Vitamin D - talk to your doctor about whether you are getting enough. Many studies (like the ones cited in this article) suggest that the sunshine vitamin can be a powerful tool in extending your life and warding off diseases like breast cancer.
1,000 IU of vitamin D is the daily dosage recommended by the Canadian Cancer Society. During the spring and summer, that can be accomplished through normal daily exposure to the sun. Some advise that as little as 10 to 15 minutes of sun exposure a day twice a week will allow the body to generate sufficient levels of the vitamin. In the fall and winter months, especially here in Canada, a vitamin D supplement may be necessary. You could also increase your consumption of oily fish or fortified foods.
But don't overdo it. 2,000 IU is the daily maximum tolerable amount as established by the U.S.-based Institute of Medicine of the National Academies. Exceeding it could be very dangerous. Health Canada warns people that taking too much can lead to kidney stones as well as damage to the heart, lungs and blood vessels.