Vitamin D Essentials

What you need to know about this important nutrient and where best to obtain it
Phillip Rhodes

Vitamin D is becoming an increasingly important player in a healthful diet. Over the past 10 years, a spate of research has linked it to an impressive and diverse array of potential benefits. In addition to vitamin D's well-known function of increasing calcium absorption and thereby helping encourage healthy bone growth, it has shown promise in helping to prevent certain cancers, heart disease, diabetes, and osteoarthritis. However, while many in the scientific community are excited about its promise, they have yet to agree on how much you need and where to obtain it. Read on to better understand the debate and learn what's best for you.

What vitamin D does
Vitamin D is unique in many ways. At the time of its discovery in 1919, vitamins A, B, and C were already identified; D was the next letter in line, so that was the name given to the compound. However, vitamin D behaves like a hormone in the body, relaying chemical messages―something no other vitamin does. For example, vitamin D signals the intestines to absorb calcium from foods and to regulate its uptake by bone cells.

Some nutrients, such as vitamin C, are put to work immediately by the body in the form in which they are consumed. Vitamin D, however, requires processing. Vitamin D begins as a relative of cholesterol. Dehydrocholesterol molecules are stored in your skin, waiting to absorb sunlight. When this occurs, dehydrocholesterol can be transformed into previtamin D3 (cholecalciferol), the most readily absorbed form of vitamin D. Another form, previtamin D2 (ergocalciferol) is derived from plant sources and can only be obtained through diet. Both previtamins are processed by the liver and kidneys into a final form called calcitriol, which then travels to the vitamin D receptors that exist in almost all, if not all, cells in your body.

"Vitamin D is a key component in helping the body respond to many different kinds of assaults and stimuli," says Robert Heaney, PhD, professor of medicine at Creighton University in Omaha, Nebraska. "In the absence of it, you're asking the body to defend itself with one hand tied behind its back."

Most of the good news about vitamin D comes thanks to improved methods of measuring the levels of vitamin D in the body. "We didn't have a good way to measure vitamin D until 15 years ago, and the test took five to 10 years to reach widespread use," Heaney says. "Previously, the only indicator of vitamin D deficiency was rickets [a disease resulting in softened bones]."

How much do you need?
The Institute of Medicine (IOM), a group that uses scientific research to formulate public health policies, currently recommends an Adequate Intake (AI) rather than a specific daily amount of vitamin D. The AI for vitamin D is 200 International Units (IU) for adults under age 50, 400 IU for those 51 to 70, and 600 IU for those age 71 and above. As new studies continue to showcase vitamin D's potential benefits, more scientists are calling for increased recommendations. Some suggest as much as 10,000 IU-currently the tolerable upper intake-daily.

Late last year, a group of leading scientists published an editorial in the American Journal of Clinical Nutrition calling for an "urgent need" to increase the AI for vitamin D. Among them was Walter Willett, MD, the widely respected chairperson of the Harvard School of Public Health's department of nutrition. "The range we are talking about-1,000 IU per day-is still a small dose," Willett says. (Consider this: A fair-skinned person can manufacture 15,000 IU or more of vitamin D in as little as 30 minutes of optimal sun exposure.)

In response to the debate, the National Institutes of Health's Office of Dietary Supplements (ODS) last year began an intensive effort to learn more about vitamin D, partnering with other federal agencies to assemble a panel to assess research needs and priorities. Their efforts may result in a new AI when the Dietary Guidelines for Americans are revised in 2010.

"Scientists agree that as the DRIs are revisited, vitamin D is one of the first recommendations that should be reconsidered," says Patsy M. Brannon, PhD, professor of nutrition at Cornell University who coordinates the ODS's vitamin D initiative. "The current recommendation is a decade old. There's been a lot of research in the last 10 years. Whether there is sufficient strength of evidence to increase recommendations is where scientists disagree."

Until they reach a consensus, you have three options for obtaining vitamin D: food, sunlight, or supplements.