''For those of us in nutritional medicine, the whole story is vitamin D is no longer just about bone health,'' says Dr. Joel Evans, founder and director of The Center for Women's Health in Darien, Conn., and co-author of the upcoming ''The Whole Pregnancy Handbook'' (Gotham Books, $20). ''It is important in gum health, it is important with inflammation, which is so important because it's a common denominator in diseases that are so disparate. … It clearly acts on the immune system, a major component of any inflammatory disease.''

Says Dr. Richard Cooper, a licensed naturopathic physician in New Canaan, Conn.: ''We're starting to get a better understanding of its full effect on the body. A lot of different conditions are associated with a sub-level of vitamin D in the body.''

There is increasing evidence that people who live farther from the equator, and therefore get less sunlight, experience a greater rate of incidence of certain maladies, including multiple sclerosis and colon cancer. Research also suggests that the vitamin plays a role in warding off breast, prostate and ovarian cancer. For Northeasterners, this is a particularly tricky time of year, when the body has used up all the nutrient it produced and stored in the summer.

Yet getting enough vitamin D naturally can pose a quandary. The body produces vitamin D naturally when exposed to sunlight. Yet exposure to the sun's rays produces its own set of problems, including skin cancer.

''There's some thought that this is a balancing act,'' Cooper says. ''On the down side, there is increased risk of certain types of skin cancer, but if you avoid the sun totally, there is the risk of being deficient in vitamin D. The fact of the matter is when you use a lot of sunscreen, you decrease the amount of vitamin D … . How much sun exposure should be based on what your risks are.''

Experts agree that you don't need more than 20 minutes of sunlight a day to produce high levels of vitamin D, but they say you should check with a dermatologist to be sure that is safe for you. Sun might be the best source of the vitamin, but it's not the only one. It can also be found in fatty fish such as wild salmon, herring and sardines; cod liver oil; fortified milk; and, to a lesser extent, egg yolks and Cheddar cheese.

Vitamin dosage is measured in International Units. Recommended dietary allowances currently call for between 200 and 600 IU of the vitamin daily (it increases with age), but doctors who advocate the vitamin push for up to five times those amounts, and some experts advise even more.

''The biggest factor is that the governing bodies that come up with recommended dietary intake purely base it on a deficiency disease to prevent things like rickets,'' Evans says. ''That's different from optimal health. That's the concept that hasn't made it into mainstream life. … There's a huge gap between what you need to prevent rickets and what you need for a healthy level. The goal is to have a good level in the blood to prevent [things such as] cancer and arthritis.'' Most multivitamins contain 400 IU of vitamin D. A tablespoon of cod liver oil contains 1,360 IU; 3 ounces of herring has 765; and an egg yolk, 25.

Suggests Erica Christ, a registered dietitian with the Healthy Living Center at Greenwich Hospital in Greenwich, Conn.: ''Go for the food first, and if your diet is compromised, then go for the multi [vitamin]. It can be a buffer. I personally feel one vitamin isn't a cure-all for a pure diet. A lot of people hear cure-all, and I think that's the wrong message. I think people should get vitamin D, as well as other vitamins, through diet to help prevent disease.''

There is a risk of overdoing vitamin intake, and anyone who is taking near 2,000 IU or more a day of vitamin D should work with a physician to monitor blood levels.

''Too much calcium in the blood can cause certain body tissue to calcify,'' says Peter McKnight, chief clinical dietitian at Stamford Hospital in Stamford, Conn. ''We don't want our heart muscle to calcify.''

''Anybody that would be looking to explore this more should be doing so with someone who knows what tests need to be run,'' Cooper advises.

There are also some who believe that vitamin D isn't actually a vitamin, because it's produced naturally through sun exposure.

''Some scientists say that it's a hormone and that you can get enough with sunshine and that we make our own,'' McKnight says.

Semantics aside, vitamin D is also seen as a good defense against Seasonal Affective Disorder, or ''the winter blues''; symptoms can range from being in a funk to full-on depression.

Proponents of the vitamin are optimistic the public will soon be made aware of its benefits. Because much of the research is fairly new and findings have been published over the past few years, some results are inconclusive and national medical groups are generally slow in recognizing natural solutions.

''I would say that this data is all within the last couple of years,'' Evans says. ''It's been within the nutrition literature but not within mainstream medical literature. It's only the most forward-thinking physicians that are aware of this stuff.''

Ray Hogan is a writer for the Los Angeles Times, a Tribune Publishing newspaper.

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