Sunlight's
Vitamin D May Help Prevent Skin Cancer
It may be puzzling, but scientists' findings suggest that
the main cause of skin cancer - sunlight - might also help protect against
the disease, says a study in Nature Immunology.
The key could lie in the amount of ultraviolet B (UVB) light
the skin absorbs - enough to stimulate a healthy, vitamin D-linked immune response
in the skin but not so much that it boosts skin cancer risk.
"I do
think that a little bit of sunlight is good for people, but I think that
one of the
problems that the American
Cancer Society and dermatologists have is, how do you define what a
little bit is?" says Marianne Berwick, Ph.D., at the University of New Mexico's
Cancer Research and Treatment Center.
"How do you tell people that it's okay to have a little
bit of sunlight but not too much?" she asks.
In 2005, Dr. Berwick's team published a controversial study
that found that melanoma patients with higher levels of daily sun exposure
actually had better survival than patients who spent less time in the sun.
"I've been searching for an explanation for that ever since," she
says.
Now, findings from a group led by immunologists at Stanford
University may provide an answer.
In their study, the Stanford team worked with cells in the
lab and discovered a biochemical chain of events that appears to link sunlight
exposure to the skin's own immune defenses.
The researchers started from the notion that an inactive
precursor of vitamin D, called vitamin D3, is generated in the skin in response
to sun exposure. That has been known for years.
Specifically, a short-wavelength form of UV light, called
UVB, is responsible for D3 generation.
D3 is inactive, however. Through contact with various enzymes
in the liver and kidneys, the body turns D3 into an active compound called
1,25(OH)2D3.
And that is where the immune-system connection kicks in,
the Stanford authors say.
In their experiments, they found that the new compound sent
a signal to (immune) T-cells, pushing them to migrate back to specific sites
in the skin's epidermis.
Once there, these powerful immune system agents stand on
guard against infection and even cancer, the researchers say.
"So, the same wavelengths of sunlight that are most potent
in inducing skin cancer - UVB - are also the wavelengths that produce this
vitamin D precursor, D3," says Dr. Martin Weinstock, chairman of the skin cancer
advisory group at the American Cancer Society.
And it is D3 that starts the whole chain of events rolling.
Dr. Weinstock stresses that the Stanford study is far from
conclusive, however, and should not be seen as an excuse to bake in the sun.
"We know that the sun is the major avoidable cause of skin
cancer," he says. "This study is interesting and points to a productive area
of research, both to confirm this in other settings and to flush out the implications
of the finding. But does it really relate to skin cancers in real live people?
We don't know.
"So, avoiding intense sun, protecting yourself when you're
out in intense sun - that's still our recommendation, and this is not going
to change that," says Dr. Weinstock.
Kathleen
Egan, Sc.D., at the H. Lee Moffitt Cancer Center and Research Institute in
Tampa,
Fla., agrees that the study findings are "tantalizing" but
need further study.
Especially
since the release of Dr. Berwick's melanoma study, "there's
been an awful lot of questions about how - or if - vitamin D has a part to
play in potentially offering some [cancer] protection under some circumstances," she
says.
"But it's very difficult to tease out, because the main
human source of vitamin D is, in fact, sunlight exposure, which is also the
most important risk factor for melanoma," says Dr. Egan.
Nutritionists have known for decades that sunlight stimulates
vitamin D production in the skin. In fact, this natural process is the body's
major source of the nutrient.
A proper
amount of vitamin D is crucial to bone health, "and
there's also a bunch of evidence that vitamin D may have a role in preventing
colon cancer, although there's still some controversy about that," Dr. Weinstock
says.
So, how much sunlight is enough to get the ideal amount
of vitamin D?
Katharine Tallmadge, a dietitian and spokeswoman for the American
Dietetic Association, suggests that most people can probably get the
US Department of Agriculture's recommended 400 daily international units
(IUs) of vitamin D by spending a half-hour to an hour outside per day.
However,
the Office of Dietary
Supplements at the National Institutes
of Health (NIH) states that 10 to 15 minutes of sun exposure
to face, hands, arms, or back without sunscreen at least twice a week is
generally enough to produce adequate vitamin D.
Dr. Egan agrees. She says it is not difficult for people
to soak up the sun's goodness without boosting their cancer risk.
In response
to even a moderate amount of sunlight, "the
skin actually creates an amazing amount of vitamin D," says Dr. Egan. "It doesn't
take much exposure to make enough of the vitamin D that's certainly needed
to preserve bone health, for example."
Always consult your physician for more information. |
