It's a key vitamin that has anti-cancer properties, and it's also been linked to a range of other cancers, including colon, bladder, prostate and colorectal.
But we have to get out into the sun to top up our levels. Only a small amount of what we need comes from food and supplements, say researchers from Zhejiang University, in Hangzhou, China.
They say their research is the closest yet to real life as it encompasses so many previously published studies. In all, they too another look at 20 studies which included more than 35,000 breast cancer patients whose levels of the vitamin in their blood were compared with those from healthy women.
They reckon that women with the lowest levels were 97 percent more likely to develop breast cancer compared with those with the highest levels of the vitamin. Every additional 5 nmol/l of the vitamin in the blood was associated with a 6 percent drop in breast cancer risk.
The evidence wasn't there to suggest the vitamin could prevent cancer, however—although it would seem to follow that it could, the researchers said.