British people - and others living in the Northern Hemisphere - need five to 30 minutes of sunlight on bare skin a few times a week to generate enough Vitamin D to prevent against diseases like cancer.
Those who had chronically-low levels of the vitamin have 30 per cent more mortality rates and are also 40 per cent more likely to get tumorous growths, a research of 96,000 people shows.
The study - which followed Danish people for 40 years to take blood samples and track their lifestyle and diet - shows that, in a country where the sun sets as early as 3.30pm in December, most people get one fifth of their Vitamin D from food and four fifths from direct sunlight.
Decreased levels of Vitamin D can also contribute to risks of heart disease, diabetes, depression and bone pain, the National Health Service says.
As the sun is not at its peak during the winter, those low in the vitamin are advised to eat Vitamin D-packed foods such as omega-3 rich fish, milk and eggs and potentially take supplements that are available from chemists or with a doctor's prescription for higher doses.
Researchers at Copenhagen University Hospital said, although additional Vitamin D is proven to be beneficial from cold months October to March, it is not yet known which way is best to produce or administer it.
Prof Børge Nordestgaard, Chief Physician at Copenhagen University Hospital, said: “Our study shows that low vitamin D levels do result in higher mortality rates, but the best way of increasing vitamin D levels in the population remains unclear.
“We still need to establish the amount of vitamin D to be added, as well as how and when it is most effective: Should we get vitamin D from the sun, through our diet or as vitamin supplements?
“And should it be added in the foetal stage via the mother, during childhood or when we have reached adulthood?”
The NHS also recommends a few minutes' exposure to UVB rays in the middle of each day to build up Vitamin D.
The study was published in the British Medical Journal.
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