Sushine Vitamin

BIOLOGY

Cell Defenses and the

Vitamin
Scientists now recognize that vitamin D does much more than build strong bones and that many people are not getting enough of it. Is widespread D deficiency contributing to major illnesses?

By Luz E. Tavera-Mendoza and John H. White

KEY CONCEPTS
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Vitamin D, long associated only with its role in bone formation, is actually activethroughout the human body, powerfully influencing immune system responses and cell defenses. It can be obtained from food or manufactured by human skin exposed to sunlight. Measures of vitamin D levels show, however, that many people have too little of it circulating in their blood to protect health. Clear associations between low vitamin D levels and cancers, autoimmunity, infectious diseases and otherconditions suggest that current daily intake recommendations for this critical nutrient need revision.
—The

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Editors

t was called the sunshine cure, and in seen micronutrients important to health the early 20th century, before the era was not yet understood by doctors. And of antibiotics, it was the only effective nearly a century would pass before scientherapy for tuberculosisknown. No one tists made the connection between such knew why it worked, just that TB patients dietary cures for rickets and the beneficial sent to rest in sunny locales were often effects of sunshine. Early 20th-century rerestored to health. The same “treatment” searchers showed that irradiated skin, had been discovered in 1822 for another when fed to rats with artificially induced historic scourge,rickets — a deforming rickets, had the same curative properties childhood condition caused by an inabil- as cod-liver oil. The critical common eleity to make hardened bone. Rickets had ment in the skin and the oil was finally been on the rise in 18th- and 19th-century identified in 1922 and dubbed vitamin D. Europe, coinciding with industrialization By then the idea of “vital amines,” or vitaand themovement of people from the mins, was a popular new scientific topic, countryside to the polluted cities, when a and subsequent research into the funcWarsaw doctor observed that the problem tions of vitamin D in the body was very was relatively rare in rural Polish children. much shaped by D’s image as one of those He began experimenting with city chil- essential micronutrients that humans can drenand found that he could cure their obtain only from food. rickets with exposure to sunshine alone. The association with rickets also steered By 1824 German scientists found that most vitamin D research for the next 50 cod-liver oil also had excellent antirickets years toward understanding the molecule’s properties, although that treatment did role in bone building and how it acts in the not catchon widely, in part because the kidneys, intestines and the skeleton itself to possibility that a food might contain un- help control the flow of calcium into and
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[THE BASICS]THE MAKING OF AN ACTIVE VITAMIN
VITAMIN D MADE BY HUMAN SKIN Ultraviolet B light D3 D2 VITAMIN D FROM FOOD

The term “vitamin D” refers to two slightly different molecules — D3, manufactured by human skin, and D2, which originates in plants and which people can obtain from food. Both versions of D must undergo stages of conversion by enzymes to reach a biologically active form, known as1,25D.

Epiderm

UVB light + 7-dehydrocholesterol

is

Dermis

Keratinocytes Melanocyte LIVER

To circulation

1 called keratinocytes when ● Vitamin D is madea by skin cells product of cholesterol, 7- ultraviolet B light and heat act on breakdown
3

dehydrocholesterol (left). Vitamin D2, found in certain foods, is derived from a similar plant sterol molecule (right). Whether...