An American study has revealed that high levels of insulin increase the risk of breast cancer in women.

Lead researchers Marc Gunter and Howard Strickler, of the Albert Einstein College of Medicine in New York, revealed that they examined the role of insulin in breast cancer while controlling for oestrogen levels.

This attains significance because, while the proneness to breast cancer has been attributed to high oestrogen levels in many obese postmenopausal women thus far, insulin has never been recognised as an independent risk factor.

During the study, the researchers examined the association between incident breast cancer and baseline fasting insulin, insulin-like growth factor-1 (a related hormone), and oestradiol levels in 835 women enrolled in the Women's Health Initiative Observational Study who developed breast cancer and a randomly-selected sample of 816 women in the study who did not develop breast cancer.

Upon dividing the women into four groups based on their fasting insulin levels, the team found that the subjects with the highest insulin levels had nearly a 1.5-fold higher risk of developing breast cancer than those with the lowest insulin levels.

When the researchers separately analysed women who were not using hormone therapy, they found that individuals with the highest insulin levels had a 2.4-fold increased risk of developing breast cancer compared to those with the lowest levels.

The finding remained unchanged even when the researchers took into account multiple other breast cancer risk factors, including oestrogen levels.