There is now increasing evidence that Vitamin-D prevents the development of both breast and colon cancer. Two studies looked at Vitamin-D levels in healthy people and then followed them up for up to 25 years.
In the Breast cancer study 1760 patients were studied and those with the lowest levels of Vitamin-D were more likely to develop breast cancer. They were able to calculate that by taking in 2000 international units of Vitamin-D per day was able to give you a 50% reduction in developing breast cancer. To obtain the 2000 units of Vitamin-D / day they calculated you would need to spend at least 15 minutes in the sun / day. To ingest this quantity of Vitamin-D / day was difficult as there are only 400 units in a pint of milk although oily fish has a high content. It was suggested that you may need to take vitamin-D supplements to reach this level of intake if you were not able to get out in the sun. | 
