Vitamin D has long been known to be crucial to bone growth, mineral metabolism, and calcium absorption. But in recent years, a deficiency of the nutrient has been linked to a surprising number of other health conditions.

The latest news, in today's headlines, suggests that Vitamin D may help prevent multiple sclerosis and rheumatoid arthritis. (Read the full story.)

Here's a look at some recent studies that have suggested vitamin D may be more important to preventing disease than we thought.

(An interesting side note: Vitamin D is actually not a vitamin at all but rather the precursor of a steroid hormone. It's found in many animal products, such as milk products and fish oil. But the best source is sunlight, when our bodies can create the steroid ourselves.)

Diabetes

Some research has linked vitamin D to a reduced risk of developing Type 1 or juvenile diabetes.

The largest study to note the link came from Finland and was published in the journal Lancet. The study followed more than 12,000 babies born in 1966 until 1997. It found those who were given the recommended amounts of vitamin D supplement had an 80 per cent reduced risk of developing diabetes. Those who had rickets -- linked to vitamin D deficiency -- in the first year of life had a three-fold risk of developing diabetes.

Finland has the highest reported incidence of type 1 diabetes in the world. In north Finland, there are only two hours of sunlight -- a key source of vitamin D -- every day in December.

Muscle pain

Deficiency of the vitamin may also be a major cause of unexplained muscle and bone pain.

Researchers at the University of Minnesota Medical School conducted a study involving 150 children and adults with unexplained muscle and bone pain. Almost 94 per cent were found to be vitamin D deficient. Of these, 55 per cent were severely deficient. Five patients unexpectedly had no vitamin D in their system at all.

The researchers concluded that patients with persistent, non-specific musculoskeletal pain should be screened for vitamin D deficiency.

Cancer

Research has also suggested an association between vitamin D and a lowered risk for a number of cancers. The thinking is that vitamin D could keep a cancer cell from multiplying out of control.

A study from the U.S. Department of Veterans Affairs Medical Center, published in the Journal of the American Medical Association, showed a link between consumption of vitamin D-containing foods and a reduced risk for polyps in the colon. The study of more than 3,100 veterans found that a diet rich in vitamin D appears to protect people from developing the potentially cancerous growths in the colon.

Patients who consumed high amounts of vitamin D fish were 40 per cent less likely to develop polyps than those who got little or no vitamin D.

Another study found that giving cancer patients vitamin D could help radiation work more effectively and could help wipe out cancer cells altogether.

Researchers from Dartmouth Medical School conducted tests on mice and found that combining a form of vitamin D with radiation allowed stubborn cancer cells to be destroyed. In those given the combined treatment, tumours were around 50 per cent smaller than in those that received radiation alone.

Heart disease

Vitamin D supplements may also help to reduce the risk of heart disease.

Researchers from the University of California at San Francisco studied 9,704 women aged 65 and older. About half were taking vitamin D supplements. They found that the women who used vitamin D supplements had a 31 per cent lower risk of heart disease death than those who did not

Vitamin D is an important regulator of calcium absorption in the body. Atherosclerosis, the accumulation of cholesterol and fat in the walls of arteries, is often associated with calcification, the build up of the mineral calcium in the arteries. It is thought that low levels of vitamin D in the blood are associated with an acceleration of this build up.

Another study made similar findings. Researchers from University of Bonn, in co-operation with the Bad Oeynhausen Heart Centre, found that levels of the vitamin are considerably lower in the blood of patients with chronic heart failure, a fatal condition that results from the failure of the heart to pump blood through the body properly.

They compared 54 patients with chronic heart failure with 34 healthy people, and found that vitamin D levels were up to 50 per cent lower in the blood of CHF patients.

Osteoporosis

Vitamin D deficiency has also been linked to osteoporosis. That's because the body can't adequately absorb calcium, which is needed for strong, healthy bones that resist fracture, without vitamin D.

Researchers at Britain's University of Cambridge School of Clinical Medicine conducted a study, published in the British Medical Journal, that found that vitamin D supplements can cut the risk of bone fractures in people over 65. They found that in those given a high-dose, slow-release vitamin D capsule four times a year, the risk of fracture at common sites, such as the hip, wrist, forearms or back, was reduced by 33 percent with vitamin D supplementation.

Schizophrenia

The chance of developing schizophrenia may be directly linked to how sunny it was in the months before a person's birth, research suggests.

A lack of sunlight can lead to vitamin D deficiency, which scientists believe could alter the development of a child's brain in the womb. according to an article in the New Scientist in 2002, research suggests people who develop schizophrenia in Europe and North America are more likely to be born in the spring.

A psychiatrist at the Queensland Centre of Schizophrenia Research in Brisbane, Australia, made similar findings, suggesting a lack of UV light during pregnancy tips the balance towards schizophrenia in genetically susceptible people.

Rickets

Nutritional rickets, a vitamin D deficiency, was once common in North America but is now thought to have been eliminated. Yet, according to a study published in the Journal of Pediatrics, nutritional rickets has been found in a number of African-American infants in the U.S.

All of the patients were African-American children who had been breastfed without receiving vitamin D supplementation. It's thought that black children may be at particular risk since their skin does not absorb as much sunlight as other races.

The American Academy of Pediatrics recommends that exclusively breastfed infants over two months of age should receive vitamin supplements until they begin taking at least 17 ounces daily of vitamin D-fortified milk. The academy recommends multivitamins containing 200 IU of vitamin D, available as over-the-counter liquid drops.

Vitamin D and the Sun

Dr. Michael Holick, a Boston University professor has been a big proponent of vitamin D for years and even runs a lab dedicated to research on the nutrient.

He has courted controversy by suggesting that to prevent vitamin D deficiency, one should spend time in the sun every day without wearing sunscreen, which would block absorption. Dermatologists have complained that promoting exposure to the sun without sunscreen will increase the risk of skin cancer.

But Holick insists that limited unprotected sun exposure during sunny months -- five to 15 minutes a day -- and then applying sunscreen on exposed skin is an ideal compromise. People with darker skins need to spend longer in the sun to get the necessary amount of vitamin D, since the pigmentation in their skins blocks UV absorption.

Holick believes most people need about 1000 IU of vitamin D each day. The recommended dietary allowance (RDA) for vitamin D varies with age, sex, but in general is 200-600 IU per day.

Although it is possible to get vitamin D through foods or supplements, Holick says it is not enough and recommends more sun exposure.

It's been suggested that boosting D in the winter might be a good way to beat the blahs and help counter seasonal depression.