It may not only be the lack of vitamin D that increases a woman's breast cancer risk but also the way in which the body utilises it, say researchers.
Milk is a source of vitamin D
Studies have shown vitamin D protects against breast cancer and a lack may contribute to the disease.
Now scientists have found women with certain versions of a gene involved in the vitamin's breakdown have a nearly twofold greater risk of breast cancer.
The St George's Hospital findings appear in Clinical Cancer Research.
Dr Michelle Guy and her team, funded by Breast Cancer Campaign and World Cancer Research Fund, looked at the vitamin D receptor which controls the action of vitamin D in the body.
A gene - the vitamin D receptor (VDR) gene - carries the blueprint for this receptor.
The researchers looked at the VDR gene of 398 women with breast cancer and 427 women without breast cancer.
The women with breast cancer were significantly more likely to have a certain versions of the gene than the cancer-free women, they found.
These certain versions increased breast cancer risk nearly twofold.
Women with these versions may also have a more aggressive form of the disease if the cancer spreads, the researchers believe.
They said the findings might provide a new way of helping to predict women's risk of developing breast cancer or of it spreading by looking at the different versions of the vitamin D receptor gene.
In turn, this would help cancer specialists to better plan an individual's treatment at diagnosis.
Dr Guy said: "While it is known that 5-10% of breast cancer cases are due to a genetic predisposition associated with well-characterised genes, like BRCA1, the underlying causes of the majority of all other breast cancers remain a mystery.
"We hope that by showing that natural variations in the vitamin D receptor gene can increase susceptibility to breast cancer, we are starting to unravel how breast cancer might develop in women who have no family history of the disease."
Pamela Goldberg, chief executive of Breast Cancer Campaign said: "This research could provide a real step forward in the future of treatment for breast cancer, which will be in risk assessment and drug regimes tailored to the individual patient.
"For patients with breast cancer the development of a test to determine how the disease will progress could provide real benefits by improving the way in which their disease is treated and managed."
Professor Martin Wiseman from World Cancer Research Fund said: "This is an important study which begins to address questions about how diet and lifestyle interact with genetic factors to influence cancer risk, and why different people respond in different ways."
Antonia Bunnin, Director of Policy and Campaigns at Breakthrough Breast Cancer, said: "Further research is needed before any firm conclusions about the role of vitamin D in breast cancer prevention can be established."
Meanwhile, researchers at Aberdeen University have found looking at a woman's genes could help to predict whether she will respond to chemotherapy treatment for breast cancer.
Dr Andrew Schofield and colleagues found specific changes in a gene called p27 made breast cancer cells resistant to the drug docetaxel.
The findings appear in Breast Cancer Research.