The Sunshine D-lemma

Sunlight causes skin cancer, but it also produces vitamin D, a substance that seems to prevent some types of cancer and possibly other diseases.

Carcinogen is a word that jumps off the page. It makes us think of nefarious chemicals and noxious pollutants.

So it's startling to see sunlight — or, to be more precise, the invisible, ultraviolet portion of sunlight — labeled a carcinogen, a term that can be applied to anything believed to cause cancer. There are some nuances to the evidence, but overall, there's not much question that exposure to ultraviolet light is a risk factor for all three types of skin cancer: basal cell, squamous cell, and melanoma.

But it's gotten more complicated lately because a steady stream of findings have linked sun exposure to lower rates of some cancers — especially colon cancer, but others as well. Sunlight may have this anticancer effect — as well as other health benefits — because when our skin is exposed to ultraviolet light it makes vitamin D, and vitamin D seems to be a powerhouse of a vitamin.

Finding the Middle Ground

The pros and cons of sun exposure have fueled an occasionally heated debate. On one side are most dermatologists, who continue to stress skin cancer prevention and the importance of avoiding midday sun and applying sunscreen. On the other are many vitamin D researchers, who believe that by so assiduously shielding ourselves from sunlight we might be missing out on the myriad benefits of their favorite vitamin. Some have come down on the side of using tanning beds, a suggestion that's anathema to mainstream dermatologists.

Fortunately, a sizable patch of middle ground exists between the two camps that acknowledges the dangers of sun exposure while giving vitamin D its due.

The ultra- in ultraviolet refers to wavelengths that are even shorter than those that humans see as purplish violet. The ultraviolet light produced by the sun that reaches the earth's surface is subdivided into ultraviolet B (UVB), which comes in wavelengths of 290 to 320 nanometers (nm), and ultraviolet A (UVA), which, at wavelengths of about 320 to 400 nm, is closer to visible light (see diagram below).

We don't see UVA and UVB because of the way the human eye happens to be engineered, but they have a major effect on the skin, especially UVB. It's UVB that causes sunburn and the damage to DNA that leads to skin cancer, although there's research implicating UVA as well.

But UVB is also responsible for the second edge that makes sunlight a double-edged sword, because it's the energy from UVB that penetrates the skin and transforms a type of cholesterol molecule into a precursor of vitamin D — call it previtamin D. A metabolic hop, skip, and jump later — a sequence that takes place in the liver and kidneys — and previtamin D becomes the biologically active form of vitamin D.

Active vitamin D helps regulate how cells grow and mature, so in some circumstances it may put the brakes on cancerous runaway cell growth. Vitamin D also helps keep the immune system in line, which may be the reason animal and epidemiological studies hint at a connection between low vitamin D levels and autoimmune disorders such as type 1 diabetes and multiple sclerosis.

Sunray spectrum   Sunray spectrum

The Winter Filter

This time of year the sun is pretty much overhead at midday, so plenty of UVB hits the skin of people who venture out uncovered.

But during the winter in the upper latitudes (roughly 40° — the latitude of New York City — or higher) of the northern and southern hemispheres, sunlight hits the earth at an angle, so it travels a longer distance through the atmosphere. Much, if not all, of the UVB gets filtered out (although the longer waves of UVA still get through). Sunburn isn't a problem, but the vitamin D levels drop to about half of their summertime levels. There's an interesting theory that dips in vitamin D levels could explain why flu epidemics are winter events.

Is Sunscreen the Anti–Vitamin D?

Theoretically, sunscreen does stand in the way of maximizing vitamin D levels. The main purpose of sunscreen is to prevent sunburn, so most products are formulated to block the very same UVB wavelengths that trigger vitamin D production in the skin. Indeed, the sun protective factor (SPF) rating is a measure of how well the sunscreen stops UVB.

At a practical level, though, sunscreens probably aren't a problem for several reasons. First, studies have shown that most people use far less sunscreen than the amounts used to arrive at the SPF, so the real-world SPF is about half of the one on the bottle. Unless you are super scrupulous about sunscreen, plenty of vitamin D–making UVB is "leaking" through.

