A new report suggests that the sunshine vitamin could cut the risk of children developing asthma by up to 40 per cent.

Researchers from Harvard University have linked increasing rates of asthma and allergies with a lack of vitamin. They note that the rise in the two conditions has occured in parallel and say that there is evidence to suggest a causal relationship.

Vitamin D is unique in that it can be manufactured by the body, specifically the skin, when it is exposed to sunlight. It is widely thought that our decreasing exposure to the sun is to blame for the increasing number of people who are deficient in vitamin D.

The theory goes that over the past 50 years as people have become more prosperous, and entertainment technology such as video games, DVDs and the internet have developed, more time is spent indoors away from the sun. It is undoubtedly true that 50 years ago more people walked rather than travelling by car and people, especially kids, spent more time outdoors playing sports and doing other activities.

Another factor which blocks the synthesis of vitamin D by the skin, and one which most people won't think about, is the use of sun block. Besides preventing sun burn these products also completely block the production of vitamin D by the skin.

The Harvard researchers also say that on top of more limited sun exposure people aren't getting enough vitamin D from their diet either. It is recommended that people living in countries far away from the equator take vitamin D supplements in winter to avoid deficiency since production from sun exposure is limited here even in people who manage to spend time outdoors.

Their research indicates that pregnant women are particularly vulnerable to vitamin D deficiency and that when this occurs their children are more likely to develop asthma and allergies.

The researchers told the Daily Mail (London): "Our studies show that higher vitamin D intake by pregnant mothers reduces asthma risk by as much as 40 per cent in children aged three to five."

Their investigations have shown that low levels of vitamin D may affect the development of the lungs and immune system in the foetus, which may lead to a higher risk for the development of asthma and allergies.

The prevalence of asthma and allergies is notably higher in Westernised nations and those further away from the equator. The researchers believe that their theory therefore explains the reasons for this geographic distribution of cases.

They conclude: "We believe these patterns can be explained by a decrease in exposure to the sun and the limited sources of vitamin D in the diet to compensate for this decrease in sun exposure, leading to vitamin D deficiency.

"Providing adequate vitamin D supplementation in pregnancy may lead to significant decreases in asthma incidence in young children."

Along with the link to asthma and allergies, vitamin plays numerous other important roles in the body. It is required for the uptake and metabolism of calcium and is therefore important in the prevention of osteoporosis and associated bone conditions.

In another link with sun exposure and distance from the equator, research has shown that vitamin D is an effective treatment for seasonal affective disorder (SAD), a form of depression which occurs mainly in winter and is associated with a lack of sunlight.

Finally, vitamin D is being recognised as an important factor in cancer prevention.

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