Monday September 8 2014
The study found bra wearing didn’t increase breast cancer risk
“Scientists believe they have answered the decades long debate on whether wearing a bra can increase your risk of cancer,” reports The Daily Telegraph.
There is an "urban myth" that wearing a bra disrupts the workings of the lymphatic system (an essential part of the immune system), which could lead to a build-up of toxins inside breast tissue, increasing the risk of cancer. New research suggests that this fear may be unfounded.
The study compared the bra-wearing habits of 1,044 postmenopausal women with two common types of breast cancer with those of 469 women who did not have breast cancer. It found no significant difference between the groups in bra wearing habits such as when a woman started wearing a bra, whether she wore an underwired bra, and how many hours a day she wore a bra.
The study had some limitations, such as relatively limited matching of characteristics of women with and without cancer. Also, as most women wear a bra, they could not compare women who never wore a bra versus those who wore a bra.
Despite the limitations, as the authors of the study say, the findings provide some reassurance that your bra-wearing habits do not seem to increase risk of postmenopausal breast cancer.
While not all cases of breast cancer are thought to be preventable, maintaining a healthy weight, moderating your consumption of alcohol and taking regular exercise should help lower your risk.
Where did the story come from?
The study was carried out by researchers from Fred Hutchinson Cancer Research Center in the US.
It was funded by the US National Cancer Institute.
The study was published in the peer-reviewed medical journal Cancer Epidemiology Biomarkers & Prevention.
The Daily Telegraph and the Mail Online covered this research in a balanced and accurate way.
However, suggestions that women who wore bras were compared with “their braless counterparts”, are incorrect. Only one woman in the study never wore a bra and she was not included in the analyses. The study was essentially comparing women who all wore bras, but starting at different ages, for different lengths of time during the day, or of different types (underwired or not).
What kind of research was this?
This was a case-control study looking at whether wearing a bra increases risk of breast cancer.
The researchers say there has been some suggestion in the media that bra wearing might increase risk, but that there is little in the way of hard evidence to support the claim.
A case-control study compares what people with and without a condition have done in the past, to get clues as to what might have caused the condition.
If women who had breast cancer wore bras more often than women who did not have the disease, this might suggest that bras could be increasing risk. One of the main limitations to this type of study is that it can be difficult for people to remember what has happened to them in the past, and people with a condition may remember things differently than those who don’t have the condition.
Also, it is important that researchers make sure that the group without the condition (the controls) are coming from the same population as the group with the condition (cases).
This reduces the likelihood that differences other than the exposure of interest (bra wearing) could contribute to the condition.
What did the research involve?
The researchers enrolled postmenopausal women with (cases) and without breast cancer (controls) from one area in the US. They interviewed them to find out detailed information about their bra wearing over the course of their lives, as well as other questions. They then statistically assessed whether the cases had different bra-wearing habits to the controls.
The cases were identified using the region’s cancer surveillance registry data for 2000 to 2004. Women had to be between 55 and 74 years old when diagnosed. The researchers identified all women diagnosed with one type of invasive breast cancer (lobular carcinoma or ILC), and a random sample of 25% of the women with another type (ductal carcinoma). For each ILC case, a control woman who was aged within five years of the case’s age was selected at random from the general population in the region. The researchers recruited 83% of the eligible cases (1,044 of 1,251 women) and 71% of eligible controls (469 of 660 women).
The in-person interviews asked about various aspects of past bra wearing (up to the point of diagnosis with cancer, or the equivalent date for controls):
- bra sizes
- age at which they started regularly wearing a bra
- whether they wore a bra with an underwire
- number of hours per day a bra was worn
- number of days per week they wore a bra at different times in their life
- whether their bra-wearing patterns ever changed during their life
Only one woman reported never wearing a bra, and she was excluded from the analysis.
The women were also asked about other factors that could affect breast cancer risk (potential confounders), including:
- whether they had children
- body mass index (BMI)
- medical history
- family history of cancer
- use of hormone replacement therapy (HRT)
- demographic characteristics
The researchers compared bra-wearing characteristics between cases and controls, taking into account potential confounders. The potential confounders were found to not have a large effect on results (10% change in odds ratio [OR] or less), so results adjusting for these were not reported. If the researchers just analysed data for women who had not changed their bra-wearing habits over their lifetime, the results were similar to overall results, so these were also not reported.
What were the basic results?
The researchers found that some characteristics varied between groups – cases were slightly more likely than controls to:
- have a current BMI less than 25
- to be currently using combined HRT
- to have a close family history of breast cancer
- to have had a mammogram in the past two years
- to have experienced natural menopause (as opposed to medically induced menopause)
- to have no children
The only bra characteristic that showed some potential evidence of being associated with breast cancer was cup size (which will reflect breast size). Women who wore an A cup bra were more likely to have invasive ductal cancer than those with a B cup bra (OR 1.9, 95% confidence interval [CI] 1.0 to 3.3).
However, the confidence intervals show that this increase in risk was only just significant, as they show that it is just possible that the risk in both groups is equivalent (an odds ratio of 1). If lower bra cup size was truly associated with increased breast cancer risk, the researchers would expect to see reducing risk as cup sizes got bigger. However, they did not see this trend across the other cup sizes, suggesting that there wasn’t a true relationship between cup size and breast cancer risk.
None of the other bra-wearing characteristics were statistically significantly different between cases with either type of invasive breast cancer and controls.
How did the researchers interpret the results?
The researchers concluded that their findings “provided reassurance to women that wearing a bra does not seem to increase the risk of the most common histologic types of postmenopausal breast cancer”.
This study suggests that past bra-wearing characteristics are not associated with breast cancer risk in postmenopausal women. The study does have some limitations:
- There was only limited matching of the cases and controls, which could mean that other differences between the groups may be contributing to results. The potential confounders assessed were reported to not have a large impact on the results, which suggests that the lack of matching may not be having a large effect, but these results were not shown to allow assessment of this by the reader.
- Controls were not selected for the women with invasive ductal carcinoma, only those with invasive lobular carcinoma.
- As most women wear bras, but may differ in their bra-wearing habits (e.g. when they started wearing a bar or whether they wore an underwired bra), this means it wasn’t possible to compare the effect of wearing a bra versus not wearing a bra at all.
- It may be difficult for women to remember their bra-wearing habits a long time ago, for example, exactly when they started wearing a bra, and their estimations may not be entirely accurate. As long as both cases and controls have the same likelihood of these inaccuracies in their reporting, this should not bias results. However, if women with cancer remember their bra wearing differently, for example, if they think it may have contributed to their cancer, this could bias results.
- There were relatively small numbers of women in the control group, and once they were split up into groups with different characteristics, the number of women in some groups was relatively small. For example, only 17 women in the control group wore an A cup bra. These small numbers may mean some figures are less reliable.
- The findings are limited to breast cancer risk in postmenopausal women.
While this study does have limitations as the authors say, it does provide some level of reassurance for women that bra wearing does not seem to increase risk of breast cancer.
While not all cases of breast cancer are thought to be preventable, maintaining a healthy weight, moderating your consumption of alcohol and taking regular exercise should help lower your risk. Read more about how to reduce your breast cancer risk.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.