"Eating bacon, sausages and other processed meats increases breast cancer risk in older women," reports The Sun. A large-scale study found processed meat – but not unprocessed red meat – was linked to an increase in the risk of getting breast cancer after the menopause.
Processed meat is defined as meat that has been preserved by smoking, curing, salting or adding preservatives. In the UK, commonly eaten processed meats include bacon, sausages and ham.
Processed meat is thought to increase cancer risk because some chemicals added to improve colour and flavour may form cancer-causing compounds.
The links between processed meat and various cancers of the digestive system have been known for some time. What is less clear is if there is a potential link between red or processed meat and breast cancer, as previous studies have had conflicting results.
This study combined a new study of a group of 262,195 UK women with the results from previous research. The combined results showed postmenopausal women who ate processed meat had a 9% higher chance of getting breast cancer than women who ate no processed meat. The new UK study on its own showed that the postmenopausal women who ate most processed meat (more than 9g a day) had a 21% higher risk of breast cancer than those who ate no processed meat.
Because of the type of study, we cannot be sure that processed meat directly causes breast cancer. But restricting your intake to an occasional treat, rather than eating processed meat every day, may benefit your health in other ways.
Where did the story come from?
The study was carried out by researchers from the University of Glasgow. It was published in the peer-reviewed European Journal of Cancer and was funded by the Glasgow University Paterson Endowment Fund.
The Times, The Sun and the Mail Online covered the study accurately, although they did not explain that the nature of the study means we cannot be sure processed meat causes breast cancer.
The Mail Online claimed the World Health Organization (WHO) says processed meat "has the same cancer-causing threat level as cigarettes, asbestos and the deadly poison arsenic", although this is based on a misunderstanding of the WHO classification.
In 2015, the WHO ranked processed meat as a cancer-causing substance (group one carcinogenic). But as we said at the time: "While any substance ranked as a group one carcinogen is known to cause cancer, this doesn't meant the risk of cancer is the same for all substances … smoking a pack of 20 cigarettes a day is far deadlier than eating a ham roll".
What kind of research was this?
The researchers carried out a cohort study using data from participants in the UK Biobank Study (an ongoing study that looks at health information provided by volunteers).
They then added the results of this latest cohort study to previous relevant cohort studies. They then pooled the results of all the cohort studies into a meta-analysis to get the best estimate of the size of the breast cancer risk associated with processed or red meat.
Cohort studies are good ways to identify links between factors, such as meat consumption and breast cancer. However, they cannot prove that one factor directly causes another. Other unmeasured factors could explain the link.
What did the research involve?
For the cohort study, researchers used data from 262,195 women taking part in the UK Biobank general population cohort. The women, aged 40 to 69 and free of cancer when recruited between 2007 and 2010, filled in food frequency questionnaires, gave information about their lifestyle and had their weight and height measured.
They were followed for an average of 7 years to see if they developed breast cancer. Researchers then looked to see whether consumption of either red meat or processed meat increased the risk of them getting breast cancer, after accounting for other factors.
Potential confounding factors taken into account included:
- sociodemographic factors such as age, ethnic group and deprivation
- lifestyle factors such as smoking, alcohol consumption, body mass index and physical activity
- diet (vegetable consumption and type of bread)
- hormone replacement therapy (HRT), for women past the menopause
For the meta-analysis, researchers looked for prospective general population cohort studies that examined the link between breast cancer and red meat or processed meat consumption. They excluded studies that looked at only one type of meat and included only one set of results for each cohort population. They included the results of the Biobank study. They analysed the results by type of meat (red or processed) and type of breast cancer (premenopausal or postmenopausal).
What were the basic results?
Among women in the UK Biobank study, those eating any processed meat had a higher chance of breast cancer than those who ate no processed meat. The increased risk ranged from 15% (hazard ratio (HR) 1.15, 95% confidence interval (CI) 1.04 to 1.28) for women eating up to 4g of processed meat a day, to 21% (HR 1.21, 95% CI 1.08 to 1.35) for women eating more than 9g a day.
However, these figures mostly reflected the risk for women after the menopause. When researchers looked at results for premenopausal breast cancer only, they found no increased risk for women who ate less than 9g of processed meat a day.
After taking account of sociodemographic, lifestyle and dietary factors, the link between red meat and breast cancer disappeared.
In the meta-analysis, researchers included 10 studies, covering 1,386,799 women, plus the Biobank study. They found:
- no increased risk of breast cancer before the menopause for women eating processed meat, based on 6 relevant studies
- no increased risk of breast cancer at any age for women eating red meat, based on 10 relevant studies
- a 9% increase in the risk of breast cancer after the menopause for women eating processed meat (relative risk (RR) 1.09, 95% CI 1.03 to 1.15), based on 6 relevant studies
How did the researchers interpret the results?
The researchers said their results suggested the association between processed meat and breast cancer "is largely driven by the risk of postmenopausal breast cancer". They said that, although the risk was higher among women who ate the most processed meat, "the largest increase in risk of breast cancer was between zero and low intake (4g/day) of processed meat".
This study adds to evidence that regularly eating processed or preserved meat such as bacon and sausages may increase the risk of cancer. The WHO has already said processed meat is likely to cause cancer, based on evidence about bowel and stomach cancer. This study suggests it may also increase the chances of getting breast cancer, at least for women after the menopause.
The study does have limitations. The women taking part in the UK Biobank study were not a representative sample of the UK population – being on average wealthier and healthier – so we don't know if the results can be generalised to the rest of the country. The results also rely on women having accurately reported how much processed meat they eat.
In addition, cohort studies can never account for all the possible factors that may affect the results. The Biobank study was able to account for many important factors, but the studies in the meta-analysis varied in the way they recorded potential confounding factors.
Should women avoid bacon and sausage, to cut their risk of breast cancer? Many factors make up a woman's breast cancer risk, from her genes to her weight, her age, whether she has children, how much alcohol she drinks and whether she takes HRT. Processed meat may be another factor to consider when thinking about your overall risk of breast cancer.
Find out more about breast cancer risk factors.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Sun, 3 January 2018
Mail Online, 3 January 2018
The Time (subscription required), 3 January 2018
Links to the science
European Journal of Cancer. Published online December 21 2017
October 2015 (PDF, 51kb)