What's new: The risk of breast cancer is apparently higher in populations living far from the equator than it is for those in the sunny tropics.
The finding: Researchers at UC San Diego reported in the recent issue of the Breast Journal that in countries where people are exposed to high levels of ultraviolet B (UVB) radiation from the sun, breast cancer rates are lower than in countries where UVB levels are low. That is, the more sunlight to which a population is exposed, the lower its breast cancer rates.
The researchers also found that breast cancer rates increase as blood levels of vitamin D decrease. Further, breast cancer rates are higher, they found, in places where people get more of their calories from meat and dairy, as opposed to vegetables and grains.
How the study was done: The San Diego team obtained 107 countries' cancer figures from a World Health Organization cancer database, information related to ultraviolet radiation from NASA and data on diet from the United Nations' Food and Agriculture Organization. The researchers then plotted breast cancer rates against latitude, UVB radiation, blood levels of vitamin D, cloud cover, cigarette and alcohol consumption, fertility and diet (among other variables).
The resulting graphs showed that as latitude, or distance from the equator, increased, so did the total number of new breast cancer cases. Breast cancer incidence also increased as the fraction of dietary calories obtained from meat and dairy increased.
Why it matters: The current findings support previous studies that have demonstrated links between breast cancer and vitamin D levels and sun exposure -- but it's the first to demonstrate these relationships on a global scale.
Across the globe, more than 1.1 million cases of breast cancer occur each year, making it the most common cancer in women. Researchers also have begun to demonstrate that vitamin D deficiency is prevalent in many countries, including several at relatively low latitudes. The current study provides further evidence that vitamin D may come to play an important role in breast cancer prevention worldwide. The findings suggest that cutting back on meat and dairy may help reduce breast cancer risk too, but the bulk of studies on this correlation have so far produced mixed results.
What we still don't know: The findings apply to countries -- not individual people -- which means there's a chance that the pattern won't hold up in studies of individuals.
Also, though scientists have proposed several hypotheses as to how vitamin D may help prevent breast cancer, and how animal foods may promote it, none has been proved. And it's still not entirely clear whether taking vitamin D supplements orally has the same effect as getting a vitamin D-inducing dose of sunshine.