While lots of folks think that milk and cheese are our main sources of vitamin D, we actually get most of our vitamin D through skin absorption of UVB rays in sunlight.
Vitamin D has several important functions in the body. Everybody associates vitamin D with bone health. It does help bone growth and repair, but also strengthens muscles. Most of its beneficial actions are related to calcium and phosphate. Vitamin D improves calcium absorption from the intestine.
Many studies have evaluated the effect of vitamin D with or without calcium supplementation on the risk of various common cancers. It’s tempting to believe that vitamin D would decrease cancer risk. It sounds healthy and clearly has benefits for our bodies. However, what seems like it should be true is not always the case.
There appears to be a protective effect of vitamin D against colorectal cancer, though studies are conflicting. The benefit of vitamin D supplementation on breast cancer remains unproven. A recent study shows weak evidence that it decreases melanoma risk, but only in women who had a prior history of nonmelanoma skin cancer. Evidence for risk reduction of other major cancers is weak or nonexistent.
The bottom line: vitamin D is important for our overall health, but it’s not a key player in reducing the risk of most common cancers. Get it naturally in small doses from the sun. There’s little or no need for most people to supplement.