The Northwest's dreary winters are infamous for inducing depression. But being starved for sunlight can do more than kick you into a psychic hole. A growing body of evidence suggests it can raise your risk of cancer, increase susceptibility to heart attack, diabetes and other disorders, and at least partly account for the region's sky-high rates of multiple sclerosis.
The reason is vitamin D, an essential nutrient produced in abundance by skin exposed to the sun's rays. Long dismissed as being important mainly for strong bones, the so-called sunshine vitamin is now recognized as a key player throughout the body, including the immune system.
Experts say vitamin D deficiency is much more common than previously believed — especially in northern climes like Washington, where solar radiation from October to March is too puny to maintain healthy levels.
"You're in a dark, gloomy place," said Bruce Hollis, a leading vitamin D researcher at the Medical University of South Carolina. "In the winter, you could stand outside naked for five hours and nothing is going to happen."
Increased use of sunscreen has turned a seasonal shortfall into a year-round condition for many people. A recent survey in Britain found 87 percent of adults tested during winter, and more than 60 percent in summer, had subpar vitamin D levels. Doctors in many parts of the world — including California — report a resurgence of childhood rickets, soft bones caused by lack of vitamin D.
While supplements offer a cheap and easy solution, Hollis and other researchers argue the recommended intake is too low to provide many health benefits. A Canadian medical organization advises that pregnant and nursing women take 10 times the amount suggested in the U.S.
"You're more likely to live longer and you're less likely to die of serious chronic disease if you have adequate vitamin D on board," said Dr. Michael Holick of Boston University School of Medicine, one of the world's top experts. "It may well be the most important nutrient of the decade."
Risks of low levels
When Lisa Hill went to her doctor complaining of joint pain, she was surprised to get a diagnosis of vitamin D deficiency. "I had never heard of it," said the 54-year-old Gig Harbor woman.
Since leaving her native Southern California, her sun exposure has dropped dramatically.
"You're like a little mole in a hole," she said. "You just don't get much sun here."
Many doctors once scoffed at the notion of vitamin D deficiency, but testing has become more routine and is covered by most insurance.
University of Washington heart surgeon Dr. Donald Miller Jr. tested 78 of his patients and found three-quarters had "insufficient" levels of vitamin D.
"It was really pretty shocking," said Miller.
In a study of 1,739 Boston-area residents reported last month, rates of heart attack, stroke and heart failure were about 50 percent higher in those with low levels of vitamin D. In addition to strengthening bones, muscles and joints, high vitamin D levels have been linked with lower rates of colon, prostate, breast, esophageal and pancreatic cancer. Harvard scientists found that high levels of vitamin D reduced children's odds of developing asthma, while researchers in Pittsburgh reported that pregnant women with low vitamin D had greater risk of preeclampsia, a dangerous form of high blood pressure.
Vitamin D also appears to be one of the reasons multiple sclerosis and other autoimmune diseases are twice as common in northern vs. southern states. Washington's rate of MS, which causes progressive nerve damage, is one of the highest in the nation. Blood samples from more than 7 million military personnel showed people with the highest levels of vitamin D were 62 percent less likely to develop MS than those with the lowest concentrations. A study in Finland found similar results.
What D can do
Formed in skin cells exposed to UVB, the invisible form of light that causes sunburn, vitamin D and its breakdown products act throughout the body. The compounds are believed to regulate as many as 1,000 genes, including genes that weed out precancerous cells and genes that slow the runaway reproduction typical of cancer. Molecular geneticist John White and his colleagues at McGill University in Montreal discovered vitamin D also switches on an arm of the immune system that kills bacteria — including the bug responsible for tuberculosis.
"It's a kind of front-line response to infection," he said.
That may explain why TB patients in the early 1900s who basked in the sun at sanitariums were often cured, added White, author of a recent Scientific American article on vitamin D.
The compound has an anti-inflammatory effect, too, which probably plays a role in preventing heart disease and autoimmune disorders.
The evolutionary angle is also being explored, with the suggestion that early people who migrated north from the equator lost skin pigmentation to maximize vitamin D production. Today, dark-skinned people in northern latitudes are among the most vulnerable to vitamin D deficiency.
Inconclusive studies
While the evidence is piling up, most of it is still based only on association. Scientists count cancer cases, infer or measure vitamin intake, then look for correlations. Some researchers advise caution until there's more data from controlled trials, where one group gets vitamin D, while another gets a placebo.
