Young women who are overweight may be less likely to develop breast cancer

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Women who are overweight in early adulthood may be less likely to develop breast cancer after menopause due to lasting changes in the structure of their breast tissue, a new study suggests.

Researchers stressed that gaining weight is not protective against cancer, highlighting that excess weight is actually linked to several cancers.

Instead, they say the finding shows how health “exposures” in earlier life can have lasting effects decades later.

Researchers from The University of Manchester and Manchester University NHS Foundation Trust said that the relationship between body mass index (BMI) and breast cancer is “complex”.

They highlighted how elevated BMI during middle age has been linked to an increased risk of breast cancer.

But previous studies have also suggested that high BMI during childhood and early adulthood is associated with a reduced risk of post-menopausal breast cancer.

They said that both these findings indicate that excess fat across different life stages may influence breast cancer “through different biological pathways”.

The authors set out to examine data on women taking part in the UK PROCAS study, which involves almost 58,000 women in the Greater Manchester area who were aged 46 to 73 when they signed up. The women were recruited to the study during routine breast cancer screenings between 2009 and 2015.

Data on BMI was collected, as well as self-reported BMI for when the women were aged 20. Overall, data was available for 33,816 post-menopausal women.

During an average follow-up period of a decade, 1,261 of these women were diagnosed with breast cancer.

The researchers found that women who had a higher BMI when they were aged 20 appeared to have a reduced risk of post-menopausal breast cancer.

But those with a higher BMI in later adulthood and higher breast density had an increased risk.

Researchers also examined breast density using data collected at the mammogram screenings – breast density refers to the amount of fibrous and glandular tissue in the breast compared to fatty tissue. Women with denser breasts face higher odds of getting breast cancer.

The research team found that women with a higher BMI when they were aged 20 generally had lower breast density later in life and were also less likely to develop post-menopausal breast cancer. Overall, they found that for every five-point increase in BMI at age 20, the risk of developing post-menopausal breast cancer fell by around 15%.

And the research suggests that lower breast density may account for 59 per cent of this effect.

Main symptoms of breast cancer in women

NHS

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Symptoms of breast cancer in women may include:

• a lump, or swelling in your breast, chest or armpit
• a change in the skin of your breast, such as dimpling (may look or feel like orange peel)
• a change in the colour of the skin on your breast – on black or brown skin it may look darker than usual, on white skin it may look red
• a change in size or shape of one or both breasts
• nipple discharge (if you are not pregnant or breastfeeding), which may have blood in it
• a change in the shape or look of your nipple, such as it turning inwards (inverted nipple) or a rash on it (may look like eczema)

The research team believe the timing of body weight gains may have an impact.

Because breasts are still developing during adolescence and early adulthood, this could potentially lead to long-term changes in tissue structure, they suggested.

“Early adult BMI may confer a lasting effect by modulating breast tissue composition and reducing susceptibility to carcinogenesis, particularly during the critical window of puberty when rapid mammary tissue proliferation occurs,” they wrote in the British Journal of Cancer.

Senior author of the study, Andrew Renehan, professor of cancer studies and surgery at The University of Manchester and programme co-lead in the Cancer Prevention and Early Detection Theme at NIHR BRC: Manchester, added: “These findings do not suggest that gaining weight is protective.

“What they offer is further insight into how breast tissue and consequent breast cancer risk may be shaped across a woman’s lifetime – and we want to understand these mechanisms further.

“Maintaining a healthy weight remains important, because excess weight in later adulthood is linked to a higher risk of breast cancer and many other serious diseases.

“But this study adds to growing evidence that exposures during early life can have lasting effects on health decades later.”

First author Dr Benoit Jauniaux, a surgical trainee in the north west, said: “Researchers in previous studies have observed that women with a higher BMI in childhood or their early adulthood appear to have a lower risk of breast cancer after the menopause, but we have not fully understood why.

“This study suggests that downstream changes to breast density may explain a large part of this effect.”