As Milwaukee heads into the darkest days of winter, some of its citizens may be approaching deficiency levels of vitamin D, a crucial nutrient derived naturally from exposure to the sun.

Vitamin D offers a host of benefits to humans, from bone strength to potential anti-cancer activities, and is easily obtained. The ultraviolet light in sunlight is naturally converted to several forms of vitamin D by mechanisms in the skin, according to Barb Troy, clinical assistant professor of biomedical sciences.

In Caucasians, sufficient levels of vitamin D can be produced by skin exposed to sunlight in as little time as 10 or 15 minutes if the intensity and angle of light is optimal, Troy said.

But that's a big if.

In winter, the sunlight in Milwaukee is both weaker and at a less advantageous angle than it is during summer, Troy said. The ozone layer also acts as a sort of screen, filtering out the vitamin D-producing sunlight, according to Robert Heaney, a vitamin D researcher with Creighton University in Omaha, Neb. Because of the condition, the conversion of sunlight into vitamin D by Milwaukeeans and other northerners proceeds more slowly in winter than in summer.

"We certainly have our months when the distance of the sun and the angle of the sunlight is not enough," Troy said.

Heaney agrees.

In winter, "people in Milwaukee would not get enough vitamin D even if the sun were shining and (they) were naked," Heaney said.

That's bad news for some ethnicities and senior citizens.

Hispanics and blacks produce vitamin D at a slower rate than Caucasians because the pigment in their skin naturally slows the transformation of sunlight to vitamin D, according to Heaney.

"The pigment in their skin absorbs the ultraviolet radiation that would convert the precursor of vitamin D into vitamin D," he said.

Even with optimal sunlight, Hispanics and blacks need about one to three hours of exposure before they produce enough vitamin D, according to Troy. And since people are less likely to be outside in winter, people of these ethnicities may not be getting enough of the nutrient because they aren't exposing themselves to enough sun.

Senior citizens are also at risk because they typically stay indoors during the winter months, Troy said, and thus may not be exposing themselves to enough vitamin D-producing sunlight.

"As people are getting older, vitamin D is becoming more of an issue," Troy said.

Luckily, sunlight is not the only source for vitamin D. Vitamin D is a fat-soluble vitamin, and so is found in the milk fat in whole and 2 percent milk. Skim milk, which contains no fat, is fortified with vitamin D, Troy said.

Coldwater fish and eggs are also good sources of the vitamin, according to U. S. Department of Agriculture dietary guidelines.

But here, too, a problem occurs. The levels of vitamin D recommended by the USDA may be insufficient, according to Troy and Heaney. For instance, the USDA currently recommends 600 International Units of vitamin D for adults over 70, but Troy thinks that number should be closer to 1,000 International Units. An International Unit is a measure of a miniscule amount of biological material that is standard worldwide. An eight-ounce glass of milk contains about 25 percent of the vitamin D an average adult needs, according to the National Milk Fluid Processing Board; the University of Michigan Health Systems puts the exact figure at 125 International Units.

Vitamin D is one of the most important nutrients the human body needs. Its main purpose in the human body is to contribute to overall bone health, according Troy.

"Vitamin D has one primary function — the absorption of calcium and phosphorous," Troy said. Sufficient levels of vitamin D — the USDA currently recommends 400 International Units for most adults — strengthen the hard calcium-containing bone matrix and so can ward off osteoporosis.

Enough vitamin D also prevents rickets, formerly a common condition in which children's bones are too soft and so become deformed with use.

Additional research suggests that vitamin D may have potential in the field of anti-cancer research. Recent articles in the medical journals Environmental and Molecular Mutagenesis and BioEssays report that preliminary studies have shown that vitamin D may encourage the immune system to attack cancerous tissue cells (among other things). This sufficient amount of vitamin D may prevent or slow the progress of colon, breast and prostate cancers.

This article appeared in The Marquette Tribune on Dec. 9 2004.