Second, it doesn't take very long to make a lot of vitamin D during the summer, so brief spells outside might give you all the vitamin D you need. Dr. Michael Holick, the Boston University researcher who has done more to push vitamin D into the spotlight than anyone else, has written that a fair-skinned person can satisfy his or her vitamin D requirement during the summer months by exposing the arms and legs to sunshine for just 5 to 15 minutes two or three times a week between 10 a.m. and 3 p.m. He recommends the use of sun protection on the face, noting that it's the area of the body that's most exposed to the sun and not a big contributor of vitamin D.

Finally, for people in the upper latitudes, the problem of running a vitamin D deficit is the most serious during the UVB-less winter months, when people aren't wearing sunscreens anyway (although skiers still need to be careful because ultraviolet light levels are more intense at higher elevations and because light reflects off snow).

Even if the UVB weren't a cause of skin cancer, added sun exposure won't be the answer to many people's vitamin D woes. With age, the skin's capacity for making vitamin D diminishes, so the effect of getting more sun is going to be limited in older folks. And the melanin that makes skin dark effectively filters out UVB, so African Americans are much more likely than whites to have low levels of vitamin D. Researchers in sun-drenched Arizona reported in 2008 that over half of the African Americans in their colorectal cancer study had low vitamin D levels, defined as 20 nanograms per milliliter (ng/ml).

Diet and Supplements

Government guidelines say people between the ages of 51 and 70 should get 400 International Units (IU) of vitamin D daily, and those ages 71 and older, 600 IU. Many experts, including Dr. Holick, say Americans should shoot for 1,000 IU, because, on average, that's how much it takes (along with incidental sun exposure) to get the levels in the blood to 30 ng/ml, the level at which prevention of cancer and other diseases seems to kick in.

When we're short on a vitamin, diet or supplements — or both — are usually the answer. But there are problems on both fronts with vitamin D. Just a few foods are natural sources of the vitamin: salmon (about 350 IU per 3½-ounce serving), mackerel, sardines, and other kinds of oily seafood are about it. In this country, milk and some brands of orange juice are artificially fortified with 100 IU per eight ounces. Some breakfast cereals are also fortified.

Traditionally, supplement makers have used vitamin D₂ (ergocalciferol), particularly in multivitamin pills. Some data show little difference in the effects of D₂ and D₃ (cholecalciferol), the form used to fortify milk and contained naturally in fish, but the conventional wisdom has been that D₂ is about a third to a fourth less effective than D₃ at increasing blood levels of vitamin D. Because D₃ has been presumed to be more potent, more vitamin D and vitamin D-calcium supplements are being made with it.

Other D-fenses

Losing weight may increase your active vitamin D levels. Vitamin D is fat soluble, so it may get locked up in extra fat tissue and become less bioavailable if we get heavy by adding body fat. In research studies, being overweight or obese has consistently been associated with low vitamin D levels.

Watching your intake of the preformed vitamin A (retinol) contained in some supplements is another way to get the most out of your vitamin D intake. Within cells, vitamin A and vitamin D compete for the same protein to be effective. Many multiple vitamin makers have reduced the amount of preformed vitamin A in their products.

And then there's the controversial question of whether tanning beds might be used to boost vitamin D levels. The tanning salon industry has jumped at the chance to portray itself as a supplier of such a healthful vitamin. Research from Scandinavia that showed a limited number of "sunbed" sessions during the winter can lift vitamin D levels to summer levels has lent their case some legitimacy.

The strong rebuttal from the American Academy of Dermatology and others is that indoor tanning usually involves exposure to UVB, that known carcinogen — not just UVA — and that there are far less risky ways of ramping up your vitamin D levels. What's more, the prime tanning salon customer is a young, fair-skinned person with both opportunity and motive to go overboard with tanning, indoor and out, for many years. So far, the dermatologists have got it right: the risks of indoor tanning aren't worth the vitamin D benefits, especially when there are alternative ways to get them.

You can get too much vitamin D, but the level where toxicity is seen is almost certainly many times higher than the upper daily limit of 2,000 IU set by the Institute of Medicine. For years, vitamin D researchers have wanted a re-evaluation of vitamin D minimums and maximums, but so far, this hasn't happened.