Dr Gillian Reeves, a co-author of the study and based at the University of Oxford, said: "Our study shows that changes in hormone levels might explain the association of established risk factors such as obesity with breast cancer risk. Other studies have found that weight and alcohol can affect hormone levels and this research confirms and adds to these findings and provides more information about how breast cancer develops."

Dr Julie Sharp, Cancer Research UK's senior science information manager, said: "There's already strong evidence that drinking alcohol and being overweight can increase the risk of breast cancer but it's important to understand why these links exist. This is an important study as it helps to show how alcohol and weight can influence hormone levels. Understanding their role in breast cancer is vital and this analysis sheds light on how they could affect breast cancer risk.

"We know that the risk of the disease can be affected by family history and getting older, but there are also things women can do help reduce the risk of the disease. Maintaining a healthy body weight and reducing alcohol consumption are key to reducing breast cancer risk."

Source: Cancer Research UK