A hormone that helps children grow may cause breast cancer, and women with high levels are at higher risk, a new study has found.
The hormone, IGF-1, stimulates cell division, especially during childhood, and is being investigated as an anti-ageing treatment. Its effect on the breast is unclear, but cancer results when cell division multiplies out of control.
The hormone has been linked to breast cancer before in smaller studies. For the new research, Cancer Research UK scientists at the University of Oxford analysed findings in 17 studies from 12 different countries, which together included nearly 5,000 women with breast cancer. The results showed that the 20 per cent of women with the highest blood levels of the growth factor were 28 per cent more likely to develop breast cancer than the 20 per cent with the lowest levels.
The effect was seen mainly in women with a certain type of breast cancer that responds to the female hormone oestrogen.
Professor Tim Key, lead author of the study, published in the journal Lancet Oncology, said: "Over the last few years there has been increasing interest in the possible link between growth factors and breast cancer, but the results have been inconsistent. Putting together all the information available worldwide gives us conclusive evidence that the higher a woman's blood levels of IGF-I, the higher her risk of breast cancer."
"We don't yet fully understand what affects blood levels of this growth factor, but it's possible that diet plays an important role."
Dr Lesley Walker, director of cancer information at Cancer Research UK, said: "This hormone has received a lot of attention, not just for its role in breast cancer, but also in prostate cancer.
"Although we'll have to wait and see if particular changes to a woman's diet can affect her levels of this hormone, this study has revealed some very interesting information that adds to our knowledge about the disease."
The finding follows the discovery last week of five new genetic sites that increase a woman's risk of developing breast cancer by between 6 and 16 per cent. They take the total number of common "low-risk" genetic sites associated with breast cancer to 18.
The increased risk from each genetic site is small and scientists still don't know which genes are responsible. But as more of them are identified it may be possible to create tests for a combination of them that together significantly increase risk.
This could help doctors make decisions about prevention, diagnosis and treatment for women who are more likely to get breast cancer.
Helen George, head of science information at Cancer Research UK, said the study, published in Nature Genetics, was the largest of its kind to explore the common genetic variations that contribute to breast cancer risk.
"This research takes us a step closer to developing a powerful genetic test for the disease. Such a test could help doctors identify women who have an increased breast cancer risk so that they can make informed decisions about how to take steps to reduce their chance of developing the disease."