A growing body of evidence in recent years has shown that lack of vitamin D may have lethal effects. Heart disease, lung disease, cancer, diabetes, high blood pressure, schizophrenia and multiple sclerosis are among the conditions in which it is believed to play a vital role. The vitamin is also essential for bone health and protects against rickets in children and osteoporosis in the elderly.
Vitamin D is made by the action of sunlight on the skin, which accounts for 90 per cent of the body's supply. But the increasing use of sunscreens and the reduced time spent outdoors, especially by children, has contributed to what many scientists believe is an increasing problem of vitamin D deficiency.
After assessing almost every scientific paper published on the link between vitamin D and cancer since the 1960s, US scientists say that a daily dose of 1,000 international units (25 micrograms) is needed to maintain health. "The high prevalence of vitamin D deficiency combined with the discovery of increased risks of certain types of cancer in those who are deficient, suggest that vitamin D deficiency may account for several thousand premature deaths from colon, breast, ovarian and other cancers annually," they say in the online version of the American Journal of Public Health.
The dose they propose of 1,000IU a day is two-and-a-half times the current recommended level in the US. In the UK, there is no official recommended dose but grey skies and short days from October to March mean 60 per cent of the population has inadequate blood levels by the end of winter.
The UK Food Standards Agency maintains that most people should be able to get all the vitamin D they need from their diet and "by getting a little sun". But the vitamin can only be stored in the body for 60 days.
High rates of heart disease in Scotland have been blamed on the weak sunlight and short summers in the north, leading to low levels of vitamin D. Differences in sunlight may also explain the higher rates of heart disease in England compared with southern Europe. Some experts believe the health benefits of the Mediterranean diet may have as much to do with the sun there as with the regional food.
Countries around the world have begun to modify their warnings about the dangers of sunbathing, as a result of the growing research on vitamin D. The Association of Cancer Councils of Australia acknowledged this year for the first time that some exposure to the sun was healthy.
Australia is one of the world's sunniest countries and has among the highest rates of skin cancer. For three decades it has preached sun avoidance with its "slip, slap, slop" campaign to cover up and use sunscreen. But in a statement in March, the association said: "A balance is required between avoiding an increase in the risk of skin cancer and achieving enough ultraviolet radiation exposure to achieve adequate vitamin D levels." Bruce Armstrong, the professor of public health at Sydney University, said: "It is a revolution."
In the latest study, cancer specialists from the University of San Diego, California, led by Professor Cedric Garland, reviewed 63 scientific papers on the link between vitamin D and cancer published between 1966 and 2004. People living in the north-eastern US, where it is less sunny, and African Americans with darker skins were more likely to be deficient, researchers found. They also had higher cancer rates.
The researchers say their finding could explain why black Americans die sooner from cancer than whites, even after allowing for differences in income and access to care.
Professor Garland said: "A preponderance of evidence from the best observational studies... has led to the conclusion that public health action is needed. Primary prevention of these cancers has been largely neglected, but we now have proof that the incidence of colon, breast and ovarian cancer can be reduced dramatically by increasing the public's intake of vitamin D." Obtaining the necessary level of vitamin D from diet alone would be difficult and sun exposure carries a risk of triggering skin cancer. "The easiest and most reliable way of getting the appropriate amount is from food and a daily supplement," they say.
The cost of a vitamin D supplement is about 4p a day. The UK Food Standards Agency said that taking Vitamin D supplements of up to 1,000IU was " unlikely to cause harm".
What it can do
Heart disease
Vitamin D works by lowering insulin resistance, which is one of the major factors leading to heart disease.
Lung disease
Lung tissue undergoes repair and "remodelling" in life and, since vitamin D influences the growth of a variety of cell types, it may play a role in this lung repair process.
Cancers (breast, colon, ovary, prostate)
Vitamin D is believed to play an important role in regulating the production of cells, a control that is missing in cancer. It has a protective effect against certain cancers by preventing overproduction of cells.
Diabetes
In type 1 diabetes the immune system destroys its own cells. Vitamin D is believed to act as an immunosuppressant. Researchers believe it may prevent an overly aggressive response from the immune system.
High blood pressure
Vitamin D is used by the parathyroid glands that sit on the thyroid gland in the neck. These secrete a hormone that regulates the body's calcium levels. Calcium, in turn, helps to regulate blood pressure, although the mechanism is not yet completely understood.
Schizophrenia
The chance of developing schizophrenia could be linked to how sunny it was in the months before birth. A lack of sunlight can lead to vitamin D deficiency, which scientists believe could alter the growth of a child's brain in the womb.
Multiple sclerosis
Lack of vitamin D leads to limited production of 1.25-dihydroxyvitamin D3, the hormonal form of vitamin D3 which regulates the immune system, creating a risk for MS.
Rickets and osteoporosis
The vitamin strengthens bones, protecting against childhood rickets and osteoporosis in the elderly.
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