A new study by scientists at the U.S. Department of Energy's
Brookhaven National Laboratory and colleagues in Norway
suggests that the benefits of moderately increased exposure
to sunlight - namely the production of vitamin D, which
protects against the lethal effects of many forms of cancer
and other diseases - may outweigh the risk of developing
skin cancer in populations deficient in vitamin D.
"We know that solar radiation is the leading cause
of skin cancer," said communicating author Richard
Setlow, a Senior Biophysicist Emeritus at Brookhaven and
a well-known expert on the link between solar radiation
and skin cancer. Setlow's group was the first to establish
that ultraviolet A (UVA) radiation and visible light are
the primary causes of malignant melanoma, the deadliest
form of skin cancer. He and his colleagues emphasize that
people need to protect themselves from the harmful effects
of sun exposure.
But solar radiation is also a major, if not the main,
source of vitamin D in humans. In the presence of sunlight,
the body converts certain precursor chemicals to active
"Since vitamin D has been shown to play a protective
role in a number of internal cancers and possibly a range
of other diseases, it is important to study the relative
risks to determine whether advice to avoid sun exposure
may be causing more harm than good in some populations."
The concern, he says, is particularly great in populations
from northern latitudes, such as Scandinavia, where sun
exposure is extremely limited.
In the current study, Setlow and his colleagues used
a model incorporating information on solar radiation intensity
and a vertical cylinder shape to represent the human body's
skin surface to calculate the relative production of vitamin
D via sunlight as a function of latitude, or distance
from the equator. The cylindrical model more realistically
represents human body sun exposure than flat surface exposure
measurements used in previous models. The scientists also
examined the incidence of and survival rates for various
forms of cancer by latitude.
According to the calculations, people residing in Australia
(just below the equator) produce 3.4 times more vitamin
D as a result of sun exposure than people in the United
Kingdom, and 4.8 times more than people in Scandinavia.
"There is a clear north-south gradient in vitamin
D production," Setlow says, "with people in
the northern latitudes producing significantly less than
people nearer the equator."
In populations with similar skin types, there is also
a clear increase in the incidence of all forms of skin
cancer from north to south. "This gradient in skin
cancer rates indicates that there is a true north-south
gradient in real sun exposure," Setlow says.
The scientists also found that the incidence rates of
major internal cancers such as colon cancer, lung cancer,
and cancers of the breast and prostate also increased
from north to south. However, when the scientists examined
the survival rates for these cancers, they found that
people from the southern latitudes were significantly
less likely to die from these internal cancers than people
in the north.
"In previous work, we have shown that survival rates
for these cancers improve when the diagnosis coincides
with the season of maximum sun exposure, indicating a
positive role for sun-induced vitamin D in prognosis -
or at least that a good vitamin-D status is advantageous
when combined with standard cancer therapies," Setlow
says. "The current data provide a further indication
of the beneficial role of sun-induced vitamin D for cancer
So, how can people get the benefits of vitamin D without
running the risk of deadly skin cancer?
"As far as skin cancer goes, we need to be most
worried about melanoma, a serious disease with significant
mortality," Setlow says.
Melanoma is triggered by UVA (the long UV wavelengths)
and visible light. Vitamin-D production in the body, on
the other hand, is triggered by UVB (the short UV wavelengths
at the earth's surface). "So perhaps we should redesign
sunscreens so they don't screen out as much UVB while
still protecting us from the melanoma-inducing UVA and
visible light," Setlow says.
Increased UVB exposure may result in an increase in non-melanoma
skin cancers. But these are relatively easy to cure and
have very low mortality rates compared with the internal
cancers vitamin D appears to protect against, Setlow adds.
Another option would be to increase vitamin D consumption
while continuing to wear sunscreen. Vitamin D is easily
accessible in many foods and liquids, such as cod liver
oil and milk, and in dietary supplements.
The study will be published online in the Proceedings
of the National Academy of Sciences the week of January