Sunny D

Our neighbors swapped houses with a family from Australia one summer and the trade had members of the block-party committee eagerly anticipating an exotic cultural exchange. “But they’re just normal,” the boy next door observed when the Aussies finally flew up from down under. “You can’t even tell they’re different.”

True, the newcomers quickly blended in, driving on the right side of the street without incident, chatting amiably about American Idol and complaining about the difficulty of finding a good babysitter—something always in short supply, whether you live in Minneapolis or Melbourne.  They might have been mistaken for Minnesotans were it not for their fanatical approach to sunscreen.

While the rest of us dabbed a squirt of it on whichever kid would stand still at the park, the Aussies applied it with a near-religious fervor, rubbing the stuff behind everyone’s ears, into the crooks of elbows, and on the backs of knees, then reapplying it a few hours later, according to the instructions on the package. When I asked the mom how she got her children to submit to such a thorough going-over, she told me they were accustomed to it—Australia is as sun-drenched as it looks on the postcards and has the skin cancer rates to prove it. In fact, she explained, two out of three Australians will be treated for some form of skin cancer in their lifetimes.

To combat skin cancer, the Australian government, in 1981, introduced an intensive media campaign called “Slip! Slap! Slop!” that reminded Australians to slip on a shirt, slap on a hat, and slop on the sunscreen before they go outdoors. Now regarded as one of the world’s most effective public health messages, the mantra made the once unfamiliar habit of slathering oneself in sunblock second nature to most Australians.

Since the Aussies returned home (after bequeathing me a leftover bottle of sunscreen they didn’t want to take on the plane), I’ve tried to make “Slip! Slap! Slop!” second nature to my family too. That is, until a few months ago, when I read that some Australian health experts were blaming their nation’s exemplary approach to skin-cancer prevention for widespread vitamin D deficiencies, which allegedly has resulted in an increase in bone fractures and a reemergence of rickets.

Of course, if you’ve walked past a newsstand recently, you know that D is the vitamin of the moment, appearing on the health pages as often as Angelina Jolie appears everywhere else. Insufficient levels of D have been linked to an increased risk of cancers of the colon, prostate, and breast, as well as to heart disease, type 1 diabetes, multiple sclerosis, bone fractures, muscle pain, and depression. High levels of D are associated with stronger immune systems, greater muscle strength, better balance, and even sunnier dispositions.

Canadians, who suffer through winters even longer than ours, are so convinced of the potential health benefits of the so-called “sunshine vitamin” that their national cancer society has begun recommending that most older adults supplement their diets with 1,000 international units of D, five times the recommended U.S. daily allowance for adults under fifty and more than twice the 400 IUs recommended for persons between fifty and seventy. Some estimates suggest that about half of all Americans are low in vitamin D.

With the long days of sunlight stretching out before us, I’m a little cloudy as to what all this means. Does the disease-fighting potential of vitamin D mean Minnesotans cooped up all winter should drench themselves in sunshine this season? Or do we risk getting too much of a good thing?

Vitamin D has been causing confusion since it was first identified, starting with its name. “It’s not really a vitamin,” explains Catherine Niewoehner, an endocrinologist at the Veterans Affairs Medical Center in Minneapolis and a professor at the University of Minnesota. “It’s a hormone that our bodies make from the action of light on the skin.”

When it was first isolated, scientists thought vitamin D was something the body didn’t produce on its own. Since then, they’ve learned it’s a precursor hormone in human skin that, when activated by ultraviolet light, produces vitamin D3, which is also called cholecalciferol. Entering the blood stream, it moves through the liver and the kidneys, changing along the way into its final, active form, calcitriol, which helps our bodies absorb the calcium we need for healthy bones. A Swiss study of women in their eighties found that those who took vitamin D and calcium supplements together had greater leg strength and suffered from half as many falls as women who took only calcium. Researchers at Creighton University in Nebraska recorded an even more encouraging outcome in a four-year, double-blind study of more than 1,000 healthy postmenopausal women. As reported last year in the American Journal of Clinical Nutrition, participants who took calcium and 1,100 IUs of vitamin D3 daily developed about 80 percent fewer cancers than those who took only calcium or a placebo.

“The science is still being worked out, but there are associations,” says Niewoehner. “Vitamin D helps to activate a lot of different genes in the body, and we’re just beginning to learn their role in cell maturation and cancer prevention.” Even so, some experts are already urging Americans to increase their daily dosage. Robert Heaney, a Creighton endocrinologist and member of the 1997 panel that created the current dosage guidelines, told Forbes magazine in March that current vitamin D recommendations are “grossly inadequate.”

Supplements of vitamin D3 and D2 (the plant-derived form) are an easy way to boost our intake, though there’s little agreement about the dosage we need to reap all the benefits we’re reading about these days. One study, reported in the American Journal of Clinical Nutrition, found that the current 400 IUs recommended daily for the over-fifty crowd resulted in “no appreciable reduction in fracture risk”; another study, reviewed in the journal Neurology, reported that the same dose—the equivalent of four glasses of milk a day—cut a woman’s chances of developing multiple sclerosis by 40 percent.

D is also found in such foods as fresh wild salmon, sun-dried shiitake mushrooms, egg yolks, and cod liver oil—which may explain why Americans have preferred, for the past fifty years, to add vitamin D to milk and other dairy products. “We’re really not intended to get it from our diets,” Niewoehner points out. “We’re intended to get it from the sun.” In fact, 90 percent of the body’s supply of vitamin D is generated by the skin’s reaction to sunlight—though there are limitations to that as well.

 In dark skin, melanin reduces the body’s ability to synthesize the vitamin from sunlight, which is one reason African Americans are more likely to be vitamin D deficient than whites. And skin thins as we age, slowing down the production of D. Sunscreens also inhibit the process, as those with SPFs as low as eight can block 95 percent of the sun’s ultraviolet rays. And for those of us living above the fortieth latitude, our sunshine isn’t strong enough during the fall and winter to give us any D at all.

“From October through March, we’re too far away from the sun,” says Niewoehner. “If you were outside for twenty-four hours a day, stark naked in February, you would die of exposure and be low in vitamin D.”