This hormone has been linked to breast cancer before in smaller studies, but this research – an analysis of the worldwide evidence to date – shows that the 20 per cent of women with the highest blood levels of this growth factor were 28 per cent more likely to develop breast cancer than the 20 per cent with the lowest levels.
The researchers also show that the effect is seen mainly in women with a certain type of breast cancer that responds to oestrogen.
Cancer Research UK’s Professor Tim Key, lead author from the University of Oxford, 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. We’ve already got research underway looking into this.”
The researchers analysed 17 studies from 12 different countries, which together included nearly 5,000 women with breast cancer.
IGF-I’s normal function is to stimulate cell division, especially during childhood development.
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.”
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Insulin-like growth factor-I (IGF-I), IGF binding protein-3 (IGFBP-3), and breast cancer risk: reanalysis of seventeen prospective studies. Key et al. Lancet Oncology.