Soon, the sun will be merely a rumor for long stretches during waking hours, and darkness will dominate our senses and moods.

As if that weren't depressing enough, here's another grim thought: You most likely will become significantly deficient in vitamin D.

Vitamin D, the so-called "sunshine vitamin," can be absorbed either through the skin via ultraviolet-B rays – by far the most efficient, plentiful source – or through pills or fortified foods.

Once thought only beneficial for preventing rickets and other bone-growth issues in children, vitamin D has been shown to help prevent heart disease, various forms of cancer, kidney disease, diabetes and a host of autoimmune conditions ranging from multiple sclerosis to arthritis. And, yes, studies have shown it helps people suffering from seasonal affective disorder.

Just two weeks ago, the American Academy of Pediatrics recommended doubling the amount of vitamin D children receive to 400 international units, though proponents consider those levels woefully inadequate.

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 an August 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 locally 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.