Behind the research: Does a higher body mass index in childhood protect women against breast cancer in later life?

A girl playing outside

Jennifer Baker Earlier this year, results from a study funded by World Cancer Research Fund were announced at the European Congress on Obesity. They found that a higher body mass index (BMI) in childhood may help protect women against breast cancer in later life, both before and after the menopause.

Intrigued by these results, we caught up with the lead author, Jennifer Baker, to find out more about what they could mean for cancer prevention and lifestyle advice.

What are the top-line results of the study?

Our preliminary results suggest that girls aged 7-13 who have a higher BMI have greater protection against pre- and post-menopausal breast cancer later in life. We are currently investigating if these associations also hold when looking at all types of breast cancer as well as whether there is any association with when puberty starts.

What do you think are some of the explanations for these findings?

BMI during childhood has a complex association with the onset of puberty, and some of these links seem to contradict each other. One plausible link, however, is through breast tissue composition.

It’s likely that the effects of growth during childhood on breast composition affect the risk of breast cancer. We know that girls with a higher BMI during childhood are more likely to have fatty breasts as adults, when compared with girls with a lower BMI – who are more likely to have dense breasts. Dense breasts increase the risk of breast cancer, so non-dense breasts could be the link between a higher childhood BMI and a lower risk of breast cancer.

Nonetheless, there are numerous factors at play and BMI is just one piece of a puzzle. It’s not yet clear when (or even if) a higher BMI turns from being protective to harmful in relation to breast cancer.

We still don’t have a full picture of body size across the lifecourse; most studies rely on people’s memories of their body size when they were younger, and there are significant age gaps. As the people involved in the current study age, we will have greater opportunities to answer this question.

What is the link between BMI and breast cancer?

Despite our knowledge about this link, the biological mechanisms remain speculative. A rush of hormones brings about the enormous changes that take place as breasts develop during puberty. High levels of adipose tissue (fat tissue) in the body are associated with inflammation and changing levels of sex hormones such as oestrogen and growth hormones. It’s possible that a higher level of adipose tissue – as indicated by a higher childhood BMI – brings about earlier differences in the breast tissue which then makes it less vulnerable to cancer.

An anatomical diagram of the female breast

What’s the message for women of any age, and parents of girls?

BMI is only one part of the puzzle when it comes to preventing breast cancer. We must remember that we can do other things to reduce the risk of breast cancer, such as:

Breastfeeding is also known to help. To me the key message is that the origins of breast health begin early in life and that girls and women have actions they can take across the lifecourse to ensure and support breast health.

How does this tally with what we know about how exercise protects against breast cancer?

Exercise fits into the total picture of having a healthy lifestyle. It improves cardiometabolic health, and body composition (eg increase in lean tissue and reduction in excess levels of adipose tissue) and these factors come together to reduce the risk of breast cancer, as well as many other types of cancer.

*Top image credit: World Obesity Federation