Most famous for making bones and teeth strong by helping the body to effectively use calcium and phosphorous, vitamin D has another notable talent. Research shows that vitamin D protects against and fights breast cancer. It helps to make your breast cells more resistant to toxins, decreases the ability of breast cells to divide, stops tumor cells from growing, causes the death of tumor cells, prevents new blood vessels from growing into a tumor, and boosts the immune system, especially the activity of Natural Killer (NK) cells.

Vitamin D is unique because your body can make its own supply. The secret catalyzing agent is not from this world; it comes from a star—the sun. Sunlight reacts with chemicals in your skin to produce vitamin D. Just fifteen minutes of sunlight a day makes enough vitamin D to reduce your risk of breast cancer by as much as 40 percent. Of course, too much sunlight isn't a good thing, because the ultraviolet radiation in sunlight damages the DNA in skin cells. If you get too much sun, especially if you have lightly pigmented skin, the damage can be severe. Serious ultraviolet-radiation damage to your skin can cause premature aging, leathery skin, deep wrinkles, discolored spots, and potentially deadly skin cancer.

But a little sunlight is important to enable you to make enough health-promoting and -protecting vitamin D. Fifteen minutes in the early morning or late afternoon—when the suns rays aren't so intense—is ideal. Combine it with a brisk walk, and you double your benefits. Research shows that regular aerobic exercise can lower your risk of breast cancer by 30-50%.

If you live in a climate that doesn't see much sun, especially during the cold winter months, taking supplemental vitamin D is a must. Fatty fish (for example, salmon and mackerel) are about the only foods with natural vitamin D. Most of the vitamin D in our diet comes from foods that are fortified with it, for example, certain dairy products and breakfast cereals. Most multivitamins have the daily recommended amount of vitamin D in them, about 200–400 international units (IU).