New findings about the vitamin's benefits come at a time when surveys show that many Americans don't get enough - with how much is "enough" varying widely among different populations. A Vitamin D primer:

Benefits of healthy levels

Recent evidence suggests possible protection against:

Heart disease.

Various cancers.

Immune system disorders such as multiple sclerosis and rheumatoid arthritis.

Infectious diseases like tuberculosis and the flu.

Mental illness including depression and schizophrenia.

Harms of inadequate levels

Long known as vital for bones, with severe deficiencies leading to rickets in children (osteomalacia in adults). Newer evidence suggests low levels might also make:

Men more likely to have heart attacks.

Colon and breast cancer victims less likely to survive.

Children more likely to develop diabetes.

Who is at risk

Modern living has reduced exposure to ultraviolet radiation that allows the skin to produce most of the "sunshine vitamin" we need. Americans now get more of it from fortified milk. At greater risk are people who:

Are exposed to less sun because they have dark skin, which blocks UV rays; live in northern latitudes; wear clothing that covers virtually all of their skin; stay indoors; or apply sunscreen liberally, protecting them against skin damage but cutting Vitamin D production.

Are elderly, when thinning skin reduces production.

Don't get supplemental Vitamin D, which is intended to make up for insufficient sun exposure. Most people get this from Vitamin D-fortified milk or infant formula; breastfed babies need Vitamin D drops.

Recommendations for all ages

Ask your doctor for a 25OHD test and discuss the level of Vitamin D found in light of recent research.

Many doctors, including policymakers at the American Academy of Pediatrics* (AAP), recommend supplemental doses to raise total Vitamin D taken in from all sources far higher than the following guidelines from the federal Food and Nutrition Board (FNB):

Age                      Adequate Intake (FNB)        (AAP)*

Birth through 18 years            200 IU            400 IU

19 to 50   years                        200 IU

51 to 70 years                          400 IU

71 years and older                  600 IU

* The new AAP guidelines, to be published in the fall, say 400 IU supplements should be given to all infants - breastfed or not - who injest less than a half-liter a day of Vitamin D-fortified milk or formula, and to all children and adolescents who do not get 400 IU of Vitamin D from a multivitamin or a half-liter of fortified milk.