Having a higher BMI at a younger age is associated with a decreased risk of developing breast cancer before menopause, major new research funded by Breast Cancer Now and other collaborators has found.

In the largest global study into the relationship between weight and premenopausal breast cancer to date, scientists from The Institute of Cancer Research, London, the University of North Carolina, the US National Institute of Environmental Health Sciences, and international collaborators, discovered that increasing premenopausal BMI was linked with a lower risk of the disease and at a greater magnitude than previously suggested.

While calling for further studies to uncover the biological mechanisms for this effect, experts have cautioned that being overweight or obese can have many adverse impacts on general health and that after menopause excess body weight increases breast cancer risk—when breast cancer most commonly develops and where obesity remains a leading lifestyle cause of the disease.

Leading charity Breast Cancer Now has today urged for women of all ages to be supported to achieve and maintain a healthy weight throughout life, to help reduce their overall risk of cancer and other diseases.

The charity has also called for greater investment from governments across the UK to help develop new public health interventions to tackle obesity, amid increasing breast cancer incidence among postmenopausal women.

New analysis of 758,592 women

Around 55,000 women are diagnosed with the disease each year in the UK, with nearly 20% of cases developing in women under the age of 50.

Previous research has established a link between increased body weight and a decreased risk of breast cancer before the menopause—but due to the lower rates of breast cancer among younger women, past studies had not been large enough to investigate the link in detail or by type of breast cancer.

A new analysis of 758,592 women from 19 prospective studies across the world including, the Breast Cancer Now Generations Study, found a 12%-23% relative breast cancer risk reduction per 5 BMI unit increase (about a 10-kg weight increase for women of average height), depending on age.

In particular, the team observed that 18- to 24-year-olds in the highest BMI category (a BMI of 30 kg/m² or more, defined as ‘obese’) were 4.2 times less likely to develop breast cancer before menopause than those in the lowest BMI category (a BMI of less than 18.5, defined as ‘underweight’).

The analysis—led by Dr. Minouk Schoemaker and Professor Anthony Swerdlow at The Institute of Cancer Research (ICR, UK), Dr. Hazel Nichols at the University of North Carolina (US), and Professor Dale Sandler at the US National Institute of Environmental Health Sciences (NIEHS)—collected information on women’s weights at different ages before following the participants for a median of 9.3 years.

A wide range of lifestyle information was also collected from participants to adjust for potential confounding breast cancer risk factors including age at start of menstruation, age at birth of first child, and a family history of the disease.

The researchers observed that 13,082 out of the 758,592 women went on to develop breast cancer, and found a strong inverse association between BMI and the risk of the disease before menopause. This applied across the entire weight spectrum, not just among those who were overweight or obese.

This effect was found to be strongest for BMI at younger adult ages, with a 23% relative risk reduction per 5-unit BMI increase for BMI at ages 18-24—compared with a 12% relative risk reduction per 5-unit BMI increase for BMI at ages 45-54.

The study observed significant associations between BMI at ages 18-24 and breast cancer risk for both hormone receptor-positive and negative breast cancer, suggesting that both hormonal and non-hormonal mechanisms may be involved.

The study has been published today in the journal JAMA Oncology.