It found women had decreased risk of low-risk breast cancer if they were overweight or suffered high blood sugar (insulin resistance), but were at higher risk of more aggressive tumours.
Experts at Umea University in Sweden carried out the study, which was funded by the World Cancer Research Fund and is published in the journal Breast Cancer Research and Treatment.
They looked at blood samples and other measurements including body mass index (BMI) taken from more than 1,000 women. Of the group, 561 women had been diagnosed with breast cancer while 561 remained free of the disease and acted as a control.
The women were aged 52 on average at the point of recruitment and the average time to diagnosis was 4.4 years.
The study found that the association of BMI, leptin (a hormone linked to body fat) and HbA1c (a blood sugar reading) and breast cancer risk differed significantly according to whether the tumour was diagnosed as early stage I or more advanced stages II to IV.
The experts pointed to previous studies which show that obesity in both pre- and post-menopausal women is associated with advanced cancer at diagnosis.
“Increasing BMI is also significantly associated with higher rates of death due to breast cancer, compared with normal weight individuals,” they said.
Tanja Stocks, lead author on the study, said: “We believe this is the first study to find a link between blood sugar levels and the type of tumour. Hopefully now this will lead to more work on how things like body fat and blood sugar levels affect the type of tumour that develops, as well as a person’s overall cancer risk.”