It depends where you live. If you are white-skinned (see below) and live in sunny Cyprus, for instance, just 40 minutes a day:

• being outside, not sitting by a window (glass filters out the ultraviolet light the body needs in order to produce D3)
• baring as much of your arms and legs to the sun as you dare (and best not to wear tights)
• not using sun lotions or any cream, moisturisers, etc. (lotions and creams filter out ultraviolet light)
• not wearing sunglasses unless absolutely essential (again, glass filters out ultraviolet light)
• walking rather than driving (ditto) all year round

would probably be sufficient for your body to generate as much D3 as it needs, as would ten minutes a side of sunbathing during the four hours the sun is at its highest.

View Vitamin Research Products' vitamin D3 1,000iu supplement

If, however, you live north of Athens (Greece) or Malaga (Spain), or south of Melbourne (Australia) or Buenos Aires (Argentina) - i.e. further than roughly 37o latitude from the equator - not only would you have to expose your skin for a longer time - an hour walking in the sun or fifteen minutes a side sunbathing - but no matter how much of this you do this between October and April it will have little effect. The sun’s rays are simply too weak to trigger much vitamin D3 production. Although vitamin D3 produced from May to September is stored in the body’s fat, two thirds of Britons, for instance, are D3-deficient by the end of the Spring, and most babies in the UK are born D3-deficient and therefore at risk of rickets. [1] The deficiency is not usually so severe as to cause the bone-softening condition osteomalacia (rickets), but could damage bone as well as increase the risk of developing one of the wide variety of illnesses listed above.

N.B. A person with black skin appears to need six times as long, a person with a brown skin somewhere in between, to generate 20,000IU, which lends some support to the hypothesis that peoples who migrated to less sunny climes evolved lighter skins to maximise D3 production.

View Vitamin Research Products' vitamin D3 1,000iu supplement