Ultraviolet B exposure triggers photosynthesis of vitamin D3 in the body. This form of vitamin D also is available through diet and supplements.

"This is the first study, to our knowledge, to show that higher serum levels of vitamin D are associated with reduced incidence rates of breast cancer worldwide," Cedric F. Garland said in a statement. "In general, breast cancer incidence was highest at the highest latitudes in both hemispheres."

The study, published in the May/June issue of The Breast Journal, said that even after controlling for known variables such as meat, vegetable and alcohol intake, cigarette consumption, weight, fertility and others, the inverse association of modeled vitamin D status with breast cancer incidence remained strong.

The authors caution that this was a study of aggregates, or countries, rather than individuals, and what might apply to aggregates may not apply to individuals.