Thursday September 25 2014
Obesity is a known risk factor for breast cancer
“Skirt size increase linked to breast cancer risk,” BBC News reports. The story comes from a UK study of nearly 93,000 postmenopausal women that looked at whether changes in skirt size since their twenties was associated with increased risk of breast cancer.
It found that going up a skirt size every 10 years was associated with a 33% increased risk of developing breast cancer after the menopause. As an example, this could be going from a size 8 at 25 years old to a size 16 at 65 years old.
It's important to stress that the initial risk of developing breast cancer, the baseline risk, is small, with only 1.2% of women involved in the study going on to develop breast cancer.
This large study used skirt size as a proxy measure for “central obesity” – the accumulation of excess fat around the waist and stomach. While overweight and obesity is known to be a risk factor for several cancers, this study suggests that a thickening waist may be an independent measure of increased breast cancer risk.
The good news is that the “skirt size effect" appears to be reversible, as losing weight and trimming your waist size may help reduce your breast cancer risk.
Where did the story come from?
The study was carried out by researchers from the Universities of London and Manchester, and was funded by the Medical Research Council, Cancer Research UK and the National Institute of Health Research, as well as the Eve Appeal.
The study was published in the peer-reviewed medical journal BMJ Open. As the name suggests, this is an open-access journal, so the study can be read for free online.
The paper was widely covered in the UK media. Coverage was fair, if uncritical.
Several headlines gave the impression that going up a single skirt size would raise breast cancer risk by 33%. Such a rise in risk would only be expected if a person went up a dress size every decade from their mid-twenties to when they were over 50 years old – the youngest age of the women recruited to the study.
Several media sources included useful comments from independent experts.
What kind of research was this?
This was a cohort study that looked at whether changes in skirt size between a woman’s twenties and the menopause was associated with increased risk of breast cancer. Skirt size was used as a proxy measure for central obesity (an excessive amount of fat around the stomach and abdomen – sometimes known as a "pot belly" or "beer belly").
The researchers say that both overall and central obesity are associated with an increased breast cancer risk in postmenopausal women, yet no studies have looked at the relationship between breast cancer risk and changes in central obesity alone.
Skirt and trouser size, they say, provide a reliable estimate of waist circumference, which may be predictive of risk, independent of body mass index (BMI), which is based on the individual’s height and weight.
What did the research involve?
The researchers recruited to their study women taking part in a large UK trial of ovarian cancer screening. The women were aged 50 or over and had no known history of breast cancer when they entered the study, between 2005 and 2010.
At enrolment, they answered a questionnaire providing detailed information on height and weight, reproductive health, number of pregnancies, fertility, family history of breast and ovarian cancer, use of hormonal contraceptives and hormone replacement therapy (HRT) – all of which influence (confound) breast cancer risk.
They were also asked about their current skirt size (SS) and what their SS had been in their twenties. Women could choose from 13 SS categories, ranging from size 6 to 30. These answers were used to calculate an increase in SS for each 10 years gone by. A “one unit” increase in SS would mean an increase from, say, 10 to 12 – as odd sizes do not exist in the UK.
The women were followed up three to four years after recruitment, when they completed a further questionnaire, providing information on education, skirt size, continuing use of HRT, smoking, alcohol use, health status and any cancer diagnosis.
The researchers used official health records to identify those women who had a diagnosis of breast cancer during the follow-up period.
They used standard statistical methods to analyse their results, adjusting these for confounders such as BMI, HRT use and family history.
What were the basic results?
The researchers report that 92,834 women completed the study and were included in their analysis. The average age of participants was 64. Participants were mainly white, educated to university degree level, and overweight at the point of entry to the study, with an average BMI of just over 25.
At the age of 25, the average skirt size had been a UK 12, and at 64 it was 14. An increase in skirt size over their lifetime was reported in 76% of women.
During the monitoring period, 1,090 women developed breast cancer, giving an absolute risk of just over 1%.
Researchers found that for each unit increase in skirt size per 10 years, the risk of breast cancer after menopause increased by 33% (hazard ratio (HR) 1.330, 95% confidence interval (CI) 1.121 to 1.579).
For those with an increase of two SS units every 10 years, the risk was increased by 77% (HR 1.769, 95% CI 1.164 to 2.375).
They also found that a reduction in skirt size since the twenties was associated with a decreased risk of breast cancer.
Changes in skirt size, they say, was a better predictor of breast cancer risk than BMI or weight generally. It should also be noted that the association of skirt size with breast cancer risk was independent of BMI.
How did the researchers interpret the results?
The researchers conclude that a change in skirt size is associated with a risk of breast cancer independent of a woman’s height and weight. They estimate an increase in five-year absolute risk of postmenopausal breast cancer from one in 61 to one in 51 with each increase in skirt size each 10 years.
Their findings, they say, may provide women with a simple and easy to understand message, given that skirt size is a reliable measure of waist circumference, and women may relate to skirt size more easily than other measures of fat, such as BMI.
They theorise that fat around the waist may be more “metabolically active” than fat elsewhere and may increase levels of circulating oestrogen – an established risk factor for breast cancer.
This study suggests that while obesity generally is a risk factor for breast cancer, an increase in waist circumference, as shown in skirt size, between a woman’s twenties and after the menopause, may be an independent measure of increased risk.
Keeping to a healthy weight is important for overall health, and for reducing the risk of several cancers. However, few women in their 60s have the same waist size as they did in their twenties – in this study, for example, the average skirt size at 25 was a 12, but at 64 it was a size 14.
The 33% increased risk of breast cancer after menopause calculated by the researchers was based on an increase in skirt size every 10 years, which could mean increasing from size 12 aged 25 to size 18 by age 55.
The study had several limitations that may affect the reliability of its results. For example, it had a short follow-up period (three to four years) and it also required postmenopausal women in their 50s and 60s to recall their skirt size in their twenties.
In addition, while researchers adjusted their results for several factors that might influence the risk of breast cancer, it is always possible that both measured and unmeasured confounders affected the results.
Finally, most of the women were white, well-educated and also overweight when they were recruited. The results may not be generalisable to other groups of women.
It’s important to maintain a healthy weight, but it would be sad if women in their sixties started needlessly worrying that they should have the same waist size as when they were in their twenties. Surely all of us are entitled to some degree of middle-age spread?
Other ways you can reduce your breast cancer risk include taking regular exercise, choosing to breastfeed rather than bottle feed, and attending screening appointments if invited.
Read more about breast cancer prevention.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.