Women living south of 30 degrees latitude have been found in an Australian study to have almost double the risk of breast cancer, compared to the risk for women living further north.
That would place all New Zealand in the higher risk zone, along with most of New South Wales, Adelaide and Perth. North Cape, the most northerly point on the New Zealand mainland is below 34 degrees south.
The Westmead Breast Cancer Institute study also analysed the geographical incidence of skin cancer and found the opposite effect.
The lead investigator of the study, clinical dietitian Kellie Bilinski, said that the researchers believed the link between latitude and breast cancer might be attributed 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.''
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, Bilinski said.
She said it might be time to adjust sun exposure messages to ensure women had enough sunlight at the appropriate times of the day.
Investigators are now recruiting almost 600 women diagnosed with early-stage breast cancer to examine the relationship between their vitamin D status and tumour growth, Bilinski said.
Should fluoride in water be the responsibility of central government?