Post-menopausal women who have Type 2 diabetes appear to have a 27% greater risk of developing breast cancer, experts say.
An international team, writing in the British Journal of Cancer, examined 40 separate studies looking at the potential link between breast cancer and diabetes.
Being obese or overweight is linked to both conditions.
But cancer experts say there may be a direct connection between the two.
These studies involved more than 56,000 women with breast cancer.
Post-menopausal women with Type 2 diabetes had a 27% increased risk of breast cancer.
But there was no link for pre-menopausal women or those with Type 1 diabetes.
The authors have also suggested that a high body mass index (BMI), which is often associated with diabetes, may be an underlying contributing factor.
Prof. Peter Boyle, president of the International Prevention Research Institute, who led the study, said: “We don’t yet know the mechanisms behind why Type 2 diabetes might increase the risk of breast cancer.
“On the one hand, it’s thought that being overweight, often associated with Type 2 diabetes, and the effect this has on hormone activity may be partly responsible for the processes that lead to cancer growth.
“But it’s also impossible to rule out that some factors related to diabetes may be involved in the process.”
Experts say a genetic test could help predict breast cancer many years before the disease is diagnosed.
Ultimately the findings, in the journal Cancer Research, could lead to a simple blood test to screen women, they say.
The test looks for how genes are altered by environmental factors like alcohol and hormones – a process known as epigenetics.
One in five women is thought to have such a genetic “switch” that doubles breast cancer risk.
The scientists from Imperial College London analyzed blood samples from 1,380 women of various ages, 640 of whom went on to develop breast cancer.
And they found a strong link between breast cancer risk and molecular modification of a single gene called ATM, which is found on white blood cells.
A genetic test could help predict breast cancer many years before the disease is diagnosed
They then looked for evidence of what was causing this change. Specifically, they looked for a chemical effect called methylation, which is known to act as a “gene switch”.
Women showing the highest methylation levels affecting the ATM gene were twice as likely to develop breast cancer compared with those with the lowest levels.
In some cases the changes were evident up to 11 years before a breast tumor was diagnosed.
Dr. James Flanagan, of Imperial College London, who led the new research, said: “We know that genetic variation contributes to a person’s risk of disease.
“With this new study we can now also say that epigenetic variation, or differences in how genes are modified, also has a role.
“We hope that this research is just the beginning of our understanding about the epigenetic component of breast cancer risk and in the coming years we hope to find many more examples of genes that contribute to a person’s risk.
“The challenge will be how to incorporate all of this new information into the computer models that are currently used for individual risk prediction.”
It is not yet clear why breast cancer risk might be linked to changes in a white blood cell gene.
But the team envisages that a blood test could be used in combination with other information about breast cancer risk, such as family history and the presence of other known breast cancer genes, to help identify those women at greatest risk of developing the disease in the future.
These women could then be closely monitored and offered pre-emptive treatment, such as surgery.