'Up to now, only a few studies have directly investigated whether insulin levels are associated with breast cancer risk,' said Geoffrey Kabat, the lead author and senior epidemiologist in the department of epidemiology and population health at Einstein.

'Our study involved analysing repeated measurements of insulin taken over several years - which provides a more accurate picture of the possible association between insulin levels and breast cancer risk,' said Kabat.

'This finding is potentially important because it indicates that, in postmenopausal women, insulin may be a risk factor for breast cancer that is independent of obesity,' Kabat said.

Kabat recommended that postmenopausal women try to keep insulin at normal levels through weight loss, regular exercise and other methods.