Women who live south of Coffs Harbour or below 30 degrees latitude in Australia are twice as likely to develop breast cancer than those living in the north, a study shows.
Researchers at the Westmead Breast Cancer Institute found women living at latitudes below 30 degrees - including cooler states like Victoria, parts of NSW, Western Australia and South Australia - are more likely to develop the disease than women living in the north.
The study, to be presented at a Sydney conference today, also analysed the geographical incidence of skin cancer and found the opposite effect.
The lead investigator of the study, clinical dietician Kellie Bilinski, said researchers believed the link between latitude and breast cancer might be down to the reduced potential to synthesise vitamin D from sunlight at lower latitudes.
The decreased risk of melanoma, which was linked to ultraviolet radiation from too much sun exposure, supported the theory, she said.
"This supports our hypothesis that the potential for sunlight exposure and, therefore, vitamin D synthesis, is lower the further south you go," Ms Bilinski said.
A similar link with vitamin D has been found in multiple sclerosis sufferers, with the disease far more common in Australia's south.
Earlier studies had linked low vitamin D levels to an increased risk of breast cancer, while studies in animals with low vitamin D had shown tumour cells grew faster, Ms Bilinski said.
She said it might be time to adjust the SunSmart messages to ensure women had enough sunlight at appropriate times of the day.
Investigators are now recruiting almost 600 women diagnosed with early breast cancer to examine the relationship between their vitamin D status and tumour growth, Ms Bilinski said.
Victorian breast cancer survivor Jo Lovelock, 54, was initially diagnosed with a malignant tumour in 2003 that was successfully treated, but recently discovered she was vitamin D deficient.
Ms Lovelock said checking vitamin D levels could be part of women's overall health strategy, along with diet, exercise, minimising alcohol consumption and not smoking.
However, she suspects vitamin D may be just one part of the breast cancer puzzle.
"It's probably only one of many factors," she said.
The research will be presented on Thursday at the Sydney International Breast Cancer Congress.
To check your vitamin D levels use an online calculator.