Having your lunch break outside, when a few rays of light appear, or taking a short stroll in the morning are not just fun but actually really productive.

First of all, it boosts your mood. Secondly, this exposure to sun light helps you to produce an essential substance- vitamin D.

Vitamin D, which the body produces when exposed to sun, is important for the metabolism of calcium and phosphate. Vitamin D deficiency may contribute to osteoporosis by reducing the amount of calcium absorption. That means you need vitamin D to make your bones strong and durable.

In addition, Vitamin D affects the immune system by promoting immunosuppression and anti-tumor activity.

A new study, recently presented to the British Endocrine Society conference, has found that low levels of the vitamin could be linked to the development of auto-immune disorders such as diabetes and thyroid disease.

Last year, scientists at King's College London found that vitamin D could help asthma sufferers who do not respond to normal treatment. It improved their responsiveness to their steroid medication.

People living in northern countries, such as UK often lack sufficient amounts of vitamin D, mainly due to low levels of exposure to sunshine. Pregnant women are particularly liable to develop vitamin D deficiency.

Sun exposure is perhaps the most important source of vitamin D because exposure to sunlight provides most humans with their vitamin D requirement, 15 minutes twice a week with only your arms and face exposed to the sun are sufficient to produce the required amount of vitamin D.

The vitamin is found in salmon, tuna and other oily fish, and is added to milk. One cup of vitamin D fortified milk supplies one-half of the recommended daily intake for adults between the ages of 19 and 50.

It is also available in supplements.