonergic activ-
ity. Deficiencies in serotonin and brain serotonergic
activity have been linked to sudden infant death
syndrome,
134
seasonal affective disorder,
133
depres-
sion,
135
schizophrenia,
136
Alzheimer disease,
137
and
migraine headaches.
138
Beta-endorphin, a neuoro-
hormone that acts as an analgesic, has been known
for many years to be released in the human body
by exercise,
139
producing a feeling of wellbeing sim-
ilar to the feeling of wellbeing induced by sun
exposure. A recent study showed that UVR expo-
sure significantly raised circulating plasma b-endor-
phin levels in a UV-exposure mouse model, leading
to suggestions that UVR exposure is addictive.
140
Alternatively, the release of b-endorphins by sun
exposure could be a natural reward mechanism
encouraging sun exposure.
The benefits of serotonin and b-endorphin, as well
as the effects of sun exposure on melatonin, photode-
gradation of folic acid, immunumodulation, photoa-
daptation, and circadian clocks, are reviewed in van
der Rhee et al. 2016.
10
Vitamin D supplements vs. sun exposure
In light of the studies discussed in this review that
found health outcomes related to sun exposure inde-
pendent of vitamin D, health outcomes dependent on
serum 25(OH)D levels but not vitamin D supplemen-
tation, and health outcomes dependent on mediators
other than vitamin D, it is apparent that vitamin D
supplements are not an effective substitute for ade-
quate sun exposure.
Balancing the risks of moderate non-burning sun
exposure against the risks of inadequate sun
exposure
The only identified risk associated with the
amount of non-burning sun exposure needed to
achieve serum 25(OH)D levels of 30 ng/mL is
some possible increased risk of nonmelanoma skin
cancer. The amount of sun exposure required to
produce this level of vitamin D varies among indi-
viduals and according to time of year, time of day
and latitude. White people with Type II skins
h
at
40 degrees latitude can obtain their annual
requirements of vitamin D by spending about 15
minutes in the sun with face, arms and legs
exposed (half that time if in a bathing suit) 2 to
3 times a week between 11 a.m. and 3 p.m. during
the months of May through October.
141
In com-
parison, nonmelanoma skin cancer is associated
withmanythousandsortensofthousandsof
cumulated hours of lifetime sun exposure.
16,52,53
Moreover, inadequate acclimatization to UVR in
daily life carries the risk of sunburn and corre-
sponding increased risk of both nonmelanoma
skin cancer and melanoma.
The risks of inadequate non-burning sun
exposure include increased risks of all-cause
mortality, colorectal cancer, breast cancer, non-
Hodgkins lymphoma, prostate cancer, pancreatic
cancer, hypertension, cardiovascular disease, meta-
bolic syndrome, type 2 diabetes, obesity, Alzheimer
disease, multiple sclerosis, type 1 diabetes, rheuma-
toid arthritis, psoriasis, non-alcoholic fatty liver
disease, statin intolerance, macular degeneration
and myopia.
People with darker skins require more time in the
sun to produce their requirements of vitamin D but
also have lower risks of nonmelanoma skin cancer,
and people with Type I skins, who are unable to tan,
require less time in the sun but have higher risks of
nonmelanoma skin cancer. All persons should avoid
sunburns, which are associated with substantial
increased risk of melanoma and nonmelanoma skin
cancer.
h
There are 6 categories of skin on the Fitzpatrick Scale: Type I Very Fair White -
always burns, never tans;Type II Fair White - usually burns, tans minimally;
Type III Cream White –sometimes mild burn, gradually tans; Type IV Brown –
rarely burns, tans with ease; Type V Dark Brown –very rarely burns, tans very
easily; Type VI Black –never burns, tans very easily.
e1248325-10 D. G. HOEL ET AL.
Conclusions
Insufficient sun exposure has become a major public
health problem, demanding an immediate change in
the current sun-avoidance public health advice. The
degree of change needed is small but critically impor-
tant. The public must be advised to obtain enough sun
exposure and vitamin D supplementation to maintain
a serum 25(OH)D level of at least 30 ng/mL. The skin
has a large capacity to produce vitamin D and a single
whole body exposure to an amount of sunlight that is
equal to 1 minimal erythemal dose is equivalent to
ingesting approximately 15,000–20,000 IUs of vitamin
D. Therefore to produce an equivalent of 4000 IUs of
vitamin D a day would require that 50% of the body
surface be exposed to 0.5 MEDs. To achieve a blood
level of at least 30 ng/ML would require ingesting
2000 IUs of vitamin D daily which would be equiva-
lent to 25% of the body surface exposed to 0.5 MEDs
2–3 times a week.
24
The amount of sun exposure
required to achieve an MED depends on skin pigmen-
tation, latitude, time of day and time of year. Warn-
ings on the dangers of sunburn at any age should be
emphasized. Periodic testing of serum 25(OH)D levels
is also reasonable especially at the end of the summer
which is when the blood level of 25(OH)D is at its
highest level.
141
Abbreviations
5(OH)D 25-hydroxyvitamin D
BCC basil cell carcinoma
CI confidence interval
CVD cardiovascular disease
HR hazard ratio
IU international units
MS multiple sclerosis
NMSC non-melanoma skin cancer
ng/mL nanograms per milliliter
NHL non-Hodgkins lymphoma
OR odds ratio
RR relative risk
SCC squamous cell carcinoma
UVR Ultraviolet Radiation (290–400 nm)
UVA Ultraviolet-A (316–400 nm)
UVB Ultraviolet-B (290–315 nm)
Disclosure of potential conflicts of interest
No potential conflicts of interest were disclosed.
ORCID
Michael F. Holick http://orcid.org/0000-0001-6023-9062
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