Vitamin D, Sun Exposure and Osteoporosis
The information given below is a summary of the consensus statements released by the Cancer Council of Australia and the NHS in the UK.
Why do We Need Vitamin D?
Vitamin D is a vitamin that is important for calcium metabolism and healthy bones. Vitamin D helps the body to absorb calcium from the intestine and then assists the body to incorporate the calcium into bones. A lack of vitamin D results in a reduced absorption of calcium and less calcium deposition in the bones. This causes rickets in children, who develop soft, bendy bones. In adults a lack of vitamin D leads to osteomalacia, where bones are weak and prone to fracture. 
Vitamin D is synthesised (made) in our skin from cholesterol molecules. Cholesterol that is exposed to ultraviolet (UV) light while passing through capillaries in the surface of our skin undergoes a change in shape. At this stage, the altered cholesterol molecule is not an active form of vitamin D. To become active, the pre-vitamin D molecule has to travel via the blood stream first to the liver and then to the kidney where further modifications are made. Active vitamin D can then circulate to perform its functions.
Vitamin D has other roles in the body besides calcium metabolism. It is important for cell growth, muscle strength and function of the immune system. A deficiency of vitamin D has recently been shown to increase ones risk of getting colon cancer. This is presumed to be a result of impaired immune function.
The only way to ensure that one has adequate vitamin D levels is either to get enough sun exposure or to take vitamin D supplements or a combination of both. The only accurate way of determining whether one is getting enough vitamin D is to measure the levels of vitamin D in one's blood. If one is deficient, then one can try to increase one's sun exposure or to take supplements. In countries with colder winters, most people need to take vitamin D supplements during winter to prevent deficiency. Studies of people living in Cape Town have found that most people become vitamin D deficient in winter.
How Much Sun Exposure is Needed to Get Enough Vitamin D to Prevent Osteoporosis?
In sunny countries, it is possible to get enough vitamin D from sun exposure alone. Clearly there is a balance between getting too much sun and increasing the risk of skin cancers and too little sun and not enough vitamin D. Fortunately, it has been shown that the average white person does not need a lot of sun to get enough vitamin D. Twenty minutes (10 minutes per side) three to five times a week, while exposing as much skin as possible usually suffices. This clearly needs to be done without wearing sunscreen because sunscreen blocks UV light. It is important to note that the sun exposure needs to occur during the hottest part of the day (i.e. midday). This goes against everything we have ever been told about safe sun behaviour, but it is only intense ultraviolet B from the midday sun that has enough energy to transform cholesterol and start the process of vitamin D production. 
The safest way to start the sun exposure process is to initially spend only a few minutes per side in the sun and to monitor what happens to one's skin. If, hours later, there is no pinkness or redness of the skin then one knows that one has had a safe dose of UV light. One can then gradually increase one's time of exposure until one is spending between 10 and 15 minutes per side in the sun. Those people who tan easily will be able to spend a little longer in the sun than those who tan poorly. On very hot days, it is probably wisest to stay indoors. Even with a mild to moderate tan, one can get burnt after only 5 minutes of sun exposure.
After a month of regular sun exposure, one can then test one's vitamin D levels. If the levels are still low and it is not practical to increase one’s sun exposure time (work commitments), then one needs to consider taking vitamin D supplements. The standard way of taking vitamin D is as a 50 000iu tablet either once or twice a week. Again, it will be necessary to check blood levels to know whether the dose is adequate.