For the first time in history, there are now more people with obesity in the world than underweight people. The tidal wave of obesity shows no signs of slowing down – a 2016 global analysis in The Lancet found that worldwide, the number of obese people has risen from 105 million in 1975 to 641 million in 2014.

While many of us now know that obesity increases the risk of diabetes and heart disease, we are less aware that obesity increases the risk of 11 cancers, according to the World Cancer Research Fund.

This includes breast cancer, the most common cancer in Australian women, according to the Cancer Council.

“There is mounting evidence that obesity is associated with a higher risk of developing breast cancer, especially in postmenopausal women, and that once diagnosed, breast cancer outcomes are poorer for women with obesity of all ages,” said Central Coast Breast Cancer Surgeon Dr Mary Ling. 

Breast cancers can be broadly classified as hormone receptor-positive or hormone receptor-negative. Hormone receptive-positive breast cancers test positive for oestrogen and/or progesterone receptors and account for two-thirds of all breast cancers. 

The effect of obesity on the risk of breast cancer is different for premenopausal and postmenopausal women. “Obesity is associated with a markedly higher risk of ER-positive (estrogen receptor positive) breast cancer in postmenopausal women,” said Dr Ling.

“However, a recent large study has found the opposite is true for premenopausal women, wher obesity is associated with a lower risk of ER-positive breast cancer. This is not a reason to try to gain weight to prevent breast cancer. It just means the pathway to developing breast cancer is different in younger women compared to older women, and more research is needed to understand this.”

once diagnosed with breast cancer though, the effect of obesity on breast cancer outcomes is independent of menopausal status. “Studies have shown that obesity along with decreased physical activity and weight gain are associated with poorer survival in patients with breast cancer at any age. 

“In contrast, exercise and weight loss are associated with reduced breast cancer risks and better [treatment] outcomes,” said Dr Ling. 

The link between exercise and improved cancer outcomes is so strong that the Clinical oncology Society of Australia (COSA) recently released a position statement stating exercise should be prescribed as an essential component of cancer treatment.

“Soon we may also see incorporation of weight loss intervention as standard part of management for patients with breast cancer,” said Dr Ling. 

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