Some scientific experts have gone so far as to call it a "super nutrient," eliciting skepticism from others who remember that appellation being given to vitamins C and E and beta carotene before extensive research reversed the thinking.
Count the National Institutes of Health's Office of Dietary Supplements among those still holding back judgment. In a position statement, the agency said vitamin D helps bone health in elderly men and women, but "for other age groups and health issues, it is too early to say conclusively whether more vitamin D may be beneficial."
Conversely, you can count the American Medical Association and the World Health Organization in asking for updated guidelines and saying that a significant percentage of American adults are vitamin D deficient.
Shedding some light is research from the U.S. Department of Agriculture's Western Human Research Center at the University of California, Davis, run by Charles Stephensen.
Stephensen, along with graduate student Laura Hall, recently presented findings on what is believed to be the first study looking at how much vitamin D that healthy adults, ranging in age from 20 to the mid-30s, receive from varying exposure to the sun during the four seasons.
The research followed 72 subjects in Davis split into four groups based on levels of sun exposure and degrees of skin pigmentation, Stephensen says.
"People at the high end of the (sun exposure) spectrum, such as student athletes or people training for triathlons were getting 1 1/2 hours of sun a day. People at the low end were those who were going back and forth to class, maybe (exposed to the sun) 20 minutes a day."
Though still analyzing data, Stephensen reports that only one of the four groups retains enough vitamin D during winter without supplements - the group with European ancestry (light skin) and high sun exposure.
Those who had high sun exposure and an African ancestry (darker skin) needed an additional 800 international units of vitamin D to reach the recommended daily amount.
Those with low exposure and lighter pigmentation needed an additional 600 to 1,000 units daily in the winter and spring, and low-exposure, dark-skinned subjects needed 1,000 to 2,000 units no matter the season.
"Part of what gets you through the winter," Stephensen says, "is that you have a lag time with vitamin D. So the exposure you get in the summer and fall carries over and becomes active in the winter."
The study, presented in April at the American Society of Nutrition conference in San Diego, suggests that during winter months, almost everyone needs to augment their intake of vitamin D.
Yet scientists debate just how much is needed daily. Leading proponents say the federal recommended daily intake of 200 to 600 units is too low. The National Academy of Sciences sets the upper limit at 2,000 units daily; the Canadian Cancer Society, 1,000 units.
"I personally take 4,000 units a day and have for years," says Dr. Bruce Hollis, professor of pediatrics, biochemistry and molecular biology at the Medical University of South Carolina.
Hollis, a leading vitamin D researcher, recently presented data to a panel that included the NIH and the Food and Drug Administration that was studying whether to raise daily allowances. "To set those levels at 200 or 400 units does nothing to help you," Hollis says. "Basically, it's enough so you don't get rickets. But it doesn't begin to address how (vitamin D) has been implicated in cancer, heart disease and autoimmune (prevention)."
But Paul M. Coates, director of the NIH's office of dietary supplements, sounded a note of caution in a written statement: "It's tempting to think that an essential nutrient is safe at any level - that if some is good, more is better. We've learned that this isn't always true, and there are potential harms associated with high levels of many nutrients."