One such trial last year found 1,000 international units (IU) a day slashed cancer risk for women. But a much bigger study found women who took vitamin D supplements had the same risk of colon cancer as those who didn't. "I would say the jury is out," said Ulrike Peters, who studies nutrition and cancer at the Fred Hutchinson Cancer Research Center.
Women in the large experiment took 400 IU a day of vitamin D — the amount in a typical multivitamin.
Hollis, the South Carolina researcher, says the results simply show that standard doses aren't enough.
The U.S. Institute of Medicine (IOM) recommends 200 IU a day up to age 50, and 400 to 600 IU for older people. The Canadian Paediatric Society recently urged pregnant and nursing women to take 2,000 IU a day — which the IOM designates as the maximum safe dose.
Vitamin D experts say much higher doses are safe. Exposing just your arms and legs to the summer sun for less than 15 minutes can generate 5,000 IU, Holick pointed out. It is possible to go overboard with supplements and trigger dangerous calcium deposits in kidneys and blood vessels, but Holick says it takes a lot: more than 10,000 IU a day for a year.
Various studies have linked low 25(OH)D levels to diseases other than cancer, raising the possibility that vitamin D insufficiency is contributing to many major illnesses. For example, there is substantial though not definitive evidence that high levels of vitamin D either from diet or from UVR exposure may decrease the risk of developing multiple sclerosis (MS). Populations at higher latitudes have a higher incidence and prevalence of MS; a review in the December 2002 issue of Toxicology by epidemiology professor Anne-Louise Ponsonby and colleagues from The Australian National University revealed that living at a latitude above 37° increased the risk of developing MS throughout life by greater than 100%.
“Scientific evidence on specific effects of vitamin D in
preventing MS or slowing its progression is not sufficient,” says
Alberto Ascherio, a nutritional epidemiologist at the Harvard School of
Public Health. “Nevertheless, considering the safety of vitamin D even
in high doses, there is no clear contraindication, and because vitamin D
deficiency is very prevalent, especially among MS patients, taking
vitamin D supplements and getting moderate sun exposure is more likely
to be beneficial than not.”
As with MS, there appears
to be a latitudinal gradient for type 1 diabetes, with a higher
incidence at higher latitudes. A Swedish epidemiologic study published
in the December 2006 issue of Diabetologia found that
sufficient vitamin D status in early life was associated with a lower
risk of developing type 1 diabetes. Nonobese mice of a strain
predisposed to develop type 1 diabetes showed an 80% reduced risk of
developing the disease when they received a daily dietary dose of
1,25(OH)D, according to research published in the June 1994 issue of the
same journal. A finish study published 3 November 2001 in The Lancet
showed that children who received 2,000 IU vitamin D per day from 1
year of age on had an 80% decreased risk of developing type 1 diabetes
later in life, whereas children who were vitamin D deficient had a
fourfold increased risk.
Researchers are now seeking to understand how
much UVR/vitamin D is needed to lower the risk of diabetes and whether
this is a factor only in high-risk groups. There is
also a connection with metabolic syndrome, a cluster of conditions that
increases one’s risk for type 2 diabetes and cardiovascular disease. A
study in the September 2006 issue of Progress in Biophysics and Molecular Biology
demonstrated that in young and elderly adults, serum 25(OH)D was
inversely correlated with blood glucose concentrations and insulin
resistance. Some studies have demonstrated high prevalence of low
vitamin D levels in people with type 2 diabetes, although it is not
clear whether this is a cause of the disease or an effect of another
causative factor—for example, lower levels of physical activity (in this
case, outdoor activity in particular).
People living
at higher latitudes throughout the world are at higher risk of
hypertension, and patients with cardiovascular disease are often found
to be deficient in vitamin D, according to research by Harvard Medical
School professor Thomas J. Wang and colleagues in the 29 January 2008
issue of Circulation. “Although the exact mechanisms are poorly
understood, it is known that 1,25(OH)D is among the most potent
hormones for down-regulating the blood pressure hormone renin in the
kidneys,” says Holick. “Moreover, there is an inflammatory component to
atherosclerosis, and vascular smooth muscle cells have a vitamin D
receptor and relax in the presence of 1,25(OH)D, suggesting a multitude
of mechanisms by which vitamin D may be cardioprotective.”
Source:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2290997/
http://seattletimes.com/html/health/2004179538_vitamind13m.html
No comments:
Post a Comment