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Shedding Light on the Controversy Over Vitamin D.

Article Excerpt
During a routine checkup, a middle-aged woman asks you whether she should stop wearing moisturizers and makeup that contain sunscreen. She has read that increased sunlight exposure enhances vitamin D production, which may prevent certain types of cancer.

What would you tell her?

Long thought of solely as a contributor to bone health, vitamin D has more recently been linked to cancer prevention, fall prevention in the elderly, the ability to ward off tuberculosis, stronger immune systems-and more. These claims have helped make the vitamin a popular topic in consumer publications, where readers have recently been encouraged to get a little "unprotected sun" each day to ensure they have a sufficient supply of D.1

But strategies aimed at raising serum vitamin D levels have engendered controversy. Some researchers urge a dramatic increase in the recommended daily intake; others advise restraint until further evidence becomes available. Then, there is the fact that the least expensive and most readily available source of vitamin D is sun exposure on bare skin. This has led to clashes between scientists who have begun to question widely accepted practices of sun avoidance and sun protection and those who fear that recommendations of even modest amounts of unprotected time in the sun will increase skin cancer rates.

VITAMIN D BASICS

Vitamin D metabolism. The molecule of vitamin D (cholecalciferol) contained in multivitamin pills or produced in the skin when exposed to UV light is biologically inert. However, vitamin D is metabolized in the liver to 25-hydroxyvitamin D (25-OHD, or calcidiol) and then undergoes another metabolic step in the kidney; the result is 1,25-dihydroxyvitamin D (1,25-[OH]2D, or calcitriol), which is now known to be a steroid hormone.

Calcitriol is an extremely potent, active molecule. In fact, Anthony Norman, PhD, Distinguished Professor of Biochemistry and Biomedical Sciences at the University of California, Riverside, refers to calcitriol as "hormone D." Dr Norman, who has studied vitamin D for more than 40 years, notes that more than 30 tissues in the body have receptors for calcitriol (Table 1). Each tissue that is equipped with these receptors is known to have or is suspected of having a response to calcitriol.

A brief history. In the past, sun exposure was the main source of vitamin D for most people. Humans are usually able to manufacture at least 1000 IU of vitamin D with just minutes of exposure to midday sun-although this varies considerably with skin color, latitude, and time of year. The only naturally occurring food source that supplies substantial quantities of the vitamin is cold-water...