Cedric F. Garland, a cancer prevention specialist at the Moores Cancer Center at the University of California, San Diego and colleagues estimate that 250,000 cases of colorectal cancer and 350,000 cases of breast cancer could be prevented worldwide by increasing intake of vitamin D -- particularly in countries north of the equator.

The researchers combined data from surveys of serum vitamin D levels during winter from 15 countries using satellite measurements of sunshine and cloud cover in countries where actual blood serum levels of vitamin D had also been determined. The data were then applied to 177 countries to estimate the average serum level of a vitamin D metabolite.

The study, published in the journal Nutrition Reviews, recommends 2000 IU/day of vitamin D for a meaningful reduction in colorectal cancer or 2000 IU/day, plus, when weather allows, a few minutes in the sun with at least 40 percent of the skin exposed, for a meaningful reduction in breast cancer incidence, unless the individual has a history of skin cancer or a photosensitivity disease.