Extra pounds – even within the overweight but not obese range – are linked to a higher risk of recurrence of the most common type of breast cancer, according to a new study in the US.

This higher risk is even present when breast cancer patients have received the best possible cancer treatment.

The new study suggests that extra body fat causes hormonal changes and inflammation that may drive some cases of breast cancer to spread and recur despite treatment.

Women who are obese when they are diagnosed with breast cancer have an increased risk of dying prematurely compared with women of normal weight.

In this new study, researchers found that increasing body mass index-a measure of the body's fat content-significantly increased women's risk of cancer recurrence and death, despite optimal treatment including chemotherapy and hormonal therapy. 

There was a stepwise relationship between increasing body mass index and poor outcomes only in women with hormone receptor-positive breast cancer, the most common type of breast cancer that accounts for approximately two-thirds of all breast cancer cases in the United States and worldwide.

"We found that obesity at diagnosis of breast cancer is associated with about a 30 percent higher risk of recurrence and a nearly 50 percent higher risk of death despite optimal treatment," said Joseph Sparano of the Albert Einstein College of Medicine's Montefiore Medical Center in New York, where the study was carried out.

"Treatment strategies aimed at interfering with hormonal changes and inflammation caused by obesity may help reduce the risk of recurrence," he added.

The study was published early online in CANCER, a journal of the American Cancer Society.