Gaining weight from early adulthood is linked to a lower risk of breast cancer before the menopause, a major global study of more than 600,000 women has found.
Being heavier as a young adult is known to lower the chances of developing premenopausal breast cancer, but it has been unclear to date what impact any subsequent weight gain (or loss) might have.
In a new long-term study, funded by Breast Cancer Now and other collaborators, women who gained 10kg or more from early adulthood (aged 18–24) up to their forties or early fifties were found to have a lower risk of breast cancer before the menopause than those whose weight remained stable, regardless of their starting weight.
The international collaboration ― led by scientists at The Institute of Cancer Research, London, the University of North Carolina and the US National Institute of Environmental Health Sciences ― found that the more weight women put on, the stronger the protective effect, with a reduction in risk of around 4% for each 5kg gained between early adulthood and ages 45–54.
The findings provide further evidence that levels of fat, or adiposity, are a fundamental factor determining a woman’s chances of getting breast cancer.
New preventive therapies
Experts today called for studies to explore why both body size and additional weight gain independently reduce the likelihood of developing breast cancer before the menopause.
It is hoped that an understanding of the biological mechanism could lead to new preventive therapies and more accurate risk tests to help guide when individual women may benefit most from breast screening.
Breast cancer is the UK’s most common cancer, with around 55,000 women and 370 men being diagnosed with the disease each year in the UK.
While the risk of breast cancer increases with age, nearly 20% of cases arise in women under the age of 50, and the disease remains the leading cause of death in women under 50 in the UK.
Previous research has established that having a higher body mass index (BMI) at a younger age is linked to a lower risk of breast cancer before the menopause, but it has been difficult to study in detail the role of weight gain due to the relatively low rates of breast cancer among younger women.
In a new analysis of data from 628,468 women from 17 studies across the world, including the Breast Cancer Now Generations Study in the UK, researchers investigated the effect of weight gain during six age intervals on the risk of breast cancer before the menopause: from ages 18–24 to 25–34, 35–44 and 45–54, ages 25–34 to 35–44 and 45–54, and ages 35–44 to 45–54.
Collecting information on women’s weight changes
The analysis — led by Dr Minouk Schoemaker and Professor Anthony Swerdlow at The Institute of Cancer Research (ICR), Dr Hazel Nichols at the University of North Carolina and Professor Dale Sandler at the US National Institute of Environmental Health Sciences — collected information on women’s weights at a minimum of two ages, and followed participants for a median of 10.1 years.
Weight changes were analysed in increments of 5kg and a wide range of information was collected from participants to adjust for other factors that influence breast cancer risk including whether they had children and when, their starting weight and any family history of breast cancer.
Where available, other lifestyle factors such as smoking, level of physical activity, and alcohol consumption were taken into consideration.
Overall exposure to excess weight over time
The researchers observed that 10,886 out of the 628,468 women had gone on to develop breast cancer before the menopause, and found that weight gain of 10kg or more from early adulthood led to a reduction in risk.
Weight gain between early adulthood (18–24) and ages 45–54 led to decrease in risk of around 4% for each 5kg gained, with weight gain specifically between early adulthood and ages 35–44 leading to decrease in risk of around 3% for each 5kg gained.
Interestingly, weight gain when started from the ages 35–44 onwards did not affect women’s chances of developing the disease before the menopause, which could suggest that it is overall exposure to excess weight over time that is linked to a lower premenopausal breast cancer risk.
Weight loss (of 5kg or more) was not found to be linked to the risk of premenopausal breast cancer once women’s starting weight had been taken into account.
Protective effect reverses after the menopause
While further studies are needed, the authors propose that the impact of a woman’s weight in early adulthood on her chances of developing premenopausal breast cancer may originate in childhood, and could be related to changes in her breast composition during puberty.
It is also thought that other factors such as altered levels of hormones or growth factors in early adulthood may play a role.
With the protective effect of weight gain reversing after the menopause (where excess body weight increases breast cancer risk and where the disease most commonly) develops, Breast Cancer Now today urged for women of all ages to be fully supported to achieve and maintain a healthy weight, to help reduce the risk of breast cancer after the menopause, and of other types of cancer and other diseases.
The study is published in the International Journal of Cancer.
Lead author Dr Minouk Schoemaker, Senior Staff Scientist in Cancer Epidemiology at The Institute of Cancer Research, London, said:
“The link between a higher body mass index and a lower breast cancer risk before the menopause has puzzled researchers for a while now. In our large-scale international study, we were able to tease out the effects in more detail than ever before.
“We found that while higher weight in early adulthood was most strongly linked to reducing breast cancer risk, later weight gain had an independent effect in bringing risk down further. But we know that the protective effect of a higher weight is reversed after the menopause, when being heavier increases women’s breast cancer risk.
“Women shouldn’t consider gaining weight as a way to prevent breast cancer – but understanding the biological reasons behind the link between weight and breast cancer risk could in future lead to new ways to prevent the disease.”
'We now urgently need to understand the biological reasons'
Baroness Delyth Morgan, Chief Executive at Breast Cancer Now, which helped to fund the study, said:
“These are really important findings that bring us a step closer to understanding the fundamental role of weight on the risk of breast cancer in younger women.
“We must be really clear that weight gain itself should never be considered to try to prevent breast cancer, particularly as excess weight raises risk after the menopause, when the disease is most common. It’s vital that women of all ages are fully supported to achieve and maintain a healthy weight, to help reduce their overall risk of cancer and other diseases.
“We now urgently need to understand the biological reasons why body size and weight gain both lower women’s risk of breast cancer before the menopause. It’s really promising that the discovery of this protective effect could help us develop new preventive therapies or better risk tests to identify those who may benefit most from screening or risk-reducing steps.
“In the meantime, everyone can help keep their risk of breast cancer as low as possible by being more active, drinking less alcohol and keeping to a healthy weight. Anyone who is concerned about their breast cancer risk can call our free Helpline on 0808 800 6000 to talk to one of our nurses.”
The Breast Cancer Now Generations Study is a landmark prospective study of the causes of breast cancer that is following over 113,000 UK women for over 40 years.
The Study – based at the ICR – has already led to a number of significant discoveries into the interlinked causes of breast cancer, including clarifying that women taking combined HRT are 2.7 times more likely to develop breast cancer than non-users, and that smoking is associated with an increased risk of breast cancer particularly if started during adolescence.
Reference: Schoemaker, et al. (2020) Adult weight change and premenopausal breast cancer risk: A prospective pooled analysis of data from 628,463 women. IJC. DOI: https://doi.org/10.1002/ijc.32892
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